PROP Jericho report - Efficiency Vermont...ATTIC AND ROOF SLOPE VENTILATION DESIGN Table of Contents...

H e n r i F e n n e l l , C S I / C D T

Box 65 Rt. 5, North Thetford, Vermont 05054 802-222-7740 page 1 of 18

ATTIC AND ROOF SLOPE VENTILATION DESIGN

Table of Contents

1. Roof performance problems in cold climates 2. Ventilation – the purposes of ventilation 3. Reducing the need for ventilation

a. Air barriers first b. Missing or inadequate insulation c. Vapor control d. Other attic and roof warming sources

4. Sizing ventilation components to provide adequate flow 5. Ventilation - attics

a. Eliminate sources of attic warming b. Maintain an evenly distributed ventilation

6. Ventilation - roof slopes a. Eliminate sources of roof slope warming b. Maintain balanced flow

7. Unvented attics 8. Unvented roof slopes

a. Eliminate sources of attic warming b. Higher R-values are required

9. Deciding when to ventilate and when not to



1. Roof performance problems in cold climates: In areas with cold climates, ice dams and moisture problems frequently occur due to poor performance of the building enclosure systems of attics and cathedral roof slopes. Without adequate insulation, air sealing, and ventilation, roof warming generates ice dams which cause roof leaks and structural damage. Performance issues also result in the accumulation of moisture inside the attic or roof slope assemblies which can result in indoor air quality problems.

Indoor humid air condenses on the underside of the roof deck in an attic with a poor air barrier at the ceiling level. Increasing attic ventilation may make this problem worse.

Snow melt high up on the roof slopes concentrates in the valley below

Air leakage low in a cathedral slope contains moisture that condenses on

the cold roof deck as it flows up to the ridge vent

Significant snow loads form large ice dams and icicles

Side view of an insulation batt showing how air flows from an interior air leak

up to the vent space above the batt in a vented cathedral slope



My experience with remediating attic and roof slope failure projects indicates that both vented and unvented approached can work well if the building envelope and the ventilation systems are designed and installed properly. Both systems have been implemented successfully in cold climate zones, even in buildings that have previously had moisture and ice dam failures. In buildings with problematic ice formations, we typically find installation flaws in one or more of the building enclosure elements. These include air barriers, insulation, and ventilation systems. We almost never find adequate attempts at making a complete, continuous air barrier (How often have you heard of quality assurance testing for the air barrier during its installation?). Most failed roofs have adequate levels of insulation, with air leakage and/or ventilation problems as the common causes of moisture and/or icing issues.

2. Ventilation – the purposes of ventilation: The original purpose for ventilating uninsulated attics and roof slopes was to remove moisture from the surfaces inside them. A side benefit of this ventilation requirement is the removal of heat from attics and roof slopes. In cold climates, this reduces melt and the subsequent accumulation of ice, and in hot climates this provides some cooling of the occupied spaces inside the structures. Today, ventilation systems still provide both of these functions, but in much more energy-efficient building enclosures.

3. Reducing the need for ventilation: Ventilation is a means of managing excess moisture and heat that is flowing through a building

assembly. Unconditioned attics and sloped roof assemblies with air and vapor permeable insulation materials typically have integral ventilation systems. Roof assemblies with air and vapor impermeable insulation materials typically are not ventilated unless the owner or the builder of the building has a bias against unvented designs. (More on the risks associated with the inappropriate use of ventilation later.)

a. Air barriers first: Providing adequate roof and attic insulation and ventilation without an adequate air barrier system can actually promote roof warming, heat loss, and moisture accumulation. If indoor conditioned air enters the attic or roof slope ventilation system,

If you think your attic is performing well, compare the snow melt pattern of the attic roof to an uninsulated roof over an unconditioned space.



which is usually below the dew point in cold climates, condensation and roof warming occur. The forces driving this outward flow include wind, interior building pressures from stack effect, and/or mechanical systems. This outward air flow easily overcomes the intended passive roof ventilation system. In any climate zone, exfiltration wastes energy.

Air barrier installations require the most attention to detail of any thermal envelope component. If the air barrier is compromised, warm, conditioned air escapes into unconditioned attics or roof slope vent spaces causing moisture accumulation, snow melt, and subsequently ice dam formation.

• Cold attics usually have the lowest up-front cost, but are the most prone to problems due to air leakage and mechanical system warming.

• Flaws in one area of the thermal envelope impact the entire attic roof area.

