What, Me Worry?: Keys to Taking Advantage Of These Uncertain Times (Through a Marketing Prism)
MEMO - Prism Marketing Group
Transcript of MEMO - Prism Marketing Group
SIKA CORPORATION • ROOFING 100 Dan Road ∙ Canton, MA 02021 ∙ USA Phone: 781-828-5400 ∙ Fax: 781-828-5365 ∙ usa.sarnafil.sika.com
MEMO TO Sika Sarnafil Outside and Inside Sales Staff; Manufacture Reps CC Sika Sarnafil Technical Staff FROM Roofing Technical Department PAGES 4 DATE January 28, 2014 Subject: Risks of Concrete Decks
Over the years, the construction industry has been aware of moisture issues due to freshly poured concrete as well as the ability of concrete to hold and absorb great amounts of water. Over time this water may migrate into the roof system, saturating the insulation and cover boards or causing adhered systems to become dis-bonded and potentially cause corrosion to metal components. Many papers and articles have been written discussing the issues of moisture and concrete. These papers identify some of the reasons and issues related to the moisture in concrete, and why they appear to be more prevalent than in the past, such as eliminating vapor retarders, especially ones that are adhered to the concrete deck and the practice of keeping the concrete forms in place, which are typically metal pans.
The most common ways excess water in concrete is generated includes;
• Mixing and pouring new concrete decks/slabs
• Interior finish work, including new concrete pours, water based construction materials including paint, plaster, and drywall application among others and heating the interior with propane or oil burners
• Concrete decks that are exposed to standing water which may come from various sources including exposure to long term leakage into existing roofs, rain or snow and other sources
CONCRETE AND WATER
Concrete is a combination of cement, aggregate (fine and coarse) and water, which typically has about 10–15% cement, 60–75 % aggregate and 15–20 % water. Studies have shown that there may be between 0.9 to 2.6 quarts (0.85 to 2.5 l)of excess water per square foot of concrete surface present in a one month old, 6 inch thick concrete roof deck. This does not include possible water from rain or snow or water from the curing process. This excess water may migrate into a roof system after the concrete has reached sufficient strength or cure which typically is 28 days. With this large amount
Sika Corporation • Roofing 2/4
of free water available it must be noted that cure time (28 days) does not mean the concrete is dry enough to cover.
In addition to normal weight structural concrete (NWSC), there is more lightweight structural concrete (LWSC) being used. The differences between the two structural concretes are the “in place density” and the type of aggregate used. LWSC has a density between 90 and 115 lb/ft3 (1440 to 1840 Kg/m3) and NWSC has a density range of 140 to150 lb/ft3 (2240 to 2400 Kg/m3). The LWSC can achieve the low “in place” densities by using a lightweight porous aggregate filled with air voids. This aggregate will absorb water so it must be saturated before mixing so as to not affect the cement to water ratio causing issues with the final concrete product. The LWSC aggregate can absorb 5 to 25% of its mass with water. To put this in perspective, the Portland Cement Association Engineering Bulletin 119 states the dry down time for LWSC is many months more than NWSC. To achieve a 75% relative humidity for NWSC it will take approximately three month. To achieve the same 75% relative humidity for LWSC it will take twice as long, almost six months according to testing noted in the PCA Engineering bulletin 119. The test was conducted with an 8 inch (20 cm) slab that had both the top and bottom sides exposed to air to dry. Consider, if a roof membrane is installed over the top surface and the bottom surface is a steel form deck (as is very common), the ability of the concrete to dry will be severely affected. The laboratory ideal conditions for the LWSC drying at six months will be much greater under field conditions.
CONSTRUCTIN GENERATED MOISTURE
Various construction activities such as newly poured concrete, water based construction materials including paint, plaster, and drywall application among others will generate and contribute to the accumulation of moisture within an enclosed building space. Additional moisture will be generated when propane or oil burning heaters are used to condition the interior of the building. This heating of the interior may be to help dry the new construction materials or make the interior space more comfortable. To put this moisture accumulation into perspective a 4 inch (10 cm) thick concrete floor slab will generate approximately 1 ton of water for every 1000 square feet of concrete. For every gallon of oil burned there will be 1 gallon of water produced and a 200 pound tank of propane will produce 30 gallons of water. All of this moisture produced and trapped in an enclosed space will affect the roofing system. Should these conditions exist, the project designer and/or the construction manager/general contractor must take steps to properly vent the moisture out of the enclosed space, or provide for a vapor retarder.
WATER ABSORBED INTO CONCRETE
Water, sitting on the concrete deck, as precipitation on new decks, or through long term leakage into existing systems being re-roofed, will typically be absorbed into the concrete deck. The top surface may appear dry, giving a false sense that a roof system can be installed. After the installation of the roof membrane, which will act as a vapor
Sika Corporation • Roofing 3/4
retarder, the moisture within the concrete will migrate into the roof system. The rate of the water migration will depend on the local climate. Often the migration of the water out of the concrete will be greater than the moisture vapor passing through the roof membrane. The accumulation of water may affect moisture sensitive products such as adhesives, paper faced insulation boards and gypsum boards.
DETERMINING MOISTURE CONTENT
The main issue our industry has regarding water and moisture in concrete is there is not a good, practical, consistent and viable test to determine the moisture content or relative humidity of a concrete roof deck. The plastic film test (ASTM D 4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method) is no longer considered a good valid test, especially with LWSC. This is also true for the calcium Chloride test (ASTM F 1869). Independent testing has shown these test methods often give misleading results.
The flooring industry, which has concerns with moisture in concrete, uses a moisture probe test, ASTM F 2170 Standard Test Method for Determining Humidity in Concrete Floor Slab using In-Situ Probes to determine if the moisture in the concrete slab has reached a level where the flooring material can be adhered. This test uses probes that are set into cores of the concrete slab and sealed for 72 hours. This test works relatively well for flooring due to the more consistent indoor temperatures and humidity. For concrete slabs that are exposed to the weather, such as roof decks, the temperature and humidity will vary, which will affect the readings from the probes. The conditioning section for ASTM 2170 states;
“9.1 Concrete floor slabs shall be at service temperature and the occupied air space above the floor slab shall be at service temperature and service relative humidity for at least 48 h before making relative humidity measurements in the concrete slab.”
Based on the conditioning statement, this test is not viable for concrete slabs exposed to the weather.
Furthermore, even if the amount of moisture could be measured easily and accurately in-situ, the industry has not determined or defined what acceptable moisture content in concrete decks is for the installation of a roofing system.
CONCLUSION
Moisture and concrete decks will continue to be an issue for the roofing industry, based on current practices of not including vapor barriers and leaving the metal pan/forms in place. In some sense we may see more issues as there are energy savings realized when the LWSC is used (reduced transportation costs, handling and weight) which may be used to accumulate some LEED points. As noted above, there is currently
Sika Corporation • Roofing 4/4
no acceptable test method to determine the moisture content or relative humidity of a concrete deck that is exposed to the weather.
The 28 day “cure” time commonly referenced with structural concrete is for testing the design compressive strength of the concrete and has no correlation with the moisture content or drying time.
The Portland Cement Association has testing that shows it takes up to 3 months to reach a 75% relative humidity level with NWSC and twice as long with LWSC. Their test was done in a laboratory setting, constant temperature and humidity levels and all sides of the concrete exposed, and without any additional moisture, which often occurs in the field due to precipitation.
Although surface dryness can generally easily be determined the remaining free moisture that is within the concrete slab cannot readily be assessed. The decision of when a concrete deck may be roofed should include the project designer, general contractor, the concrete contractor and suppliers as they will have more knowledge of the concrete formulation, and moisture release rates. This design and management group should communicate with the roofing contractor when they can proceed. The designer of projects that include concrete decks, should strongly consider including in the roofing specification an adequate vapor retarder on the top side of the deck to prevent any water that may be retained in the concrete from migrating into the roofing system over time.
ATTACHMENTS
• SPRI Industry Info. Bulletin No. 2-13 • Moisture in concrete roof decks, M. Graham NRCA • What You Can’t See Can Hurt You, S. Condren SGH • Reducing The Risk Of Moisture Problems From Concrete Roof Decks, G. Doelp SGH
NO:
Industry Alert SPRI, RCI, and PIMA would like you to be aware that:
The roofing industry is increasingly experiencing roof system performance issues
when roof systems are installed over lightweight structural concrete roof decks.
The potential for high moisture content in this type of deck, coupled with the need for
extended drying times, can pose significant risk to long‐term performance and
possible premature roof failure.
This risk can be significantly increased by the standard practice of installing these
decks over non‐removable, form deck or other non‐permeable substrates.
