Double Wall Framing Technique – An Example of High Performance, Sustainable Building Envelope Technology

Jan Kosny*, Andi Asiz**, Som Shrestha***, Kaushik Biswas***, Nitin Shukla*

*Fraunhofer CSE – Cambridge MA, USA

**Prince Mohammad Bin Fahd University, Kingdom of Saudi Arabia

***Oak Ridge National Laboratory, Oak Ridge, TN, USA

ABSTRACT:

Double wall technologies utilizing wood framing have been well-known and used in North American buildings for decades. Most of double wall designs use only natural materials such as wood products, gypsum, and cellulose fiber insulation, being one of few building envelope technologies achieving high thermal performance without use of plastic foams or fiberglass. Today, after several material and structural design modifications, these technologies are considered as highly thermally efficient, sustainable option for new constructions and sometimes, for retrofit projects. Following earlier analysis performed for U.S. Department of Energy by Fraunhofer CSE, this paper discusses different ways to build double walls and to optimize their thermal performance to minimize the space conditioning energy consumption. Description of structural configuration alternatives and thermal performance analysis are presented as well. Laboratory tests to evaluate thermal properties of used insulation and whole wall system thermal performance are also discussed in this paper. Finally, the thermal loads generated in field conditions by double walls are discussed utilizing results from a joined project performed by Zero Energy Building Research Alliance and Oak Ridge National Laboratory (ORNL), which made possible evaluation of the market viability of low-energy homes built in the Tennessee Valley. Experimental data recorded in two of the test houses built during this field study is presented in this work.

1. INTRODUCTION

A significant number of super insulated wall technologies has been developed, built, and investigated in North America and Europe since the energy crisis in the early 70’s, listing double walls among the best performing constructions suitable for these regions. At the same time, wood based technologies are widely considered as primarily building materials for low-environmental-impact buildings [4, 30, 38, 48]. For example, current research performed by the Mid Sweden University demonstrated that both the primary energy consumption as well as the CO2 emission generated during production of materials used in building construction are significantly lower in case of wood-framed constructions than for concrete buildings [14].

In the U.S.A., a detailed experimental and numerical analysis of over 150 wall technologies including advanced wood-framed walls was performed by ORNL. This information is available as the Whole Wall R-value Database [19, 20, 21, 22]. Some of these walls have

effective R-values exceeding RSI-4.4 m2 °K/W, including several double wall material configurations. In another study conducted for the U.S. DOE Building America Program by Straube and Smegal [42] thermal performance characteristics of high R-value walls have been investigated for North American residential applications. One of the conclusions was that moisture tolerant thermal insulation may be primarily choices for these applications due to potential exterior sheathing durability problems in high-R-value assemblies. Finally, several high performance insulation options including vacuum insulations and aerogels have been recently analyzed and tested by Fraunhofer Center for Sustainable Energy Systems (CSE), U.S.A. in wall retrofit applications [26, 40].

In Canada, thermal performance of buildings has become a dominant focus of changes to construction practices with special requirements for prescriptively built housing and small buildings being approved in 2012 (Part 9 of the National Building Code), and a more generalized National Energy Code of Canada for Buildings,1 2011. National Research Council of Canada (NRC) has conducted several projects developing high thermal performance wall assemblies that can be applied in extreme northern climates [35]. Based on this study, NRC has recommended using double stud wall, standoff truss wall, structural insulated panel, or structural insulated concrete wall system in wall housing assemblies to reach R-value larger than RSI-5.3 m2 °K/W. Some of these walls have been tested in the NRC laboratory for further performance verification.

In principle, thermal performance of wall frame assemblies can be increased by either: applying thicker and wider insulation space in wall cavity; installing insulating sheathing; improving thermal resistance of used insulation; mitigating of thermal bridging; and finally applying air/moisture-tight construction techniques. It is good to notice that double walls represent in very practical way a combination of the above measures in order to reach high R-value and sometime to improve other performance aspects such as durability, constructability, and costs. The main objective of this paper is to discuss a short history of development, structural variations, and thermal performance of double wall assemblies applicable in North American residential and small commercial buildings that have or could have R-values larger than RSI-3.5.

2. PROSPECT REDUCTIONS OF WHOLE BUILDING ENERGY CONSUMPION AS A FUNCTION OF IMPROVEMENTS IN WALL R-VALUE Double wall assemblies make possible reaching relatively high R-values only with use of

low cost traditional fiber insulation and without a need for expensive exterior foam sheathing insulation. An improvement of the wall R-value is achieved in this case thanks to increased thickness of the wall and by an overall system design, making possible effective mitigation of thermal bridges. In order to illustrate an impact of wall thermal performance on the whole building energy consumption, a series of parametric simulations has been performed on a single-family residence. A small single-story house selected for this purpose has been the subject of previous energy efficiency modeling studies (Huang et al. 1987) serving in development of the ASHRAE 90.2 standard. The heating and cooling loads generated from this numerical analysis have been used to estimate the relation between wall R-value and whole building energy consumption in conventional wood-framed house. As depicted in Figure 1, about 60% wall R-value raise between most-commonly used in the U.S. RSI - 2.2 m2 °K/W residential walls and

1 http://www.nationalcodes.nrc.gc.ca/eng/necb/index.html

expected in the close future RSI - 3.6 structures, may generate between 5% and 8% changes in the building-envelope-generated whole building energy consumption for space conditioning. Considering that in most of North American residential buildings, walls may generate in average up to 25% of total loads associated to building envelopes (U.S. DOE Building Energy Data Book2 ), it is a substantial upgrade of the whole building performance generated by changes in only a single building enclosure component (wall). In that light, wall framing improvements can be considered as an important source of future energy savings in residential buildings.