Cathedral slopes

Attics

The left image is an infrared scan of the roof at the bottom of the valley under depressurization. The intersection of sheets of air barrier membrane are leaking here outward under ambient conditions. The right image was taken looking down the roof slope. It shows the snow melt occurring in the vented roof bay above the valley intersection.



Air leakage into attics and vented roof slopes causes large areas of roof to be warmed (the entire attic or the entire vented roof bay). Air leakage in unvented roofs assemblies affects only the limited area at the leakage site, usually a small, isolated melt area that can only create localized icing. In many cases, adding ventilation to roofs or attics with significant air barrier problems will increase the warm air leakage from the building into the roof slope or attic and increase the icing. In order for increased ventilation to be an effective ice dam or moisture remediation strategy, air sealing must be completed first, allowing only outside unconditioned air to promote drying and carry away heat loss through the insulation. Good air barrier systems also reduce the heat loss associated with air leakage and can impact indoor air quality by preventing mold growth within roof slope or attic assemblies. DON'T VENT YOUR ROOF WITH CONDITIONED INDOOR AIR.

Dropped soffit under batt insulation with no air barrier under

the batts

Dropped soffit under batt insulation with air barrier

repair under the batts (Courtesy of CSG)



b. Missing or inadequate insulation:

Missing or inadequate insulation can usually be detected visually. Gaps in insulation or measured insulation thicknesses can be used to quantify the R-value in most building enclosure assemblies. The most recent code versions are useful guidelines for what to use as the minimum cost-effective R-values. Increasing R-values above the code minimum will reduce energy costs, up to a point. This point is usually governed by the available space in the framing, or the cost to install new or additional insulation. The level of insulation will always be less important than large amounts of air leakage. Unwanted ventilation that degrades the performance of roof insulation is called wind washing.

Both of these roofs have R-38 batt insulation. The air barrier on the lower-right roof has been repaired, while the air barrier on the upper left roof has not been repaired.

Temperature monitoring of fourteen attic zones shows which attic areas are performing well and which need more air sealing

Wind washing can occur in some types of insulation. Draft stops and air sealing can reduce convection in batt insulation. Rigid foam board and foamed-in-place insulation is air impermeable.



c. Vapor control: Vapor transport by diffusion is not typically a major issue in this discussion. Code compliant rules

of thumb for ventilation (1/300 with a VR, and 1/150 without VR) are adequate to remove moisture entering roof assemblies by diffusion. The largest source of moisture in attics and roof slopes is vapor that is transported by air movement; therefore, air barrier systems and ventilation that moves moisture and heat are the focus of this paper. Source control can be another factor in moisture management in attics and roof slopes. A damp crawl space, swimming pool, or other space with a high humidity level in the occupied space can be a source of moisture in attics and roof slopes. Eliminating the moisture source or sealing the air flow leakage path will prevent moisture from migrating into spaces where a dew-point temperature can occur.

d. Other roof warming sources: Both vented attics and cathedral roof slope ventilation spaces suffer from a number of other

interior and exterior heat sources, usually originating in mechanical systems. These include exhaust from through-wall heaters and poorly insulated or isolated mechanical systems. Ventilation systems are rarely adequate to meet the additional loads these real-word sources would require.

i. A problem that can plague roof ventilation systems is often caused by external heat sources. Exhaust from through-wall heating systems, and leaky wall-to-roof air barrier transitions can be sources of warm air that flows up and into soffit vents, warming and wetting the roof sheathing in the vent chutes instead of keeping it cool and dry. Localized heat/moisture sources immediately below ventilation system inlets can require closing the soffit vents in these locations and proving alternate sources of inlet ventilation air.

Heat escaping from exhaust vents flows up into the roof slope vents. Steam condenses into water inside the vent chutes on surfaces that are below the dew point.



ii. Roof-top exhaust ventilators should be up-draft, not down-draft.

iii. South and west-facing walls (solar gain) in attics can provide attic warming.

iv. Internal heat sources occur where mechanical systems pass through ventilated attics and cathedral roof slopes. Uninsulated chimneys in attics and chimney chases that are open into roof vent spaces are common culprits. Plumbing vents can also introduce warm conditioned air into attics and roof vent spaces.