These moisture issues are not unique to the roofing industry. The flooring industry
has experienced parallel moisture issues with lightweight structural concrete, and
those slabs are not subject to periodic rewetting from being exposed to weather, as
roof decks are.
Roofing stakeholders, including designers, property owners, roofing contractors, and
roofing manufacturers can be at significant risk when installing roofing systems over
lightweight structural concrete roof decks with elevated moisture levels.
Determining when a deck is ready for roofing
Test methods include (but are not limited to):
The spot application of hot bitumen;
Electrical impedance;
ASTM D4263 (Plastic Sheet);
ASTM F1869 (calcium chloride); and
ASTM F2170 (relative humidity probes).
Latent moisture However, latent moisture in the deck material may still be present:
Latent moisture may not be measured by the tests noted above and can affect the
long term performance of roofing systems placed over lightweight structural concrete
decks.
There is no industry agreement concerning methods to detect this latent moisture or
level of moisture that may be tolerable.
Loss of adhesion Experience has shown that high moisture content can lead to compromised adhesion:
Adhesive applied or self‐adhering products may show acceptable adhesion, but can be
comprimised due to high/elevated moisture content and upward vapor drive.
Exposed to high/elevated levels of moisture, insulation facers can deliminate from the
substrate or the insulation core and membranes that appear to be initially adhered
can lose adhesion due to moisture migration.
Date: 07/31/2013
No: 2-13
INDUSTRY INFORMATION BULLETIN
To: Roofing stakeholders, including designers, property owners,
roofing contractors, and roofing manufacturers
Topic: Moisture Concerns in Roofing Systems Applied Over Lightweight
Structural Concrete Roof Decks
Industry Info. Bulletin No. 2-13 Date: 7/31/13
Loss of R Value Upward vapor drive that results in entrapped moisture in insulation can:
Result in significant loss of insulation value; and Possibly increase a buildings energy use.
Mold growth potential
Mold growth can occur:
High/elevated moisture levels can create conditions consistent with mold growth
within the roof system.
Water-based adhesive curing issues
Elevated moisture in these roof decks:
Could compromise the cure time of adhesive; or
Cause rewetting of water–based (low‐VOC) adhesives.
Corrosion of roof fasteners and other ferrous-containing roof components
Mechanical fasteners used to attach roof insulation and membranes to lightweight structural concrete roof decks.
There is the potential for the occurrence of fastener and steel plate corrosion due to the presence of elevated moisture levels.
FM Global FM Global has not specifically addressed the moisture in lightweight structural concrete issues:
It is important to note the lightweight structural concrete does not meet FM’s definition of “structural concrete”.
In the June 2012 version of the FM 4470 standard, FM’s defines structural concrete as having a “density of approximately 150 lbs/ft3”.
Lightweight structural concrete has a density of 90 – 120 lbs/ft3.
Conclusion Because of these performance issues and the potential risk for roof system failure, SPRI, RCI, and PIMA urge building designers to select roofing components and system with great care. Our organizations are continuing to study possible roofing solutions which mitigate the risks associated with the use of lightweight structural concrete. We hope to provide further guidance for proper roof design in the future.
\' ~ r :-·
Moisture in concrete roof decks
Concrete's curing and drying rates can affect roof systems
by Mark S. Graham
lATELY, NRCA has experienced an increase
in reports of moisture-related problems
with low-slope membrane roof systems
applied to newly poured, normal-weight
or lightweight structural concrete roof
decks.
In the reported instances, significant
amounts of water have been found within
roof systems within several months to up
to three years after construction. In most
of the situations reported, it was deter
mined the roof membrane was watertight
and not the source of moisture infiltration.
Nevertheless, NRCA has some recommen
dations for avoiding such problems.
Concrete decks
When mixed, poured and formed, normal
weight and lightweight structural concrete
contain significant amounts of water. As
concrete cures and hardens, it consumes
large amounts of this water through hy
dration and evaporation. For example,
a 4-inch-thick concrete slab will release
about 1 quart of water for each square
foot of surface area.
Historically, the roofing industry has
used a minimum 28-day period as a guide
line for applying roofing materials over
newly poured concrete roof decks. The
28-day period coincides with the curing
time of concrete before it is tested for de
sign compression strength. There is little
correlation between this 28-day period
and concrete's true "dryness."
Professional Roofing February 20 l 0
In some instances, a plastic sheet test has
been used to determine concrete's dryness.
With this test, a plastic sheet (4-mil-thick
polyethylene) is taped to the concrete surface
and the plastic sheet's underside is moni
tored for the presence of condensation.
Up to the publication of The NRCA
Roofing and Waterproofing Manual, Fourth
Edition in 1996, NRCA recommended
the plastic sheet test as a method for deter
mining a concrete surface's dryness.
However, with the publication of The
NRCA Roofing and Waterproofing Manual,
Fifth Edition in 2001 and continuing with
the publication ofThe NRCA Roofing
Manual this year, NRCA no longer con
siders the plastic sheet test a viable assess
ment of concrete's dryness.
Similar to the roofing industry, the con
crete industry has seen significant advances
in technology regarding concrete mix de
sign, placement and curing.
For example, the use of concrete additives
in concrete mix designs and curing com
pounds during concrete placement greatly
can accelerate or retard concrete's curing and
release of free moisture. Similarly, weather
conditions, covering newly poured concrete,
timing of concrete form removal, and tem
porary heating or ventilating of a build
ing's interior after concrete placement can
affect the rate of concrete's upward or
downward release of free moisture.
For these reasons, NRCA no longer sup
ports the 28-day drying period or plastic
sheet test.
Tech Today
NRCA's recommendations
NRCA considers the decision of when it
is appropriate to cover a newly poured
concrete substrate to be beyond roofing
contractors' control. Because of the nu
merous variables associated with concrete
mix design, placement, curing and drying, roofing contractors are not privy
to and may not be knowledgeable of
the information necessary to make
such a decision.
Also, though a roofing contractor can
assess the dryness of concrete's topmost
surface, he or she cannot readily assess
any remaining free moisture within
the concrete and its likely direction
of release.
NRCA recommends the decision of
when a newly poured concrete substrate is
ready to be covered with a new roof sys
tem be made with the project or roof sys
tem designer, roof system manufacturer
and roofing contractor. It also would be
useful for designers to consult structural
engineers, general contractors, concrete
suppliers and concrete placement contrac
tors who likely have more knowledge of
concrete's curing and moisture-release
rates.
Additional information regarding con
crete roof decks is contained in The NRCA
Roofing Manual: Membrane Roof Systems-
2007. ~·1ft.
MarkS. Graham is NRCA's associate
executive director of technical services.
23
, •. ;' :~··.:cpy~,§lfl!f-' 'lf' .• . JJIII::r - .
1~!- 1:'!".~l':'l"~ l,!;~·WJ L!r·~.f-'WJ ~ .. . . ~·-~~ ll!L·:::::i: ·lll.. \'(1 •. ..
. ,,~ · :...· .. ,~·· ··:~··
~:w ·~,·~ r:: ~ Total cementitious material
650 710 710 779 pounds per cubic yard (lb/ cy)
Woter~cxementitious material ratio 0.39 0.43 0.43 0.50
Added botch water (lb/cyl 242 308 308 384
1 percent fine aggregate 11 13 13 n/o
absorption (lb woter/cy)
Coarse aggregate 10 73 145 n/o absorption (lb woter/cy)
Total water in botch (lb/cyl 263 394 466 384
Water consumed in 163 178 178 195
hydration reactions (lb/cy)
Water remaining after hydration 100 216 288 189
(lb/cy)
Water remaining in 6-inch con-0.9 1.9 2.6 1.7
crete deck (quart per square foot)
Figure 1 : Water added to and remaining in example concrete mixes
Ll' . -W ~: ~ ~·~· ·-:.!'•· !I!' ·;.II 1~·- . =····· . ::..U.· ~· ~r:Ll.llD II"'. .. [· .:Ir· ::.il I :: llLil) \1' ·:~
.. m. . . . I u;; " 11'· ... ·::JI ~;.;.·:~;:'lr" ·...:...· ~ ~~·
... ,.. Cold-moist climate 0.7 2.2 0.5 International Falls, Minn. (80 percent) (87 percent) (3 3 percent)
Worm-humid climate 0.7 2.2 0 .6
Miami (80 percent) ( 1 87 percent) (35 percent)
Worm-dry climate 0.6 2.1 0.3
Phoenix (70 percent) (83 percent) ( 15 percent)
Figure 2 : Water remaining in concrete roof decks aker one month of exposure
Times have cha nged
Although many modern roof membranes and insulations
are moisrure-resisranr, modern insulation board ofren is
faced with moisture-sensitive paper facers and is frequenrly
adhered ro substrates wirh moisture-sensitive adhesives.