Typically, the amount of structural members incorporated into the total wall area is called a framing factor. It used to be expressed as a percent of the total wall area (Syed, Kosny, 2006). It is good to remember that in conventional wood framed walls, each framing member represents thermal short worsening overall thermal performance. For conventional wood-framed walls, the wall area represented by framing members (framing factor) has been considered to be between 10% and 14%. However, according to the report prepared by Enermodal Engineering for the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE), an average 25% framing factor is representative for all US residential buildings (ASHRAE, 2001). To illustrate a scale of thermal bridge effects generated by framing in conventional wood- and steel-framed assemblies, R-values of five structural configurations were numerically analyzed - (Kosny et al. 2014). The simulation results are shown in Table 1. It can be observed that an increase in framing effect coefficient in wood frame wall is almost consistent with increase in percentage of framing, whereas in light-gage steel walls this effect is more prominent.

Figure 1: Total whole building energy consumption for twelve lightweight wood-frame walls calculated using whole building energy simulations for one story rancher.

2 http://buildingsdatabook.eren.doe.gov/

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40

Wall R-value

Total Annual HVAC Energy [GJ]

Atlanta

Bakersf ield

Chicago

Denver

Houston

Knoxville

Miami

Minneapolis

Phoenix

Seattle

Washington DC

GJ per year

Wall RSI-value

1.8 3.5 5.3 7.0 [m2K/W]

Calculations made for ~150 m2 single-story house

Table 1. R-values and Framing Effect Coefficients for different percentage of framing in wood and steel framed walls

Percentage of Framing

R-Value m2.°K/W Framing Effect Coefficient, f (%)

Wood Steel Wood Steel 5 % Framing 2.13 1.64 7.13 28.79 8 % Framing (~ 610 mm. o.c) 2.05 1.34 10.75 41.49 11 % Framing (~ 400 mm. o.c) 1.97 1.21 14.27 47.40 11 % Framing (~610 mm. o.c with track) 1.97 1.19 14.07 48.21 14 % Framing (~ 400 mm. o.c with trck) 1.90 1.09 17.16 52.70

As mentioned earlier, one obvious solution to increase wall R-value is to use a thicker insulated cavity. In addition to many available structural options using oversized framing members, it can be also achieved through a usage of two sets of wall framing, either via two 2x4 studs, a set of 2x4 and 2x3 studs, or a set of 2x6 and 2x4 studs. In practice, this type of wall is called double wall or double stud walls. Sometimes it is called party wall because it can be used as interior bearing walls that can reduce sound transmission in multi-family buildings through small gaps provided between the interior and exterior wall frames. Because of these wide wall cavities (empty or filled), double wall system can provide more fire resistance relative to the traditional wall frame assemblies. Figure 2 presents an example of double wall systems being assembled for constructing zero net energy housing in Canada (Riverdale Net Zero Project, 2007).

Figure 2. Double wall construction (Riverdale Net Zero Project, 2007)

3. CONSTRUCTION OF DOUBLE WALL The double wall represents a relatively low-tech method of building energy-efficient

walls with use low-cost materials and well-known wood framing technique. It was introduced in 1970-ies during the time of energy crisis. In addition to the new building applications, double wall construction technique can be used to retrofit an existing building, and this can be done via adding interior or exterior non load bearing walls to the existing load bearing assembly. As

mentioned before, this construction method virtually eliminates thermal bridging within the wall assembly, although there still can be thermal bridges present at sills, top plates, and window and door openings. Additional advantage of double walls is their excellent acoustic performance. Double walls are usually built from two parallel layers of framing. Next, both stud walls and the space between them are filled with continuous insulation. Depending on the insulation thickness provided, a whole-wall R-value larger than RSI-5.3 m2 °K/W) can be reached. It should be noticed that whole-wall R-value is used here (rather than clear-wall R-value) to indicate the effects of framing elements and interfaces or junctions within wall assembly. For example, double (2x4) wall with 240 mm RSI-6.0 m2 °K/W insulation has whole-wall R-value of RSI-5.3 m2 °K/W (Straube and Smegal, 2009). Any type of insulation materials can be used to fill out the wall cavity, but blown cellulose insulation is preferred for better durability performance due to capability of storing and redistributing small amounts of moisture in addition to the fact that it is an environmentally friendly material made from recyclable paper and cartoon products.

Figure 3. Cross section of double wall framing system.

The wood studs in double wall can be placed either in-lined or staggered order (the studs in the first wall are offset from the studs in the second) with studs spacing 600 mm on each side – see Figure 3. By placing studs in staggered way, thermal bridging is reduced further due to wider distances between structural members. Either interior or exterior stud-frame wall can be designed as load bearing (structural) wall, depending on the architectural need (e.g. interior space dimension). In general, structural design procedure for the load bearing wall is similar to that of the standard wall system. Gravity and lateral loads (wind or earthquake) should be transferred fully to the structural wall by designing adequate nailing between the sheathing to stud frames.

6.4 mm OSB or plywood

0.60 m

0.60 m 0.60 m

Single or double top-plate can be used for the structural wall depending on adequacy of the wall chord (i.e. top plate) in carrying and transmitting in-plane lateral load to the wall and foundation (Figure 4). For the exterior non load bearing wall, structural adequacy with respect to out-of-plane wind load should be checked including the cladding and components attached to it. Normally the wall cladding and component system will be the determining factors instead of the wall frame. On top of the top-plate, gusset plate made of plywood or OSB (~9.5 mm thick) can be used to connect the interior and exterior wall frame. As for framing members, single or double top plate would have less impact on the whole R-value because significant thermal breaks can be provided between the exterior and interior frames. One of the concerns of this wall with respect to durability is that the sheathing is kept very cold, and has little drying potential if the sheathing is wetted by air leakage or rain water penetration.

(a) (b)

Figure 4. Inter-storey junction in double wall system (platform v.s. balloon framing)

For two-story or higher constructions, platform system can be used with insulation material inserted inside the rim joist to minimize thermal bridging (Figure 4a). As one can see, in the platform system thermal bridge is reduced into a very small area, i.e. gusset plate. Balloon frame system also can be constructed for the exterior non load bearing system to eliminate the work of insulating the rim joist as in the platform system (Figure 4b). Stronger and stable engineered wood composite such as Laminated Veneer Lumber (LVL) or Laminated Strand Lumber (LSL) can be used for constructing long and slender studs in balloon framing. Using balloon framing can increase R-value of the double wall system due to elimination of thermal bridging suffered in un-insulated rim joists of the platform framing. The main constraint about reaching higher R-value is interior space demand, and if this is not an issue for architect or home owner, R-value larger than RSI-8.8 m2 °K/W can be reached simply by increasing the space between the two stud lines and filling it out with insulation. As shown in Figure 5, two layers of framing can be separated with fiber fabric to enable an application of two different types of thermal insulation.