Down-draft exhaust fans melt snow around the fan

Up-draft exhaust fan

South facing walls in ski resort attic: 60F inside surface, -20F outside temperature



4. Sizing ventilation components to provide adequate flow: Once a good air barrier is established and adequate insulation has been installed, ventilation can be used to supplement the insulation in preventing roof warming and reducing moisture accumulation in the enclosure assemblies. Ventilation systems are a combination of inlet and outlet devices, combined with a cavity in the assembly that acts as a conductor of the air flow between the inlet and the outlet. These cavities must not be isolated from the insulation in a way that prevents moisture from migrating out of the assembly and into the air flow stream. To maximize the effectiveness of ventilation systems, inlet and outlet opening areas should be properly sized. If insulation values are not high enough to effectively eliminate heat loss, air flow rates must be adequate to carry away the escaping heat to keep the roof below freezing and prevent snow melt and the resultant ice dam formation. Most soffit and ridge vent hardware/device systems are designed to have the appropriate size and ratio of inlet and outlet area. Air flow opening areas are reported in square inches of net free area. Most products provide code-compliant air flow; i.e., the manufacturers plan for the net free area of one foot of ridge or shed vent (outlet) to be the same as or more than the net free area of one foot of soffit vent (inlet). Refer to the code jurisdiction for the particular location to see how to calculate the total net free vent area with respect to the roof area or the attic floor area being vented. The use of a vapor barrier in the attic floor or the cathedral roof assembly typically impacts how this area is determined. The distribution or ratio of low-to-high (inlet to outlet) net free area is also detailed in the codes.

a. The net free area of a ventilation device (louvers, ridge vents, soffit vents, etc.) is the effective flow area of the accumulated openings in the device. Screening and opening shape affect the flow, so the net free area is not just the gross length times the width of the louver. This area is published in the product data and often printed on the device itself. Adding screens, especially with tight-weave spacing, to louvered devices significantly reduces the net free area reported by the manufacturer. Contact the manufacturer for guidance on sizing ventilation if these devices are modified in any way.

b. In cathedral roof slopes that are larger than typical residential roofs, soffit-to-ridge vent runs are longer. The longer the roof run, the more total area there is in each rafter bay

Warm chimney surfaces inside vented attics (infrared images)



that adds to the heat loss and moisture load that the ventilation air must carry away. For this reason, the size of the ventilation passages must be increased, and inlet and outlet openings must increase in size proportionally to provide adequate flow to keep the ventilation air below freezing until it reaches the top/outlet of the vent chute. Most soffit and ridge vent systems are designed for average-sized residential roofs. An example of how to provide adequate flow for a large commercial roof would be to use a double row of soffit vent strips, a full-width 2” deep air passage in each rafter bay, and a higher ridge vent product with a larger net free area than standard residential ridge vents.

Even larger ridge vents are ineffective when snow covers shed and ridge vents when ventilation is needed the most to keep the roof surface cool and dry. Eventually heat loss through the roof and solar gain will melt the snow and open the vents; but, some icing will occur and related ice dams will form during the “vents-closed” condition. The length of time it takes for the ridge vents to melt open is related to outdoor temperatures, wind direction, and snow depth. The deeper the snow (and the more insulation value it provides above the roof sheathing), the more melting will occur before ventilation can resume removing heat from the underside of the roof deck.

a. In attics, using gable vents and/or evenly spaced cupolas or vent hoods with outlets that are above the local design snow depth (aka snow load) can address this vent blockage problem. In cases where adequate inlet and outlet net free areas cannot provide adequate passive air flow, mechanical ventilation may be required. A combination of downdraft inlet fans with distribution ducting and balanced outlet cupolas is an effective strategy. Significant depressurization or pressurization of the attic can increase air leakage into or out of the conditioned space.

b. In cathedral slopes, high-rise ridge vent caps that have outlets above the design snow depth are effective, but often are objectionable aesthetically. Another approach is to frame the roof so that the tops of the vent chutes terminate in a duct or gallery immediately under the ridge. This allows the ventilation air to flow along the ridge to gable end vents or evenly spaced outlet cupolas or vent hoods that are not susceptible to closure by snow on the roof. A product designed to allow this framing configuration is the SnowVENTTM system which is a bracket system that makes a structural connection at



the top of the rafters while maintaining a continuous air passage along the length of the ridge.