The change ro maisrure-sensirive adhesives is a result of
recent vola rile organic compound (VOC) regulations
rhar caused manufacturers ro convert ro either I 00 per
cent solids, low-VOC or warer-based formulations.
Moisture migrating into roof sysrems from concrete
decks also carries alkaline salrs and high pH from rhe
concrete. When the moisture, salrs and high pH react
with moisture-sensitive materials, rhe paper facers may
grow mold, decay and lose cohesive strength; the water
based adhesives may revert to a liquid; and rhe curing of
foam-based adhesives can be altered or delayed. Gypsum
30 www.professionolroofing.net AUGUST 2012
and wood-fiber based cover boards may lose cohesive
strength. Mold rends to develop when rhe relative humid
ity is above 80 percenr and temperatures are above 41 F. Inrernal condensation can occur daily or seasonally as
roofing materials are exposed ro varying temperatures.
Condensed water can degrade adhesives and facers on
insulation boards, as wel l as gypsum-based and other
water-soluble materials. High moisture levels in cold
regions can lead to freeze-thaw damage of some roofing
materials. Concrete mixes also have changed. In general, concrete
mixes made wirh Ay ash or slag are more durable and may
contain admixtures (chemicals added to the concrete to
cause a specific performance change) rhat reduce the water
content. These improvements in concrete technology result in concrete mixes being less permeable, which slows
rhe drying of moisture from rhe concrete. Increased use
of lightweight structural concrete, with irs pre-wetted
aggregates, contributes additional moisture ro concrete that further extends a concrete deck's drying period.
p
In addition, roof deck design has changed. Cast-inlace concrete roof deck design may now include com
osil:e.CQ.O.St.tl.iCUOILwiclw:hc:.s.teeLfoans..(mer.aldeckUefr.. n place. The meral deck prevenrs the drying of moisture
rom rhe deck's underside. Although some metal decks
contain perforations along the ribs (Aures) on rhe corru
n
i
fJ
g
c a
ated forms, rhe perforation area is only 0.25 (o 1.5 per
ent of rhe meral deck surface, allowing only a minimal
mounr of moisture evaporation through the forms.
And because currenr construction schedules usually
require a roof sysrem be installed within three ro four
weeks of placing rhe concrete roof deck, once a modern
roof system is installed, a majority of rhe concrete mois
ture will remain entrapped within the completed roof
system. A metal deck form inhibits moisture loss from rhe
concrete deck's underside, and the roof membrane inhib
its moisture from leaving through the top of rhe roof
system. Modern roof systems are susceptible to internal
condensation, and rhe wer materials will be ar risk of
moisture degradation, mold and decay.
Concrete moisture content
Concrete is a mixture of portland cemenr, aggregates, air
voids, warer and other additives. A portion of the mix warer reacts wirh rhe cement and any poz.zolanic additives
present to form rhe hydra red binder of rhe hardened
ce
y
s
concrete. Additional water provides the fluidity for place
ment and finishing of freshly mixed concrete. Although
chemical admixtures may reduce the amount of addi
tional water required, all freshly placed concrete will
contain surplus water that is not consumed in hydration
reactions.
The mix water added to the concrete mixture is ex
pressed as the water-to-cementitious material ratio (w/cm).
Most concrete mixes are hatched at 0.40 to 0.55 w/cm.
The cement hydration process, most of which occurs
with in the fi rst month, consumes about 0.25 w/cm.
T his leaves an additional 0 . I 5 to 0.30 w/cm, which will
remain in the concrete as liquid water.
In addition, the aggregates absorb water- 0 .1 to 2
percent for normal weight aggregates and between 10
and 30 percent for lightweight aggregates. T his water
also will be contained within the concrete after it has
been placed .
To demonstrate how much water remains, we calcu
lated the tree water in concrete after hydration for typical
roof deck concretes (see Figure 1) .
Our calculations show 0.9 to 2.6 quarts of excess water
per sq uare foot of concrete surface will be present in a
typical one-month-old 6-inch concrete roof deck with
out accounting for any additional water from the curing
process or rain. This is the estimated water that will be
available to migrate into a roof system installed after the
concrete has developed design strength.
Concrete drying
The freshly placed concrete's interior voids (pores, capil
laries and aggregate porosity) initially are saturated with
water. When d rying begins, concrete loses water by evap
oration from the deck's surface. The concrete surface may
appear dry, but it may be an illusion. As the surface loses
water, d iffusion begins, drawing water and alkaline salts
from the wet interior to the surface. W hen roofing mate
rials are installed, the water evaporating from the con
crete's top surface becomes sealed within the roof system.
Our company used WUFI, a one-dimensional, validat
ed heat and moisture transport software application, to
estimate the drying of concrete decks cast over vented
metal decks during the one month before roof system
installation and during the two-week exposure as required
by the cellular concrete manufacturers. We selected three
climate zones across the U.S. as benchmarks for our cal
culations and assumed there was no added water from a
curing process or rain during the initial drying period. To
estimate the best conditions fo r drying, we based the d ry
ing simulatio n on local climate data beginning July I .
We found the amount of water evaporating from
exposed concrete roof decks during the month before
installing a roof system, with the exception of cellular
concrete, is only a small portion of the available excess
water remaining in the concrete roof decks. The water
that remains in the deck after this initial period of evapo
ration is shown in Figure 2.
Our results show there is no location in the' U.S. chat
will dry all the excess water contained in che concrete
within 30 days after placement.
100 r 95 90 85
] 80 75 t i 70
r ~ 65 ... 60 J:! 55 ~ 1 1
50 45
I 0 40 1l 35 ~
30 £ 25 20 15 10
C.a...lcucOI'IC.r.,. lntefno~ fols, M.nn. ,
I ! Cc-!tvklr con<re.,., Miomi
Notmol weight Con<tdt Mioft'li
tigf'ltwefght (Oft(t~hr MOotroi
I~<J .. rwhQhl co•~• ~ !rkofMhonnl ~,., .. ~ M· 11
I --------------------~0-3~0 60 90 120 150 180 210 2...,.40~2~7~0-3~0~0-330 360 Number of days
Note: Cellular concrete must be roofed over between three and 14 days (value in figure reported at 14 days)
Figure 3 : Water loss by evaporation from vented concrete decks exposed for a year (assuming a July 1
summer costing dote and no rain during the entire year)
Figure 3 shows how much d rying can occur if the
evaporation time is extended to one year. The graphs are
based on the assumption that a deck can be left exposed
for one year without precipitation, an unlikely condition
without providing temporary protection. Normal weight
and lightweight concrete decks will contain water if al
lowed to dry for one year in all regions.
In the real world, water from curing and precipitation
must be taken into account, which means the free water
content within the concrete can be expected ro remain
significantly higher for prolonged time periods.
To evaluate the effect free water in a concrete deck
has on roofing materials, we need to consider the vapor
AUGUST 20 1 2 PROFESSIONAL ROOFING 31
ON tin· WEB
To read more about
moisture in concrete roof
decks, log on to www
. professianalroofing. net.
Phoenix (hot-dry)
International Foils, Minn. (cold-moist)
Miami (hot-humid)
drive of rhe moisture into rhe roofing materials from the
concrete. We use relative humidity to evaluate the vapor
drive. When concrete begins drying, the relative humid
ity within the concrete is 100 percent. As drying contin
ues, the relative humidity decreases.
After roofing materials are installed, the moisture in
the concrete raises the relative humidity of the air trap
ped within the roof system above the concrete deck and
begins to cause problems. Typical organic roofing com
ponents are susceptible to mold and ocher decay organ
isms when the relative humidity is greater than 80 per
cent for prolonged rime periods. Relative humidity also
is important for determining when condensation may
occur within a roof system and when condensed water
may begin deteriorating moisture-sensitive roofing
components.
To demonstrate the moisture drive from concrete to
roofing materials, we calculated the relative humidity
withi n one-month-old concrete at the nme the roofing
materials are installed. We found the relative humidity
within concrete was above 95 percent in all locations (see
Figure 4), indicating the roofing materials will be exposed
to deleterious amounts of m-oisture.
94 98 98
97 99 99
97 99 99
Figure 4: Relative humidity of one-month-old concrete roof deck
Because most roof decks receive a roof system within
one month of placement (when concrete is still wet), an
alternative approach is needed to address the entrapped water.