Regarding the fire safety regulations which apply to the double walls, the International

Residential Code (IRC section R302.11) requires draft-stopping in double-stud assemblies every 3.m (10ft.) (minimum) along the length of the wall, from bottom plate to top plate and covering the full depth of the double cavity, using 1-cm (1⁄2-in.) gypsum drywall or 1.6-cm (3⁄4-in.) plywood. The code also requires fire blocking to keep the top of the wall assembly separate from the floor framing or attic spaces above. If full-depth top plates that span across both stud walls

are not used, it is recommended that 1-cm (1⁄2-in.) drywall or 1.6-cm (3⁄4-in.) plywood between the top plates, and fire-caulk the joints are installed.

(a) (b) Figure 5: Construction details of double wall framing system designed by Jan Kosny for Oak Ridge, TN U.S.A. field experiment (Miller et al. 2010); (a) Internal layer of framing, (b) Two layers of framing separated with fiber fabric.

4. TRUSS WALLS – AN ALTERATION OF DOUBLE WALL DESIGN

Another North American wall assembly that is similar to the double wall is called the truss walls or standoff truss wall system. Sometime it is also referred to the Larsen truss wall system because it was developed first by John Larsen in Canada in the early 80’s (Christian and Kosny, 1996). Figure 6 illustrates an example of Larsen truss wall construction. In principle, Larsen truss wall is a double wall assembly plus vertically 1.9 cm gusset plates (OSB or plywood) spaced at 60-90 cm o.c. that tie the exterior and interior studs frame (Figure 6). This creates stiffer out-of-plane wall frame system. Unlike in some of the double wall systems, the interior wall frame of Larsen truss wall is designed as a load-bearing wall, which does not compromise interior space demand. Figure 8 shows typical cross sections of the Larsen truss wall assembly. The truss wall cavity is normally filled with environmentally friendly dense blown cellulose insulation. The exterior frame can be extended below the rim joist in the wall-to-foundation junction to provide insulation space that eliminates thermal bridge which is commonly occurred in this junction. Due to reduced impacts of thermal bridges associated with structural intersections and wall opening details, Larson truss walls, demonstrate lower differences

between clear wall and whole wall R-values comparing to other conventional wood-based technologies (Kosny and Desjarlais, 1994).

-

Figure 6: Larsen truss wall construction

Depending on the insulation thickness provided, whole-R-value higher from RSI – 6.2 m2

°K/W can be reached. For example, truss wall with 0.30 m RSI-7.6 m2 °K/W cellulose insulation has whole-wall R-value of RSI- 6.4 m2 °K/W (Straube and Smegal, 2009). The three layers of low permeable materials (drywall, dense pack cellulose, house-wrap) make the walls virtually impermeable to infiltration. As in the double wall, a significant cavity thickness plus cellulose insulation in the truss wall provide a good fire resistance and sound insulation.

The structural design procedure for the load-bearing wall subjected to gravity and lateral loads is similar to that of the standard wall system. Because of laterally stiffer relative to the double wall system, the exterior non-load bearing wall can carry heavier out-of-plane wind load. Depending on the structural load demand, single or double top plate can be used for the interior bearing wall with horizontal plywood or OSB plate closure tied to the exterior wall frame for each storey. The exterior wall also can be balloon-framed to minimize thermal bridging between the floors (Figure 6). The studs’ frames can be spaced 0.40 m or 0.60 m o.c. depending on the load requirement, since the whole-wall R-value is not influenced by this spacing requirement due

siding

interior finish

vapor/air barrier

interior studs (e.g. 2x4)exterior studs (e.g. 2x3)

ledger boardfoundation wall

floor joist

gusset plate (OSB orPlywood)

cellulose insulation

6.4 mm gusset plates (OSB or plywood)spaced vertically 060 to 0.90 m o.c.

Continuous insulation using long studs for inter-storey junction

to significant thermal break provided between the interior and exterior wall frames. Window framing on the interior load bearing wall can be designed as in the standard wall construction, i.e. using double header, since again this will not notably affect the wall R-value due to thermal break applied between the walls. To make it airtight in opening areas, plywood is applied to tie typically deep window box frames. To anchor each truss at the top of the walls of the house, the upper most gusset plate is extended 38 mm and nailed to adjacent roof trusses or rafters. The ceiling joists are cantilevered outward to form the soffit and meet the rafter tails (ceiling joists and rafters are lapped to opposite sides of each stud) with a 50 mm block in between to create a thrust-resisting triangle.

As in other double wall systems that have very thick insulation layers, in winter conditions, a relatively cold area can easily develop on the interior side of the exterior sheathing. This fact may lead to moisture deposition issue if drying system ability is not sufficiently facilitated. However, OSB exterior sheathing can be eliminated and replaced with t-bracing and 19 mm drop siding over the house wrap. Let-in metal t-bracing in exterior and interior load-bearing walls and wooden under-rafter diagonal bracing sufficiently stiffens the structure, particularly once the sealed drywall is installed. Due to the amount of framing involved and time needed for assembly, the Larsen truss can be prefabricated off-site. For retrofitted wall envelopes, a vapor barrier could be applied over the wall sheathing before installation of trusses without fear of condensation as long as 2/3 of the R-value of the envelope is outside of the barrier.

Other variation of Larsen truss wall is to use ‘an actual’ standoff truss system (Hefner, 2007). Instead of using gusset plate to tie the interior and exterior wall frames, 2x2 diagonal braces are applied to the cords (exterior and interior studs) with metal gang-nail plate connectors at certain spacing and angles. It is claimed by Hefner (2007) that this truss wall could reach RSI-8.8 m2

°K/W or more with 0.30 m insulation cavity.

5. LABORATORY AND FIELD TESTS OF HIGH-R-VALUE DOUBLE WALL ASSEBMLY

A unique configuration of the double wall, making possible an application of two different types of blown cavity insulation, was developed by Oak Ridge National Laboratory (ORNL), U.S.A. (Kosny et al. 2014). As depicted in Figure 7, this wall was constructed with two lines of staggered composite wood studs, separated from each other with about 5mm. distance. In this space between two lines of studs a separation liner fabric was installed. In this wall design, the internal wall cavities were filled with cellulose fiber insulation mixed with microencapsulated PCM. The exterior side cavities were field with conventional blown cellulose. Table 2 shows thermal properties of conventional cellulose insulation and two different PCM-cellulose blends which can be used for internal side insulation of the double wall - as shown in Figure 7. Thermal conductivity for PCM-cellulose blend was investigated earlier by Kosny et al. (2007) as a function of amount of added Micronal PCM from BASF. Dynamic thermal performance characteristics of the 25% PCM-cellulose blend are presented in Shukla et al. (2013).

Table 2. Steady state thermal properties of PCM-enhanced cellulose, as tested by Fraunhofer CSE with use of the heat flow meter apparatus.

Material configuration

Used PCM type Material density Medium test temperature

Thermal conductivity

25% weight percent blend of cellulose with PCM

Shape stabilized PCM product, from Syntroleum

42.0 kg/m3 12.5 oC 0.03774 W/mK

42.0 kg/m3 40.0 oC 0.04141 W/mK

Compressed 22% weight percent cellulose -PCM blend

Microencapsulated PCM from Microtek

94.7 kg/m3 15.0 oC 0.04216 W/mK 94.7 kg/m3 25.0 oC 0.04333 W/mK 94.7 kg/m3 35.0 oC 0.04426 W/mK 94.7 kg/m3 50.0 oC 0.04591 W/mK

Figure 2: Schematic of the double wall containing two types of cellulose fiber insulation, designed by Jan Kosny for the ORNL field experiment (Kosny et al. 2014).

In order to measure steady state R-value of the described above double wall, the hot box testing procedure (ASTM C1363) was utilized. For the purpose of the hot-box testing, which was performed by ORNL, a 2.4 m by 2.4 m. wall specimen was constructed. During the series of hot-box tests an average density of the conventional cellulose insulation was about 56 kg/m3, while for the 22% -by weight cellulose-PCM blend, it was about 75 kg/m3. Steady-state hot box tests were performed with three different sets of temperature conditions yielding in average the wall R-value of R- 3.8 m2K/W (21.2 h-ft2-F/Btu). A summary of hot-box test results is presented in Table 3 below.

Thermal performance of discussed above configuration of the double wall was also investigated in full whole building scale. Four high thermal performance wall technologies constructed in four identical homes have been monitored and analyzed in Oak Ridge, Tennessee,

200 mm masonry foundation

19 mm floor deck board

16 mm gypsum board

2x4 0.6 m wide cavity with blown PCM insulation

2x4 0.6 m wide cavity with blown cellulose insulation100 mm

13 mminterlocking

Studs corner arrangement

U.S.A. (Miller et al 2010). This project was a partnership between ORNL and building materials and construction companies. It was sponsored by Tennessee Valley Authority and the U.S. Dept of Energy to demonstrate and develop new energy efficient technologies for homes. Detailed energy performances comparisons for all four tested houses are discussed in Shrestha et al. (2012). Tested opaque envelopes included structural insulated panel (SIP), optimum value engineering (OVE), double wall with phase change material (PCM) insulation in house, and exterior insulation and finish system (EIFS) in house (Kosny et al. 2014).

Table 3. Steady state thermal resistance of double wall as tested by ORNL.

Test Temperatures oC (oF) Measured heat flux

Measured R-value

Meter side Climate side Temperature difference W/m2 (Btu/h-ft2)

m2K/W (h-ft2-F/Btu)

1 22.5 (72.5) -0.2 (31.7) 22.7 (40.8) 5.84 (1.85) 3.89 (22.1) 2 24.7 (76.4) 15.9 (60.7) 40.6 (15.7) 2.27 (0.72) 3.83 (21.8) 3 37.1 (98.7) 16.4 (61.6) 53.5 (37.1) 5.52 (1.75) 3.73 (21.2)

Figure 8 shows the test house constructed with double walls, which were filled with blown cellulose (external layer) and PCM-enhanced cellulose insulation (internal layer). The house is two-story with total floor area of 253 m2. The wall studs are made of 2x4 Laminated Strand Lumber (LSL) and are spaced 0.60 m o.c. (Figure 5). In the tested double wall, two layers of studs are staggered by 0.30 m. The interior framing is supported on top of the bottom plate that is fastened through floor sheathing and floor truss, while the exterior framing is supported on the sill plate and is fastened to the floor truss. A top plate was used to tie the two walls together for lateral strength. It is anticipated that this double wall system function as composite system (Figures 5 and 7). The interior frame acts as the structural wall component responsible for transmitting gravity and lateral force to the foundation. However, since the two double top plates are mechanically connected and wall sheathing is provided in the exterior wall, a portion of lateral load is carried by the exterior walls as well.

Figure 8: Oak Ridge field experiment – installation of the cellulose insulation inside the exterior wall cavities and front view of the test house with double walls (Miller et al.

2010). As shown in Figure 7, thermal bridging at the corners is minimized by applying double

stud corner practice with insulation surrounding it. The exterior wall OSB sheathing has a built-in protective weather resistive barrier (WRB) overlaid at the factory to eliminate the need for house wrap. All joints were taped to make the sheathing air tight. A high-density polyethylene sheet having about a 6.4 mm high dimpled profile was also installed on the exterior of the sheathing to ventilate the exterior part of the wall. It provides drainage of transient moisture migrating through the wall and creates two independent air flow streams to dry out both the cladding and the concealed wall cavities. The product eliminates the impact of solar driven moisture problems, and reduces the impact of interior moisture loading at the same time.