Testing for moisture
Roofing contractors have limited ability to evaluate rhe
amount of water present in roof deck concrete and relate
that moisture ro potential effects on the proposed roof
system's durability.
There are several hand-held surface meters that purport
to measure concrete's moisture content. These meters
supply numeric values, and some indicate the values
ro be percentages of moisture. This data is best used as
arbitrary numbers to compare readings with a concrete
32 www.professionolroofing.net AUGUST 2012
surface known robe dry. The problem is char though the
meters may d isplay an acceptable value (suggesting a con
crete surface might be dry), the concrete's interior is likely
to be wet, and this moisture will migrate to the surface
after rhe deck is covered with roofing materials. Moisture
meter readings at the surface can provide a false impres
sion concrete is dry.
More generally, there are no adequate rests that can
determine acceptable moisture content of roof deck con
cretes for installing specific roof systems. ASTM Inter
national provides standard test methods for determining
moisture-related properties of concrete under controlled
interior conditions, such as interior concrete floors. These
methods include ASTM Standard F 1869, "Standard Test
Method for Measuring Moisture Vapor Emission Rare of
Concrete Subfloor Using Anhydrous Calcium Chloride,"
and ASTM Standard F2170, "Standard Test Method for
Determining Relative Humidity in Concrete Floor Slabs
Using in situ Probes." T hese methods are difficult to use
on roof decks because of solar radiation, temperature
fluctuations and precipitation.
Another standard, ASTM 04263, "Standard Test
NfeiliOc!TorTI1dicaring Moisture in Concrete by Plastic
Sheer Method ," determines the presence of capillary
moisture buc will not provide moisture levels. The results
of rhe ASTM International rests are extremely .difficult
to interpret for evaluating the amount of moisture in
roof deck concrete rhar will migrate into a roof system
as the roof system undergoes substantial temperature
fluctuations.
Currently, NRCA and rhe Midwest Roofing Contrac
tors Association (MRCA) state a concrete deck is likely to
contain levels o f water that could be detrimental to a roof
system. Methods for resting moisture are discussed in
their publications; however, they acknowledge the roof
ing industry has no acceptable moisture values for the
safe application of roofing. MRCA recommends a roof
system should include a complete vapor retarder seal.
NRCA cautions the decision of when it is appropriate to
cover newly placed concrete is beyond roofing contrac
tors' control.
We believe the only reliable method to predict the
potential for excessive moisture within a roof system is
ro perform hygrorhermal modeling simulations for the
specific environment and roof system being evaluated.
Several computer modeling progranlS are available for
conducting these simulations.
e
1 ·
ly
se
St
,f
:s
Hygrothermal rnodeli11g We conducted hygrothermal modeling of modern roof
systems in three climate zones using WUFI. The pro
gram model~ building envelope behavior on an hourly
basis (transient) according to interior and exterior cli
mate conditions. We modeled our roof systems for two
to five years. To develop an effective model, we consid
ered the local climate because changes in temperature
play a major role in moisture migrating out of the con
crete and into roofing materials.
Our studies show concrete deck moisture diffuses into
roofing materials under all condirions of exterior temp
erature and humidity in all the geographical locations
studied. In all cases, moisture diffused out of the concrete
faster than it could diffuse out of the roof system when a
vapor retarder is not included. This trend causes moisture
to accumulate within the roof system and could result in
the deterioration of moisture-sensitive materials in the
roof system.
The moisture diffusion in the modeled systems per
formed differently because the rate at which moisture dif
fuses out of concrete is based on the temperature profile
across the deck system, and the rate at which moisture
diffuses from the roof system is based on roof membrane
permeance. Other factors include membrane color (which
affects solar loads and radiant cooling), type, thickness and
the length of the condensation cycles a roof system experi
ences. Regardless, all the roof systems modeled became wet.
The next step of our hygrothermal modeling included
studying the same roof systems but including a vapor
------ retar er:'.()ur goal was tO mm1m1ze the potennaJ of any
components within the roof system from reaching pro
longed relative humidity levels greater than 80 percent.
Keeping the relative humidity levels below 80 percent
would minimize the potential for mold and condensation
within the roof system. Our modeling considered the
concrete deck's initial moisture content, the roof mem
brane's moisture permeance, and the temperature profile
between exterior and interior surfaces the system will
experience during its service life.
We varied the materials and components in the roof
system until we found systems that will perform in the
specific climates. We reduced the moisture emission out
of concrete by adjusting the vapor retarder's permeance
of the vapor retarder at the concrete surface. We found
a vapor retarder reduces rhe moisture collection within
the roof system to acceptable levels but only if the vapor
retarder's permeance is less than the roof membrane's
Water condensing on the underside of the membrane dripped bock onto the concrete deck.
This roof did not leak.
Roofing material was applied to a concrete deck without a vapor retarder in the mid·
Atlantic region. The membrane lost bond at the wet insulation facer. This roof did notleok.
permeance. Vapor retarder selection must be coordinated
with the type of roof membrane, and the system must be
evaluated for the influence of the local climate.
We did not find systems that will perform equally in
all climates. In other words, a system that maintains low
moisture within the roof system in a hot-humid climate
may not work in a hot-dry climate or cold-moist climate.
AUGUST 2012 PROFESSIONAL ROOFING 33
We found each geographical location required some fine
tuning of.rhe roof system ro make it work.
StL iy conclusions
Typical concrete roof decks installed on steel forms will
conrain I ro 2\12 quarts of free water per square foot of
deck area after rhe decks have cured . Our moisture analy
sis demonstrates much of rhis warer will remain in the
concrete deck following roof system installation.
Because of construction schedules, roof systems are
being installed onro wet concrete even though rhe con
crete's surface may appear dry. Over time, the internal
warer in the concrete will migrate to the roof system .
Without a vapor retarder, a roof membrane will perform
as a vapor retarder and trap moisture within the roof
system. The enrrapped moisture will degrade moisture
sensitive materials. T he rate and amounr of water migrat
ing inro the roof system will depend on the local climate.
Effective roof systems incorporating a vapor retarder can
be designed for specific installations and climatic condi
tions to minimize the derrimenral influence of concrete
------moisture-on roofingmaterial . Dcvcloping"fhese d-esigns
Apparent mold growth on insulation facers in a roof located in the Northeast. The roofing material was applied to a concrete deck without a vapor retarder and became loose. This roof did not leak.
' ' ,,
....
. ...... ' ' ..
"" '- .... ' V•
' .....
. -- { <
' .
Deterioration in a gypsum·based cover boord in a roof system in the M idwest. This roof did not leak.
34 www.prafessianalraofing.net AUGUST 2012
requires systematic evaluation of all componenrs in the
roof and involves hygrorhermal modeling calculations.
T he numbers presenred are based on the id~al condi
tions of placing, finishing and curing the concrete without
adding water during construction or rewerring by rain .
In reality, cement will nor be fully hydrated and a deck
likely will be wet-cured or subjecred to some precipita
tion before application of roofing materials. These condi
tions will increase the amount of water entrapped in the
deck by the rime roofing materials are applied .
F ecomrnendol ions
More research is needed ro determine the maximum
amounr of moisture roofing materials can safely toler
are. In the meantime, roof systems should be designed
and installed with vapor retarders to mitigate moisture
entrapped in concrete roof decks. "•"~~-
STEPHEN J. CONDREN, P.E., is a senior project manager
at national engineering fi rm Simpson Gumpertz & Heger Inc.,
Boston; JOSEPH P. PINON, P.E., is a senior project man·
ager at Simpson Gumpertz & Heger, San Francisco; PAUL C. SCHEINER, Ph.D., is staff consultant at Simpson Gumpertz & Heger, Boston.
REDUCING THE RISK OF MOISTURE PROBLEMS FROM CONCRETE
ROOF DECKS
by
Gregory R. Doelp, P.E. and Philip S. Moser, P.E.
ABSTRACT
In recent years, the roofing industry has become increasingly aware of the problems caused by
moisture in concrete roof decks that migrates into the roofing system. Installing a vapor retarder
over the concrete deck is the primary method of addressing this problem. This paper summarizes
some of the challenges associated with incorporating a vapor retarder into the roofing system.
For example, selecting a vapor retarder of the appropriate vapor resistance is challenging due to
the shortage of published data on the acceptable moisture limits of roofing materials. We explore
the question of acceptable moisture limits through an extensive review of published literature,
product-specific recommendations from manufacturers, and some preliminary laboratory testing
of some common roof cover boards. This paper is based on the authors’ experience as designers
and investigators of roofing systems, literature review, and laboratory testing.