In order to illustrate thermal performance advantage of the double wall over the conventional wood framing assembly, Figure 9 presents measured wall heat fluxes in the double wall house and the identical house with 2x6 wood stud walls. In conventional wall 14-cm (5.5-in) wood studs are spaced 60-cm. (24-in.) from each other and wall cavity is insulated with R-3.3 m2K/W (R-19 h-ft2-F/Btu) fiberglass batts. Presented below recorded heat flux data shows walls’ performance comparisons during three winter days – see Figure 9. It can be observed that the double wall generates significantly lower heat losses comparing to the conventional 2x6 wall. In addition, significant dynamic effects can be observed in all wall orientations. During the winter time, they are mainly caused by thermal loads generated by fenestration and by the thermal mass response of the double walls. More detail analysis will be presented in following publication.

South walls16% heat flow

difference

North walls17% heat flow

difference

East walls35% heat flow

difference

West walls54% heat flow

difference

Double wall, - - - - - Conventional 2x6 wall

Btu/

ht-f

t2Bt

u/ht

-ft2

Btu/

ht-f

t2Bt

u/ht

-ft2

-6.3

-5.0

-3.8

-2.5

-9.5

-7.9

-6.3

-4.7

-4.7

-7.9

-6.3

-3.2

-1.6

-7.9

-6.6

-5.4

-4.1

W/m

2W

/m2

W/m

2W

/m2

Figure 9: Oak Ridge field experiment –heat flux data recorded for all four wall orientations during three days in January 2011.

6. CONCLUSIONS It can be observed that many different high R-value double wall assemblies have been

developed and successfully tried out using many different insulation materials with different wall thicknesses and framing members. Traditionally several configurations of double walls and truss walls have been considered as energy efficient solution for low-energy buildings. A review of existing practices of residential double wall construction techniques was conducted to identify wall assemblies, structural members, and insulation materials of high thermal performance. Current research demonstrated that R-values of these walls can easily exceed RSI-5.3 m2 °K/W. Results of analytical and experimental research work performed at ORNL and Fraunhofer CSE were presented. In addition, test data recorded during the four test houses experiment in Oak Ridge, Tennessee, U.S.A. were utilize to compare filed performance of double walls and conventional 2x6 wood framing. Measured wall heat fluxes in the double wall house were compared to the test data from identical house with 2x6 wood stud walls. It was observed that during the winter days of consideration the double wall generated significantly lower heat losses comparing to the conventional 2x6 wall.

Acknowledgements

Financial supports provided by Oak Ridge National Laboratory (ORNL) through the Oak Ridge Institute for Science and Education (ORISE) and by Frunhover CSE are greatly acknowledged. Thanks are also extended to ORNL and Fraunhofer CSE research staffs for providing technical information about ZEBRAlliance test houses and materials used in the Brunswick, ME test house.

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[15] Kośny J. and Desjarlais, A. O. (1994), Influence of Architectural Details on the Overall Thermal Performance of Residential Wall Systems - Journal of Thermal Insulation and Building Envelopes, Vol. 18, July 1994.

[16] Kośny J., Desjarlais A.O., Christian J.E. (1995) -Thermal Performance of “Energy Efficient” Metal Stud Wall Systems -ASHRAE, BETEC, U.S.DOE VI Thermal Envelope Conference, Dec. 1995.

[17] Kośny J., Christian J.E., Desjarlais A.O., Kossecka E., Berrenberg L. (1998) -Performance Check Between Whole Building Thermal Performance Criteria and Exterior Wall Measured Clear Wall R-value, Thermal Bridging, and Airtightness - ASHRAE Transactions 1998, v.104 pt 2.

[18] Kośny J., Syed A. M. (2004) - Interactive Internet-Based Building Envelope Materials Database for Whole-Building Energy Simulation Programs – IX Conference - Thermal Performance of the Exterior Envelopes of Buildings, December 2004 Clearwater, Florida.

[19] Kośny, J., Yarbrough D., and Wilkes K. 2006. “PCM-Enhanced Cellulose Insulation: Thermal Mass in Light-Weight Fibers,” presented at International Energy Agency and Department of Energy Ecostock 2006 Conference, May 31, 2006.

[20] Kośny J., D. Yarbrough, T. W. Petrie, and A. Syed 2007a. “Performance of Thermal Insulation Containing Microencapsulated Phase Change Material,” presented at 2007 International Thermal Conductivity Conference, June 24–27, 2007.

[21] Kośny J., D. Yarbrough, W. Miller, T. Petrie, P. Childs, and A. Syed 2007b. “Thermal Performance of PCM-Enhanced Building Envelope Systems,” presented at Thermal Envelopes X Conference, December 2007

[22] Kośny J, Fallahi A, Shukla N., – 2013 “Cold Climate Building Enclosure Solutions” – U.S. DOE Building America Program report, NREL/SR-5500-55875, January 2013, http://www.nrel.gov/docs/fy13osti/55875.pdf

[23] Künzel H., 2003. A Retrospective Look at 50 Years of the Outdoor Testing Field in Holzkirchen. Journal of Thermal Envelope and Building Science 27, 1, 5–14.

[24] LBNL – (1993) -DOE-2 version 2.1E - Energy and Environment Division, Lawrence Berkeley Laboratory University of California, Bekeley, CA 9420, Nov. 1993.

[25] Miller, W., Kosny, J., Shrestha, S., Christian, J., Karagiozis, A., Kohler, C., Dinse, D. (2010) Advanced Residential Envelopes for Two Pair of Energy-Saver Homes, 2010 ACEEE Summer Study on Energy Efficiency in Buildings, Pacific Grove, August 15-20, California, 18p.

[26] NAHB (2008) Advanced framing: An examination of its practical use in residential Construction, Report Prepared for the Partnership for Advanced Technology in Housing, Upper Marlboro, MD, 21p.

[27] Oikos (1994) Engineered Wall Displays Strength and High R-value, Energy Source Builder #36, Iris Communications, Inc. http://oikos.com/esb/36/Daviswal.html

[28] Petrie T.W., J. Kosny, A. Desjarlais, J. Christian (2002) - Lessons from the Habitat/ICF/ORNL Whole House Demonstration Project - ACEEE Summer Study August 2002.

[29] Rousseau, M.Z., S.M. Cornick, M.N. Said, W Maref, and M.M. Manning (2008) PERD 079 Project - Report Task 4 -– Review of Work Plan & Selection of Wall Assemblies, National Research Council Canada, Ottawa, Canada.