1. BACKGROUND
1.1 Consequences of moisture
While the primary function of a roofing system is to prevent water from passing through it into
the building or structure below, water or moisture vapor that collects within the roofing system
can also be detrimental, both to the roofing system’s immediate performance and its long-term
durability. Besides leakage to the interior, moisture in roofing systems can have numerous
negative consequences including the following:
Reduced thermal resistance of insulation.
Loss of strength of the insulation, cover board (Photos 1 and 2), adhesive, or fasteners
(Photo 3); leaving the roofing system vulnerable to uplift damage from wind or
crushing from foot traffic or hail.
Page 2
Photo 1- moisture damaged gypsum cover board.
Photo 2 – moisture damaged fiberboard cover board.
Photo 3 – corrosion of roofing fastener
Deterioration of the structural deck.
Page 3
Dimensional changes in the substrate, which can in turn damage the roof membrane.
Blistering or weakening of the roof membrane itself, especially BUR.
Mold growth.
1.2 Sources of Moisture
Moisture in the roofing system can come from a variety of sources, such as:
Installation of wet materials (i.e., insulation that was not properly protected from
weather while stored on site).
Water leakage through the roof membrane, flashing, or adjacent construction.
Moisture vapor from interior humidity –
Moisture vapor from interior humidity may migrate into the roofing system by
diffusion if no vapor retarder is installed. The need for a vapor retarder to limit
the migration of interior moisture into the roofing system is generally
acknowledged to depend on the local climate and the interior conditions of the
building. Later sections of this paper include additional information about
determining when a vapor retarder is needed.
Moisture vapor from interior humidity may be carried into the roofing system
by air leakage if there is no air barrier in the roofing system, particularly if the
building is positively-pressurized due to stack effect or the operation of the
HVAC system. This is generally addressed by including an air barrier in the
roofing system and connecting it to the air barrier in the wall system to provide
a continuous barrier.
Concrete deck – When a new concrete deck is poured, some of the mix water is used up
in chemical reactions as the concrete cures and some evaporates, but the rate of
evaporation is slow, so large quantities of water remain stored within the pore structure
of the concrete for extended periods of time. While the concrete itself is generally not
damaged by this moisture, the moisture may migrate into the roofing system where it is
absorbed by materials that are more sensitive to moisture. Historically, roofing systems
were adhered to concrete decks in hot asphalt; the hot asphalt (often with reinforcing
felts) provided the additional benefit of limiting the rate of moisture migration from the
concrete into the roofing system. However, modern single-ply roofing systems are
often installed today without any vapor retarder on the concrete deck.
Comparison can be made to the flooring industry, which has also suffered detrimental
effects from moisture diffusing out of concrete, and as a result has developed consensus
test methods for measuring the internal relative humidity (RH) of concrete or moisture
evaporating out of concrete. Flooring manufacturers typically specify acceptable RH or
moisture vapor emission limits for the concrete as a condition of their warranty. By
Page 4
contrast, the roofing industry, while it has begun to focus more attention on this issue1,
had not (as of 2011) “established any benchmarks or acceptance levels” for moisture in
concrete2.
A 2012 research update3 proposes that “Until we have more data, 75% relative humidity
appears appropriate for normal weight concrete”, and recommends monitoring the RH
of concrete according to ASTM F21704 to determine when it is “dry” enough to roof
over. However, one limitation of F2170 testing is that the standard requires
conditioning both the concrete slab and the air above it to a constant “service
temperature” and relative humidity for at least 48 hrs before making measurements, but
constant temperature and RH do not exist for an in-service roof – the conditions vary
constantly with the weather. Furthermore, the effect that concrete moisture has on the
roofing system will depend on the specifics of the roofing system and the local climate,
so it may be difficult to establish an industry-wide “acceptable” level of moisture for all
concrete roof decks.
Another recent study5 found that concrete retains significant amounts of water after
months of drying, and therefore, high moisture levels are likely to still be present when
the roofing system is installed. In most roofing installations, it is impractical to wait for
the concrete deck to “dry” fully; it is often faster and more reliable to install a vapor
retarder over the concrete deck to inhibit the migration of moisture from the concrete
into the roofing system. Specifying the vapor retarder presents several challenges, as
discussed in the next section.
2. VAPOR RETARDER CHALLENGES
2.1 Wind Uplift Rating
In many roofing systems installed over concrete decks, the roof insulation is adhered to the
concrete (often with ribbons of low-rise foam adhesive). Incorporating a vapor retarder into the
roofing system means adding another layer that needs to be adhered to the concrete, and to which
the insulation needs to be adhered. In this situation, the vapor retarder can affect the wind uplift
resistance, so it is crucial that the vapor retarder be part of the tested assembly. Most roofing
system manufacturers now offer tested assemblies that include adhered vapor retarders, but the
relative number of options for this system is more limited than those without a vapor retarder.
1 Graham, Mark S. Moisture in Concrete Roof Decks, Professional Roofing, February 2010
2 Structural Lightweight Concrete Roof Decks, MRCA T&R Advisory Bulletin 1/2011. September 2011.
3 Dupuis, Rene. Research Continues on Moisture in SLC Roof Decks, Midwest Roofer, June 2012
4 ASTM F2170-11, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs
Using in situ Probes. ASTM International, 2011. 5 Condren, S; Piñon, J; and Scheiner, P. What you Can’t See Can Hurt You – Moisture In Concrete Roof
Decks Can Result in Premature Roof System Failure. Professional Roofing, August 2012.
Page 5
Recent searches of FM Global’s RoofNav online database6 found that there are 2.5 to 3 times
fewer tested systems with a vapor retarder as the number of systems without a vapor retarder.
These searches included both adhered and mechanically-attached insulation systems; in the
authors’ experience, roofing system designs with adhered insulation and a vapor retarder have
even fewer options.
If the vapor retarder is going to be adhered, moisture in the concrete deck can affect the adhesion,
so the question of acceptable moisture content in the concrete deck still needs to be addressed.
Similar to adhering a plaza waterproofing membrane or deck coating to concrete, it is advisable to
contact the manufacturer for recommendations and use a mockup as the final criteria for
evaluating whether good adhesion can be achieved.
Fortunately, designers have other options for securing roof insulation besides adhesive. Roof
insulation (or membranes) can be mechanically attached through the vapor retarder into the
concrete deck, which avoids the difficulty in finding a tested system that relies on adhesion of
(and to) the vapor retarder. Alternatively, roofing systems in some regions can be ballasted;
however, building codes prohibit stone ballast in some high-wind regions.
2.2 Product Selection
Another challenge is determining the appropriate vapor permeance for the vapor retarder. Some
designers rely on past experience and rules of thumb or the minimum requirements of the
building code, while others use moisture vapor transmission calculations to predict the in-service
moisture contents of any moisture-sensitive materials in the roofing system. Rules of thumb and
calculations are discussed in more detail below.
2.2.1 Rules of Thumb
One rule of thumb7 that is sometimes used is that every layer in the system should be 10 times
more permeable than the vapor retarder to avoid creating a vapor trap. Table 1 lists the typical
range of permeance of some common roof membranes and vapor retarders. Because of the low
6 www.roofnav.com searched September 2012 with the following search terms: single-ply roof system,
new roof application, adhered cover, structural concrete deck, board stock insulation, and wind uplift
ratings ranging from 60 to 300. 7 Like any rule of thumb, this is a generalization, and there are some conditions where it may not be
suitable.
Page 6
permeance of most roof membranes, using a vapor retarder often violates the rule of thumb for
avoiding a vapor trap. This means many roofing systems have very limited ability to self-dry any
water that leaks through the membrane. Even a small membrane defect, which may not produce a
large enough volume of leakage to appear on the inside of the building, can cause water to
accumulate in the roofing system over time and cause concealed damage. Desjarlais explains the
disadvantages of compact roofs in several of his publications8
9
10 and recommends avoiding
vapor retarders where they can be shown to be unnecessary. However, in many cases including
new construction with concrete decks, vapor retarders have been shown to be necessary11
.
Table 1 – Permeance of Selected Roof Membranes and Vapor Retarders
Material Thickness
(mils = 0.001
inch)
Permeance (US perms)
- ASTM E96
Procedure B12
Sources13
Roof Membranes
BUR Not reported 0.0 ASHRAE14
EPDM 60 mils 0.03 – 0.04 2 manufacturers
PVC 60 mils 0.05 - 0.22 4 manufacturers
TPO 60 mils 0.01 – 0.05 3 manufacturers
Vapor Retarders – Loose Laid
Polyethylene or
“polyolefin” sheet –
various grades
6 mils 0.059 - 0.13 2 manufacturers
7-8 mils 0.038 1 manufacturer
10 mils 0.019 – 0.039 3 manufacturers
15 mils 0.009 – 0.021 2 manufacturers
Polyolefin – aluminum
composite
14-15 mils total 0.000 1 manufacturer
8 Kyle, D; Desjarlais, A. Assessment of Technologies for Constructing Self Drying Low Slope Roofs,
CON 380, Oak Ridge National Lab, p. 82, May 1994. 9 Desjarlais, A; Byars, N. A New Look at Moisture Control in Low Slope Roofing, 4
th International
Syposium on Roofing Technology, pp. 341-346, NRCA, September 1997. 10
Desjarlais, A; Karagiozis, A. Review of existing criteria and proposed calculations for determining the
need for vapor retarders. North American Conference on Roofing Technology, pp. 79-83, NRCA,
September 1999. 11
Condren et al, 2012. 12
The conditions at which permeance is measured can significantly affect the result, but most
manufacturers only report data for standard conditions. Some manufacturers do not report the test
procedure, but most do. Those that do report a procedure report Procedure B. 13
Manufacturer’s data is from a combination of published data sheets and telephone conversations with
technical representatives, September 2012. 14
ASHRAE Fundamentals, American Society of Heating, Refrigerating and Air Conditioning Engineers,
2009.
Page 7
Material Thickness
(mils = 0.001
inch)
Permeance (US perms)
- ASTM E96
Procedure B12
Sources13
Vapor Retarders – Self Adhered
Polyolefin -
bituminous composite
32 mils total
.017 2 manufacturers
2.2.2 Moisture Vapor Transmission Calculations
Calculations provide a more sophisticated analysis than simple rules of thumb, but have their own
challenges.
Moisture moves between components of the roofing system by diffusion over time. The moisture
content of a particular layer may vary with daily or seasonal weather variations, and there may be
a net wetting or drying over the long term (multiple years). The construction industry has been
developing and publishing methods for evaluating moisture vapor flow for many years. Hand-
calculation methods developed specifically for the roofing industry include two published in
198015
and 198916
; other criteria exist in ASHRAE and NRCA publications. In the past two
decades, exponential increases in available computing power have made state-of-the-art computer
programs for predicting moisture vapor flow (e.g., MOIST and WUFI) readily available. These
programs have made moisture vapor transport easier to quantify and are more accurate than
previous hand calculations. These state-of-the-art computer programs are now in widespread use
by building envelope consultants.
WUFI17
, a computer program by the Fraunhofer Institute for Building Physics (Germany),
calculates transient one-dimensional heat and moisture transport, and can be used to
quantitatively predict how moisture levels within a building envelope assembly vary over time.
WUFI uses historical, hourly weather data for user-selected project locations, to simulate time-
15
Condren, S.J., Vapor Retarders in Roofing Systems: When Are They Necessary?, Moisture Migration in
Buildings, ASTM STP 779, M . Lieff and H.R. Trenschell, Eds., American Society for Testing and
Materials, 1982, pp. 5-27. 16
Tobiasson, W. Vapor Retarders for Membrane Roofing Systems. 9th
Conference on Roofing
Technology, NRCA, pp. 31-37, May 1989. 17
A free educational version is available online at http://www.ornl.gov/sci/btc/apps/moisture/ ; the
professional version is for purchase.
Page 8
varying exterior conditions (temperature, relative humidity (RH), solar exposure, etc.) during the
course of the simulation. An example output screenshot is shown in Figure 1.
Figure 1 – Example WUFI simulation screenshot for a roofing system, showing the results for a typical year in
International Falls, MN. The physical layers of the roofing system from top (exterior) to bottom (interior) are
listed from left to right. The red shading indicates the range of service temperatures, the green indicates the
range of relative humidity, and the blue indicates the water content.
WUFI can also simulate several years of moisture migration to analyze seasonal variations and
year-to-year cumulative wetting or drying trends. The output data can be easily reviewed to
determine, for each layer in the roofing system, moisture-related data such as (1) maximum
annual moisture content, (2) quantities of condensate (if any), (3) number of occurrences of
moisture content exceeding established thresholds, and (4) number of freeze-thaw cycles.
A recent study18
used WUFI to analyze a variety of roofing systems in a variety of U.S. climates
and found that in new construction, all roofing systems constructed over cast-in-place concrete
decks accumulated problematic levels of moisture within the roofing system unless a vapor
retarder was included. The study further found that selection of an effective vapor retarder
depends on the roof membrane; on typical buildings, the vapor retarder should have a lower
permeance than the roof membrane. The selection of an effective vapor retarder also depends on
18
Condren et al, 2012.
Page 9
the local climate; the study “did not find systems that perform equally in all climates… each
geographical location required some fine-tuning of the roof system to make it work”.
One limitation of WUFI analysis is that it focuses on vapor diffusion and generally does not
account for bulk air or water leakage; therefore, the accuracy of the results depends on the roof
membrane and air barrier to be functioning properly. Also, interpretation of WUFI results
requires knowledge or assumptions regarding the acceptable moisture limits of the materials
being considered; the study recommends more research “to determine the maximum amount of
moisture that roofing materials can safely tolerate.” Quantitative knowledge of moisture content
and acceptable moisture limits would also be valuable when one is asked to evaluate existing
roofs to determine whether problematic levels of moisture are present.
3. ACCEPTABLE MOISTURE CONTENT OF ROOFING MATERIALS
Several industry sources have recognized the need for more research to determine acceptable
moisture limits for roofing materials. The recommendation in the concrete moisture study
discussed in the previous section echoes an earlier recommendation by Kyle and Desjarlais19
that
“researchers must establish a set of moisture limits with a reasonable safety factor by means of
well-controlled experiments.” This need was discussed even earlier by Tobiasson20
, who stated:
“For most roofs in most locations, the objective is to limit seasonal wetting to an acceptable level.
This level will vary with the moisture-sensitivity of the materials present… However, developing
moisture limit states for each of the myriad roofing systems on the market is a sizable task that
has not yet been accomplished.” Kirby21
similarly concluded that “the roofing industry does not
have a consensus evaluation method for determining whether insulation is wet.”
This section discusses information on acceptable moisture limits for roofing materials based on
three sources: (1) limits proposed in industry publications, (2) manufacturers’ recommendations,
and (3) recent laboratory testing conducted in support of this paper.
19
Kyle and Desjarlais, 1994. 20
Tobiasson, W. Condensation Control in Low-Slope Roofs, Proceedings of Workshop on Moisture
Control in Buildings, Building Thermal Envelope Coordinating Council (BTECC), Washington, D.C.,
September 1984. Also available as CRREL Misc. paper 2039. 21
Kirby, J. Determining When Insulation is Wet, Professional Roofing, February 1999.
Page 10
3.1 Industry Publications
A wide variety of information and opinions on acceptable moisture content are available,
illustrating the lack of consensus. We reviewed technical literature spanning the past 35 yrs and
found several conflicting theories regarding acceptable moisture limits in roofing materials. The
theories for acceptable moisture limits discussed below are generally listed in order of least
stringent to most stringent, although there is ambiguity in some of the criteria.
Thermal Resistance Ratio (TRR) 80% maximum: The thermal resistivity (R value) of
insulation is reduced when it becomes wet. In 1991, researchers at the U.S. Army
Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL)22
,
proposed the following criterion: “The ratio of a material’s wet thermal resistivity to its
dry thermal resistivity is termed its TRR... Insulation with a TRR of 80 percent or less
is, by our definition, ‘wet’ and unacceptable.” The paper acknowledges that “For some
insulations, less moisture than that required to reduce the TRR below 80 percent can be
detrimental for other reasons (e.g., delamination, rot and corrosion of fasteners). It is
not yet known what those moisture “limit states” should be… As additional information
on other moisture “limit states” becomes available, it is expected that maximum
acceptable moisture contents for some materials will decrease below the 80 percent
TRR values.” The paper further acknowledges that EMC (discussed below) is a more
appropriate pass-fail criterion for new materials to be installed; TRR is proposed
primarily for deciding when to replace existing insulation. Table 2 summarizes some
the EMC [equilibrium moisture content] and TRR data for some other insulation
products still in use today.
Seasonal Wetting of 1-2% by volume23
. For foam insulation with 2 pcf density, this
equates to 31-62% moisture content when calculated as a % of dry weight.
No Visible Liquid Water: Kirby24
states, “one factor can never be ignored – if liquid
moisture is present in existing insulation, the insulation is too wet to be left in place or
re-covered.”