[30] Riverdale Net Zero Project (2007) Technical presentation to Canada Mortgage and Housing Corporation, http://www.riverdalenetzero.ca/, Alberta, Canada.

[31] Riversong, R. (2008) Modified Larsen truss system, http://www.builditsolar.com/Projects /SolarHomes/LarsenTruss/History.htm

[32] SBCA (2010) Network Looking at Mid- and High-rise Construction Opportunities, http://sbcacarolinas.com/common/kb/kb_fsc_newsdetails.php?KBID=15617

[33] Shukla N. C., Fallahi A., Kosny J., Harasim S., and Blair C. –2012 “Aerogel for Thermal Insulation of Interior Wall Retrofits in Cold Climates” - The Building Enclosure Science & Technology (BEST), AIA Conference, Atlanta, GA, April 2012.

[34] Smith, M., Mooney, T. (2006) Mooney wall, http://www.buildsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm

[35] Straube, J., and Smegal, J. (2009) Building America Special Research Project: High-R Walls Case Study Analysis, Research Report-0903, Building Science Press, MA, 64p.

[36] Swedish House Co (2008), Specification for component materials, http://www.swedishhouses.com/

[37] Strzepek, W. R., (1990) - Thermal Resistances of Metal Frame Wall Constructions Incorporating - Various Combinations of Insulating Materials - Insulation Materials, Testing and Applications, ASTM/STP 1030, 1990

[38] Syed Azam Mohiuddin, Jan Kośny, (2006) -“Effect of Framing Factor on Clear Wall R-value for Wood and Steel Framed Walls” – Journal of Building Physics, Vol. 30, No. 2, 163-180, 2006

[39] Trethowen, H. A., (1988) - Thermal Insulation and Contact Resistance in Metal_Framed Panels - ASHRAE Transactions, Vol. 94, Part 2, 1988.

[40] Trus JoistTM (2002) 1.9E Microllam® LVL Headers and Beams, Specifier’s Guide 2510. [41] ZEBRAlliance (2010) ZEBRAlliance: Building Smart, http://www.zebralliance.com/ [42] Zirkelbach D., Künzel HM., Sedlbauer K. 2004. Einsatz von Wärmedämm-

Verbundsystemen in anderen Klimazonen (Application of External Wall Insulation Systems in Different Climate Zones). Bauphysik 26, 6, 335–339.

National Institute of Building Sciences Provider #G168

BEST4 Conference Nibsbest4 • April 13-15, 2015

Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request. This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. ___________________________________________ Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

Participants will : 1. Learn how to link the performance of individual building enclosure components in a holistic framework to achieve high-performance buildings. 2. Explore, through built case studies, how building envelope design determines overall energy conservation and sustainability capabilities 3. Learn innovative practices for avoiding heat loss as well as moisture and air infiltration in enclosure design for healthy new and existing buildings. 4. Understand the role of building enclosure commission- ing in the design, construction, and operation and maintenance of commercial facilities.

Learning Objectives

© Fraunhofer USA

Double Wall Framing Technique – An Example of High Performance, Sustainable Building Envelope Technology

Jan Kośny Ph.D. Building Enclosure Program Lead

3

Co-authors: Prof. Andi Asiz– Prince Mohammad Bin Fahd University, Kingdom of Saudi Arabia Drs Som Shrestha and Kaushik Biswas – ORNL Dr. Nitin Shukla – Fraunhofer CSE, Boston, MA

BEST 4 Conference April 12-15, 2015, Kansas City, Mo

Outline

Background Prospect Reductions of Whole

Building Energy Consumption as a Function of Improvements in Wall R-value

What is Double Wall and Construction Details of Double Wall

Truss Wall an Alteration of the Double Wall Design

ASTM C1363 Hot-Box tests of Double Wall

Field Testing of Residential Building Having Double Walls

Summary

VARIETY OF RESIDENTIAL WOOD-FRAMED WALL TECHNOLOGIES IS AVAILABLE NOW IN

NORTH AMERICA • Different lumber

profiles and sizes • Different spacing

between studs • Different architectural

details • Different fabrication

environments: • In field • In-shop - system

scale • In factory – whole

building scale • Different types of the

exterior structural sheathing

• Different insulation types and installation methods

AN EXAMPLE OF ANATMIC ANALOGY WHICH IS SOMETIMES MISIMPRETTED:

How to Insulate the interior of a skeleton?

I am really cold. Need some insulation

Strategy # 1 Insulation between ribs

I am still cold

Strategy # 2 Insulation on top of skeleton

This is a perfect solution

or

WHAT STRATEGY WAS SELECTED DURING THE EVOLUTION PROCESS?

Strategy # 1 + Strategy # 2 Insulation between ribs and on top of skeleton

Strategy # 3

REALITY CHECK IN CASE OF SELECTION MADE BY MOTHER NATURE DURING EVOLUTION

• Ribs represent approximately 20%-30% of the total ribs cage heat transfer area

• External layer of insulation is usually 2 to 3 times thicker from the thickness o ribs cage

• A combination of the fat and the muscle tissue serves as a great majority of thermal insulation both between the ribs and on top of the ribs cage

• Bone thermal conductivity is between 0.53 and 0.58 W/mK – see Davidson and James – “Measurement of thermal conductivity of bovine cortical bone.” Med Eng Phys. 2000 Dec;22(10):741-7.