Only Small Amounts of Condensation - Tobiasson25
stated that “A small amount of
moisture may condense then without doing any real harm.” He also stated that “A little
condensation on the coldest day of the winter will do no harm, but when condensation
occurs for many days, weeks, or months, the amount of moisture deposited can create
major problems.”26
Condren took a similar approach but quantified his acceptable
22
Tobiasson, W.; Greatorex, A; VanPelt, D. New Wetting Curves for Common Roof Insulations,
International Symposium on Roofing Technology, pp. 383-390, 1991. 23
Tobiasson, 1984, cites the following source: Hoher, K. “Environmental and Climatic Factors in the
Specification of Roofing Membranes” 1982, Sarnafil, Canton, MA. 24
Kirby, 1999. 25
Tobiasson, 1989 26
Tobiasson, 1984.
Page 11
“small amount” of condensation, proposing a limit of no condensation deeper than 1/16
in. or 1/32 in. below the membrane, depending on the substrate material27
. Desjarlais
and Karagiozis28
base their analysis on the criteria that the insulation should not have a
relative humidity of 100% (i.e., condensation occurring) for more than 24 hrs, which is
consistent with one of the three criteria in ASHRAE 160 (discussed below).
No Condensation – Desjarlais and Byars29
contend that “moisture accumulation in a
roof system must not be large enough to cause condensation within the roof, since this
can damage the insulation and reduce its effectiveness.” They also state that “moisture
accumulation should not be great enough to cause degradation of the insulation material
or membrane. To pass this requirement, there must be no condensation under the roof
membrane.”
Equilibrium Moisture Content (EMC) 40%, 45%, or 90% maximum – The concept of
EMC as a metric for determining acceptable moisture levels in roofing materials was
first proposed in a 1977 paper30
regarding roofing felts, but later expanded to include
insulation and other materials. The 1977 paper proposed 40% EMC as the limit for
roofing felts that were sufficiently dry to avoid the appearance of blisters when hot
asphalt is applied to the felts in construction of a built-up roofing membrane. In
addition to the blistering concerns, the 1977 paper found that conditioning organic and
coated organic roofing felts in a moist environment reduced their tensile strength
(compared to oven-dried samples) by 6-18% when conditioned at 40% RH for 6 weeks,
and by 11-38% when conditioned at 90% RH for 9 weeks. The effects of moisture on
roofing felts were also studied by several other researchers around the same time31
. The
1977 paper also examined the EMC of insulation materials, and notes that “The test
results do not allow definite conclusions as to the “tolerable” moisture level of the
insulations. However, we expect any “excess” moisture in the insulation to be available
to influence the roofing membrane… in view of our field experience and pending
further research results, it seems prudent to assume that insulation moisture will not
damage the system, if the moisture does not exceed the equilibrium moisture content
attained by 40% RH storage.”
A 1985 article32
provides equilibrium moisture content (EMC) for various roofing
materials conditioned at 20° C (68° F) and both 45% and 90% RH. The article notes
that “When the material contains more water than its EMC, it is wet and may donate
water to surrounding air or materials”. The EMCs for unfaced polyisocyanurate
27
Condren, 1982 28
Desjarlais and Karagiozis, 1999 29
Desjarlais and Byars, 1997. 30
Schwartz, Thomas A., and Cash, Carl G. Equilibrium Moisture Content of Roofing and Roof Insulation
Materials, and the Effect of Moisture on the Tensile Strength of Roofing Felts, Symposium on Roofing
Technology, National Bureau of Standards and National Roofing Contractors Association n 28 p 238-243,
September 1977. 31
Busching, H; Mathey, R; Rossiter, W; Cullen, W. Effects of moisture in BUR: A state of the art
literature survey. Tech Note 965, National Bureau of Standards, July 1978. 32
Cash, Carl G. Moisture and Built-Up Roofing, published in “A decade of change and future trends in
roofing - proceedings of the 1985 International Symposium on Roofing Technology”, September 1985.
Page 12
insulation were updated in 200333
. EMCs for selected products that are still in use
today are summarized in Table 2, along with TRR data.
Table 2 – EMC and TRR of selected roofing materials
Material EMC, Mass% @ 20° C34
Moisture content,
Mass % @ 80%
TRR35
45% RH 90% RH
Faced Polyisocyanurate 1.1 2.9
Unfaced Polyisocyanurate 1.7 5 262
Expanded polystyrene (1pcf) 1.9 2 383
Extruded polystyrene 0.5 0.8 185
Perlite board 1.7 5 17
Wood fiberboard 5.4 15 15
Gypsum 0.4 0.6 8
D226 Asphalt-Organic Felt 4.1 - 4.3 7.9 - 8.2
D2178 Glass Felt 0.5 - 0.9 0.6 - 1.1
A material that contains less than the 45% EMC is considered dry; between the 45%
EMC and 90% EMC is considered moist; and over the 90% EMC is considered wet.
Avoid Three Conditions Favorable to Mold Growth - There is broad consensus that
mold growth should be avoided in buildings due to potential health impacts and because
mold growth implies at least some level of decay. International Energy Agency (IEA)36
states that susceptible surfaces are in danger of developing mold growth if the relative
humidity at the surface rises above 80% RH for a sustained period of several days.
Temperature also plays a role; below 32°F, fungal cells may survive but rarely grow,
and above 104°F, most cells stop growing and soon die. ASHRAE 16037
is generally
consistent with IEA but provides more specific recommendations. ASHRAE
recommends avoiding the following humidity conditions in order to minimize problems
associated with mold growth on surfaces of components of building envelope
assemblies; these criteria apply when the running average surface temperature (for the
duration of interest) is between 41°F and 104°F:
30-day running average surface RH >/= 80%
7-day running average surface RH >/= 98%
24-hr running average surface RH >/= 100%
33
Cash, Carl G. Roofing Failures, Spon 2003, p. 46. 34
Extracted from Cash, 1985 and Cash, 2003. 35
Extracted from Tobiasson et al, 1991. 36
International Energy Agency (IEA) Condensation and Energy Sourceboook, Report Annex XIV, Volume
1, 1991 37
ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings, 2009.
Page 13
In summary, a wide range of theories have been proposed regarding acceptable moisture limits in
roofing materials. Most of past physical testing on the effects of moisture on roofing materials
has focused on two issues: (1) loss of thermal resistance of insulation, and (2) weakening, decay,
and dimensional instability of built-up roofing. In our experience, loss of strength of the
insulation (or its facers) and cover board are also significant concerns, because they affect the
ability of the roofing system to resist common loads such as foot traffic, hail, or wind uplift.
Moisture-induced degradation of water-based membrane bonding adhesive has also been
reported38.
However, we are not aware of any industry publications containing data on how
moisture affects the strength of insulation, cover boards, or bonding adhesive, with the exception
of some limited data on gypsum cover boards discussed in the next section.
3.2 Product-Specific Manufacturer Recommendations
We contacted manufacturers of isocyanurate insulation and gypsum cover boards to ask for
recommendations on acceptable in-service moisture limits for their products. This section
summarizes the information provided by manufacturers.
3.2.1 Isocyanurate Insulation
We contacted four manufacturers of isocyanurate roof insulation and inquired about acceptable
in-service moisture limits. One manufacturer was unresponsive, and two said they do not have
any data or recommendations. The fourth manufacturer cited 3% moisture content as a rule of
thumb, but did not provide any supporting data39
.
3.2.2 Gypsum Cover Boards
A 2001 article40
provides data on the water absorption of a gypsum cover board product after
24 hrs conditioning at 95% RH, 2 hrs exposure to surface moisture, 2 hrs submersion, and 24 hrs
submersion. The article also provides peel adhesion data for hot asphalt application to boards at
ambient conditions and after 7 days at 95% RH. The article does not provide any
recommendations for acceptable moisture limits; its main focus is on avoiding heat damage to the
gypsum during installation of roofing membranes set in hot asphalt or torch application.
38
MRCA T&R Advisory Bulletin, Noteworthy Limitations of Water Based Bonding Adhesives, 2/2011. 39
Telephone conversations with manufacturers, September 2012. 40
Murphy, C.; Mills, R. Dens-Deck Roof Board. Interface, March 2001
Page 14
In addition, we contacted two manufacturers of gypsum cover boards and inquired about
acceptable in-service moisture limits. One manufacturer stated that it manufactures its gypsum
board to less than 2%, and recommends that its product does not become “wet”, but could not
define what moisture content it considers to be “wet” or provide any recommendations for in-
service moisture limits 41
. The other manufacturer stated that its product often has less than 1%
free water as delivered from the factory, and cited a variety of thresholds of concern for moisture
content in service including 2%, 4%, and 5%, but without clear recommendations for an
acceptable level for long-term performance42
. Also, the manufacturer did not provide any
supporting data on the strength of their material at those moisture levels.
In summary, very little information is available from manufacturers of roofing materials
regarding their products’ resistance to moisture degradation.
3.3 Laboratory Testing
To collect some initial data on the moisture resistance of roofing materials, we selected three
insulation cover board products for testing. Two of the products are gypsum-based, the third is
high-density isocyanurate, and all are nominally 1/2 in. thick. Insulation and bonding adhesive
are also of interest, but are excluded from this initial testing.
3.3.1 Description of Testing Program
We exposed the samples to a variety of moisture conditions, and tested their flexural strength.
Flexural strength is relevant to the wind uplift resistance of intermittently-attached cover boards.
Tensile strength and compressive strength perpendicular to the plane of the board are also of
interest; these properties are excluded from this initial testing, but should be considered in future
testing.
Three samples of each of the three products were exposed to each of the following ten different
moisture conditions (90 samples total):
Oven dried at 100° F (below the 110 to 115° F maximum recommended by one of the
gypsum manufacturers).
41
Telephone conversation with manufacturer, September 2012. 42
Private correspondence from manufacturer, 2010 through 2012.
Page 15
Standard laboratory conditions (50% RH and 74°F).
High humidity (95% RH and 74°F).
High humidity (95% RH and 74°F) followed by oven drying.
Water immersion for 30 minutes.
Water immersion for 30 minutes followed by oven drying.
Water immersion for 1 hr.
Water immersion for 1 hr followed by oven drying.
Water immersion for 24 hrs.
Water immersion for 24 hrs followed by oven drying.
Following conditioning, we tested the flexural strength of the samples according to a modified
ASTM C47343
method B. The test procedure generally followed C473 but the sampling
procedures and the definition of breaking load were modified to suit the goals and scope of our
test program. Our test method is briefly summarized as follows, and is depicted in Photos 4 and
5:
Samples were cut to 12 in. x 16 in., with the 16 in. dimension parallel to the long
dimension of the board. Some samples included the factory edge of the board. All
samples were tested face up.
The flexure fixture was set up with supports at a 14 in. span and a center loading nose.
Load was applied at a constant crosshead rate of 1 in./min. until the load-resistance of
the sample decreased; the maximum load was reported.
43
ASTM C473-10, Standard Test Methods for Physical Testing of Gypsum Panel Products. ASTM
International, 2010.
Page 16
Photo 4 – Flexural test setup
Photo 5 – Flexural test in progress.
The isocyanurate board has anomalies in the foam structure, known as “knit lines”, where the
ribbons of foam came together during the manufacturing process. The knit lines are parallel to
the long dimension of the board. By testing only samples spanning parallel to the knit lines, we
avoided testing the weaker orientation.
Page 17
3.3.2 Results
The results of our testing are summarized as follows. We refer to the two gypsum products as
“GYP #1” and GYP #2”, and the isocyanurate product as ISO. Our test results are not suitable for
design strength values; our testing was intended only to explore the trends of strength-loss caused
by exposure to moisture.
Moisture conditioning had the following effects on the flexural strength:
50% RH - None of the three products was significantly weakened by conditioning to
standard laboratory conditions (50% RH) compared to oven-dry conditions.
95% RH - The ISO was unaffected at 95% RH, but the two gypsum products lost some
strength. GYP #1 was reduced to 70% of its standard-laboratory strength, and GYP #2
was reduced to 80%.
Water Immersion - The three products showed differing rates of water absorption when
immersed. After 24 hrs, GYP #1 had gained 4% weight, GYP #2 had gained 24%
weight, and ISO had gained 11% weight. (Figure 2) All three products rapidly lost
strength when immersed in water (Figure 3); most of the strength loss occurred in the
first hour of immersion. In the first hour, the two gypsum products were reduced to
approximately 60% of their standard-laboratory strength, and ISO was reduced to
approximately 70% of its standard-laboratory strength. The rate of loss slowed
significantly after 1 hr of immersion; at 24 hrs, the two gypsum products retained
approximately 50% of their standard-laboratory strength, and ISO remained around
70%.
Page 18
Figure 2 – Moisture content (% of original mass) vs duration of water immersion (hrs) for three cover board
products. Each data point indicates an average of three or more samples.
Figure 3 - Flexural strength (lb) vs duration of water immersion (hrs) for three cover board products. Each data
point indicates an average of three or more samples.
0.000
5.000
10.000
15.000
20.000
25.000
0 5 10 15 20 25
mo
istu
re, %
of
ori
gin
al m
ass
Soaking Time (hrs)
Moisture Content vs Duration of Water Immersion
GYP 1
GYP 2
ISO
0.000
20.000
40.000
60.000
80.000
100.000
120.000
140.000
160.000
0 5 10 15 20 25
avg
load
(lb
f)
Soaking Time (hrs)
Flexural Strength vs Duration of Water Immersion
GYP 1
GYP 2
ISO
Page 19
Moisture Content - When we plot strength versus moisture content rather than wetting
time (Figure 4), we see that the most dramatic strength loss occurs in the range of 0 to
3% moisture content by weight. We also see more of a difference between the three
products. At 6% moisture content, GYP #1 is reduced to approximately 45% of its
standard-laboratory strength, GYP #2 is reduced to approximately 63%, and ISO to
73%.
Figure 4 – Percent of original flexural strength retained vs moisture content (% of original mass) for three cover
board products. Each data point indicates one sample.
Oven Drying after Wetting - All three products regained all of their original strength (to
within the level of accuracy of the test method) when oven dried after one wetting
cycle.
3.3.3 Discussion of Test Results
Our testing showed that some common cover board products lose strength quickly and at
relatively low moisture contents (less than 5% by mass) when wetted. This result is consistent
with our field observations on existing roofs with wet cover boards, and is also consistent with
statements from one gypsum board manufacturer that moisture content over 2-5% is a concern.
The isocyanurate product was less affected by moisture than the two gypsum products.
0%
20%
40%
60%
80%
100%
120%
0 3 6 9 12 15 18 21 24 27
% o
f st
ren
gth
ret
ain
ed
moisture content (% of original mass)
% Strength Retained vs Moisture Content
GYP 1
GYP 2
ISO
Page 20
Our testing further showed that some common cover board products exposed to one relatively-
short wetting cycle will regain virtually all of their original strength after oven-drying. While this
result supports the concept that a small amount of short-term condensation may be acceptable in
some circumstances, it does not provide sufficient data to define those acceptable circumstances.
On many roofs, the wetting is longer term and the drying is less complete than in our tests. In
addition to longer-duration wetting, other factors that could result in unacceptable strength loss in
cover boards include: (1) a greater number of wetting and drying cycles, (2) freeze-thaw cycles,
which are common because the coldest weather coincides with the peak moisture levels directly
under the roofing membrane, and (3) loading from wind or foot traffic during one of the wetting
cycles when the materials are temporarily weakened may result in permanent strength loss or
even immediate failure. There is no guarantee that loading will occur only when the materials are
dry and at their maximum strength.
4. CONCLUSIONS AND RECOMMENDATIONS
We conclude the following regarding roofing system design for moisture in concrete decks:
Most roofs on new concrete decks need vapor retarders; this poses some challenges
with wind uplift rating and product selection, but these challenges can be addressed
through careful design. As additional roofing systems that include an adhered vapor
retarder are developed and tested for wind uplift, designers will gain more flexibility.
State-of-the-art analysis tools are available to predict time-varying moisture levels due
to vapor migration through the roofing system and assist selecting an appropriate vapor
retarder. Interpreting the results of these analyses requires knowledge of the acceptable
in-service moisture limits of roofing materials.
We conclude the following regarding acceptable in-service moisture limits:
Past publications have proposed various different acceptance criteria for moisture
content of roofing materials. An industry consensus does not exist, and product-
specific data and recommendations from manufacturers are lacking.
Our test data shows that some common cover board products lose strength quickly and
at relatively low moisture contents (less than 5% by mass) when wetted. However, our
data is insufficient to establish acceptable moisture levels.
We recommend additional research into the acceptable in-service moisture limits of
roofing materials. In the interim until additional data become available, roofing
professionals will have to continue to rely on their experience and judgment.
Page 21
Acknowledgments
The authors would like to thank the management and shareholders of Simpson Gumpertz &
Heger, Inc. for their financial support of the laboratory testing described in this paper.
The authors would also like to acknowledge the significant contribution made by Stephen
Condren, Joseph Piñon, and Paul Scheiner, in their August 2012 work referenced in this paper.
Their work has lent much-needed clarity to the issue of moisture in concrete roof decks, and
provided a clear direction for future research needs.