• Thermal conductivity of human fat is around 0.23 W/mK, while muscles yields 0.46 W/mK - see El-Brawany MA et al. “Measurement of thermal and ultrasonic properties of some biological tissues.” - J Med Eng Technol. 2009;33(3):249-56

• Assuming that in combination of the fat and the muscle tissue the mix proportions are 50% to 50%, an average thermal conductivity should be about 0.35 W/mK

• The above gives thermal conductivity ratio of about 0.63 between the ribs cage structure and its insulation

REALITY CHECK IN CASE OF A TYPICAL RESIDENTIAL WALL CONSTRUCTION PRACTICE

• Studs represent approximately 11%-14% of the clear wall heat transfer area

• External sheathing insulation represents usually 1/3 to 2/3 of the thickness of framing

• Fiber insulations are the major choice as a wall cavity insulation

• Plastic foams, which are appr. 4 to 6 times more expensive per R-value of the unit thickness, are the most typical exterior sheathing insulation

• Wood stud thermal conductivity is between 0.12 and 0.14 W/mK

• Thermal conductivity of fiberglass batts is about 0.039 W/mK,

• Thermal conductivity of wall cellulose insulation is about 0.041 W/mK,

• Thermal conductivity of plastic foams is between 0.022 and 0.035 W/mK,

• The above gives thermal conductivity ratios of about 0.40 and 0.17-0.27 between the studs structure and, respectively, wall cavity and foam wall sheathing insulations

THERMAL BRIDGE EFFECTS COMPARISON A % reduction of a nominal wall R-value, caused by thermal bridge

Human body – ribs cage: Proportion of the structural layer thickness to the thickness of external insulation is 1-to-3 The same thermal insulation is used for structural layer and external insulation Structural profiles represent even 30% of the structural layer area

2x4 wood stud framing – most common wall technology in North America: Proportion of the structural layer thickness to the thickness of external insulation is often close to 3-to-1 Two different thermal insulations are used; the first one for the structural layer (cavity insulation) and the second one as the external insulation| Structural profiles represent 14% of the structural layer area

External insulation

Structural Layer Rib

External insulation

Structural Layer

Framing Effect 15.2%

Framing Effect 12.8%

CONCLUSION AFTER COMPARISON BETWEEN HUMAN BODY INSULATION AND A TYPICAL

RESIDENTIAL WALL CONSTRUCTION PRACTICE • Very similar thermal bridge effects can be achieved using completely

different structural/insulation strategies • In comparison to North American builders, Mother Nature uses

• Significantly more dense structural system, but it is utilizing a significantly lower thickness of the structural layer

• Thermal conductivity of the skeleton is pretty close to combined thermal conductivity of the fat and muscle, while in walls, wood studs are at least 3 times more conductive from the cavity insulation

• North American builders, have to use an additional layer of a second better performing and more expensive insulation, to reach the expected wall thermal performance target (two different insulation techniques and two different installers at the site are needed at two different times)

• At the same time, Mother Nature uses a single, widely available insulation material, combined with smart overall structural design

• Mother Nature is a clear winner in this theoretical

contest with North American builders

LESSON LEARNED FROM MOTHER NATURE

• Improvements in thermal performance of an exterior diaphragm can be achieved through improvements in overall structural design

• An application of a single and widely-available insulation material can be a way-to-go

• Thermal conductivity of structural components should be as close as possible to thermal conductivity of thermal insulation

IS BULDING INDUSTRY LEARNING IN NORTH AMERICA FROM MOTHER NATURE?

In the U.S.A., a detailed experimental and numerical analysis of over 150 wall technologies including advanced wood-framed walls was performed by the U.S. DOE ORNL. This information is available as the Whole Wall R-value Database. Some of these walls have effective R-values exceeding RSI-4.4 m2 °K/W (R-25), including several double wall material configurations. U.S. DOE Building America Program conducted a study focused on double walls. National Research Council of Canada (NRC) has conducted several projects developing high thermal performance wall assemblies that can be applied in extreme northern climates. Based on this study, NRC has recommended using double stud wall, standoff truss wall, structural insulated panel, or structural insulated concrete wall system in wall housing assemblies to reach R-value larger than RSI-5.3 m2 °K/W (R-40).

Outline

Background Prospect Reductions of Whole

Building Energy Consumption as a Function of Improvements in Wall R-value

What is Double Wall and Construction Details of Double Wall

Truss Wall an Alteration of the Double Wall Design

ASTM C1363 Hot-Box tests of Double Wall

Field Testing of Residential Building Having Double Walls

Summary

RELEVANCE OF HVAC LOADS GENERATED BY RESIDENTIAL WALLS

-1000

-500

0

500

1000

1500

2000

solar equip people floor Infiltration Roof Walls Windows

Load Distribution in Residential Buildings in Trilion BTU's/year by J.Huang 1999, 2002, Infiltration Distribution by Dickerhoff 1982

infiltration as separate bar infiltration distributed

15%

35% 32%

18%

Infiltration %

Windows

Walls

Floor

Roof

8% more than windows 39% more than roof 30% more than floors

Infiltration

Loads as separate bars Loads plus infiltration

WE HAVE SOMETIMES A PROBLEM WITH UNDERSTANDING WHAT IS AN IMPACT OF WALL

THERMAL INSULATION ON WHOLE BUILDING ENERGY CONSUMPTION

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40

Wall R-value

Total Annual HVAC Energy [GJ]

Atlanta

Bakersf ield

Chicago

Denver

Houston

Knoxville

Miami

Minneapolis

Phoenix

Seattle

Washington DC

GJ per year

Wall RSI-value 1.8 3.5 5.3 7.0 [m2K/W]

Calculations made for ~150 m2 single-story house

ACCORDING TO THE ASHRAE RP904* AND CEC**, THE AVERAGE WALL FRAMING FACTOR IN U.S. RESIDENTIAL BUILDINGS IS 25%, WHILE

ONLY IN CALIFORNIA IS 27% No framing

6% - 9% framing

11% - 14% framing 25% framing

*Carpenter S.C. Schumacher C. (2003) - “Characterization of Framing Factors for Wood-Framed Low-Rise Residential Buildings” ASHRAE Transactions v 109, Pt 1. Feb. 2003. **CEC (2001) - “Characterization of Framing Factors for Low-Rise Residential Building Envelopes in California” - Public Interest Energy Research Program: Final Report, Publication Number: 500-02-002, Dec 2001.

WHAT DOES REALLY MEAN AN INCORRECT INTERPRETATION OF WALL FRAMING FACTOR?

Whole building energy MBTUs/Year

26 28 30 32 34

R-14.5 walls R-8.8 walls

Let’s assume a framing factor between 25% and 27%: • R-value for 2x4 wall insulated

with R13 fiberglass batts (nominal R-value of RSI - 2.6 or R-14.5) is in the range of RSI- 1.5 -1.6 (R- 8.5 to 9.0).

• Which is a 35 – 40% shortage in clear wall R-value

• It can be translated as an approximate need of extra 10-12% energy to provide heating and cooling.

• In whole-country scale it means additional and unnecessary 26.7-33.4 GWy or (0.8–1.0 Q) of energy consumed by residential buildings

• It would take an additional 1-1/2-in. of EPS sheathing to negate this R-value discrepancy

RSI-2.6 walls RSI-1.5 walls

1500 ft2 one-story rancher located in Bakersfield, CA

Outline

Background Prospect Reductions of Whole

Building Energy Consumption as a Function of Improvements in Wall R-value

What is Double Wall and Construction Details of Double Wall

Truss Wall an Alteration of the Double Wall Design

ASTM C1363 Hot-Box tests of Double Wall

Field Testing of Residential Building Having Double Walls

Summary

VARIETY OF DOUBLE WALL CONFIGURATIONS

Inline studs, small distance between wall layers

Inline studs, increased distance

between wall layers

Staggered studs, small distance between wall layers

Balloon framing

Truss wall construction

CONSTRUCTION DETAILS OF DOUBLE WALL 6.4 mm OSB or plywood

0.60 m

0.60 m 0.60 m

Insulated rim joist

Continuous studs (balloon framing)

Corner framing example

EXPERIMENTAL DOUBLE WALL SYSTEM BUILT IN OAK RIDGE, TN - ZEBRA ALLIANCE HOUSE

Outline

Background Prospect Reductions of Whole

Building Energy Consumption as a Function of Improvements in Wall R-value

What is Double Wall and Construction Details of Double Wall

Truss Wall an Alteration of the Double Wall Design

ASTM C1363 Hot-Box tests of Double Wall

Field Testing of Residential Building Having Double Walls

Summary

LARSEN TRUSS WALL CONSTRUCTION DETAILS

siding

interior finish

vapor/air barrier

interior studs (e.g. 2x4)exterior studs (e.g. 2x3)

ledger boardfoundation wall

floor joist

gusset plate (OSB orPlywood)

cellulose insulation

6.4 mm gusset plates (OSB or plywood)spaced vertically 060 to 0.90 m o.c.

Continuous insulation using long studs for inter-storey junction

I-BEAM WALLS – ADOPTED EUROPEN WALL DESIGN;

http://www.greenbuildingadvisor.com/blogs/dept/musings/klingenberg-wall

Outline

Background Prospect Reductions of Whole

Building Energy Consumption as a Function of Improvements in Wall R-value

What is Double Wall and Construction Details of Double Wall

Truss Wall an Alteration of the Double Wall Design

ASTM C1363 Hot-Box Tests of Double Wall

Field Testing of Residential Building Having Double Walls

Summary

DOUBLE WALL CONFIGURATION FROM THE ZEBRA ALIANCE HOUSE WAS TESTED BY ORNL

USING ASTM C1363 HOT BOX APPARATUS

Double layer wall framing

After installation of the PCM layer

Instrumentation

Installation of the FG fabric

WALL R-VALUE TESTING WITH USE OF THE ASTM C1363 PROCEDURE

Cellulose no-PCM

2X4 studs @ 60-cm. o.c.

2X4 studs @ 60-cm. o.c.

Fiberglass fabric ~1.0 cm.

Cellulose with PCM OSB board

OSB board

Test Temperatures oC (oF) Measured heat flux

Measured R-value

Meter side Climate side Temperature difference W/m2 (Btu/h-ft2)

m2K/W (h-ft2-F/Btu)

1 22.5 (72.5) -0.2 (31.7) 22.7 (40.8) 5.84 (1.85) 3.89 (22.1) 2 24.7 (76.4) 15.9 (60.7) 40.6 (15.7) 2.27 (0.72) 3.83 (21.8) 3 37.1 (98.7) 16.4 (61.6) 53.5 (37.1) 5.52 (1.75) 3.73 (21.2)

Outline

Background Prospect Reductions of Whole

Building Energy Consumption as a Function of Improvements in Wall R-value

What is Double Wall and Construction Details of Double Wall

Truss Wall an Alteration of the Double Wall Design

ASTM C1363 Hot-Box tests of Double Wall

Field Testing of Residential Building Having Double Walls

Summary

ZEBRA ALIANCE HOUSE WAS TESTED IN FIELD CONDITIONS DURING 2010/11

Example of measured wall heat fluxes – winter time comparison with performance of the conventional 2x6 wood stud wall

South walls16% heat flow

difference

North walls17% heat flow

difference

East walls35% heat flow

difference

West walls54% heat flow

difference

Double wall, - - - - - Conventional 2x6 wall

Btu/

ht-f

t2Bt

u/ht

-ft2

Btu/

ht-f

t2Bt

u/ht

-ft2

-6.3

-5.0

-3.8

-2.5

-9.5

-7.9

-6.3

-4.7

-4.7

-7.9

-6.3

-3.2

-1.6

-7.9

-6.6

-5.4

-4.1

W/m

2W

/m2

W/m

2W

/m2

Summary; double wall systems, if well designed, may be a part of many future success

stories about low-energy buildings It can be observed that many different high R-value double wall

assemblies have been developed and successfully tried out using many different insulation materials with different wall thicknesses and framing members.

Traditionally several configurations of double walls and truss walls have been considered as energy efficient solution for low-energy buildings.

Current research demonstrated that R-values of these walls can easily exceed RSI-5.3 m2 °K/W.

Steady-state hot-box R-value of double wall tested by ORNL was R- 3.89 (22.1) m2K/W (h-ft2-F/Btu)

Field test data recorded during the four test houses experiment in Oak Ridge, Tennessee, U.S.A. were utilize to compare filed performance of double walls and conventional 2x6 wood framing. It was observed that during the winter days of consideration the double wall generated significantly lower heat losses comparing to the conventional 2x6 wall.

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jkosny@fraunhofer.org | Office: 617-714-6525 | Cell: 865-607-6962

Contact Information:

BEST 4 Conference April 12-15, 2015, Kansas City, Mo

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