5. Ventilation - attics: Attics are enclosed areas located directly under the roof. Attics can be conditioned or

unconditioned. Conditioned attics have the building enclosure at the roof plane and are therefore cathedral roof slopes (see below). Unconditioned attics are spaces where the building enclosure is at the floor of a ventilated attic space. In attics, each local zone in the attic must have adequate and balanced inlet and outlet net free areas for cooling/drying to be effective over the entire roof area of that attic. Attics isolated by fire walls or overbuilt slopes should be treated as individual ventilation zones. Specifically, isolated sections of attics that have little or no inlet or outlet will be "dead zones" in the airflow pattern, providing little cooling or drying.

In attics where roof configurations make it impossible to provide adequate local air movement, mixing the air can be an effective strategy. This involves using fans to circulate air from well-ventilated areas into and out of the "dead zones." If there are no well-ventilated areas, supply fans can distribute outside air to the dead zones with outlets being passive or active, depending on the situation. Inflated duct systems are inexpensive and effective. The last strategy for difficult-to-vent areas is to use a completely unvented design which does not require balanced flow in these problematic areas. See Unvented/unconditioned attic strategy, below. Vented and unvented strategies can be used in a single structure, as long as the strategy zones are isolated from each other.

When designing a ventilation system, don’t just compare the gross inlet area and gross outlet area for an entire building. Ventilation should be evenly distributed to keep the entire roof surface that is served by paired inlet and outlet cool and dry. Remember, it is the temperature of the roof surface that determines if melting and drying will occur in that specific area. Ventilation in adjacent framing bays usually cannot help keep another bay cool or dry. Specifically, attic areas with only an inlet or only an outlet will not have effective air flow.

Each local area of the roof or attic must have adequate and balanced inlet and outlet net free areas for cooling/drying to be effective over the entire roof area. Remember, it is the temperature of the roof surface that determines if melting will occur in that specific area. Likewise, adequate flow is required to reduce moisture accumulation. In attics where roof configurations or obstructions make

Soffits below dormers do not contribute to roof slope or attic

ventilation. In this project, only one or two soffit vents between the dormers were expected to provide adequate air

inlet flow for the long roof slopes in addition to the entire attic above.



it impossible to provide adequate local air movement, mixing is an effective strategy. This involves using fans to circulate air from well ventilated areas into the ventilation dead zones.

a. Evenly distribute the inlet and outlet openings over the length of

each given attic or sloped roof zone.b. Attics and roofs should be

considered as isolated systems between barriers such as fire walls, dormers, valleys, hips, and

penthouses.c. Mechanical ventilation may be

required when rates are too low or zones are not balanced.

In this design, the inlets for attic ventilation were in the balcony ceilings. They were large, adding up to enough inlet area, but inadequate in two of the three attic zones isolated from each other by party walls. There were dead zones on the sides away from the balconies and no inlets in Zone A4.

In this design, the cathedral slopes and the attic above the central 4-way valley have virtually no ventilation inlet area. This problematic attic condition required mechanical inlet ventilation

and the addition of large gable outlet vents.



6. Vented roof slopes:

Reducing heat loss through the roof assembly is important to ventilation performance. Adequate R-value and a continuous air barrier reduce the load on the ventilation system. Minimize thermal bridging by reducing the framing area or by including a thermal break in the roof design. There are two kinds of air barrier system at play in vented cathedral slopes. The first is the air barrier that separates the air in the conditioned space from the vent space - usually in the plane of the interior finishes. The second air barrier component is the air barrier that separates the outdoor air from permeable insulation. This barrier prevents wind washing under and around the insulation and reduces R-value drift in the insulation. This usually starts at the bottom of the roof slope and continues up and over the insulation along the vent chute. A combination of baffles at the rafter tails and vent chutes up the slopes is one option for reducing wind washing.

In cathedral roof slopes, each roof bay should have a discrete ventilation system unless bays are interconnected. Otherwise, each framing bay should have balanced (an equal flow rate under ambient conditions) inlet and outlet openings. This is often impossible to achieve in roof configurations where valleys, hips, dormers, skylights, etc. are desired by the designer. Cathedral roof slopes with valleys will typically have significantly more outlet area (ridge vent length/area) than inlet area (soffit vent length/area). A hip roof will have much less outlet area at the main ridges when compared to the available inlet area provided by much longer lengths of soffit vents. These complex roof configurations require careful planning to maintain the appropriate inlet-to-outlet ratio (see balanced distribution below) in each roof slope area. Hip and valley conditions usually require more than just the area provided by conventional passive ridge and soffit vent details. Areas above valleys are the worst places to have melting as the melt water is concentrated in a narrow flow area and valleys are therefore the most susceptible to problematic ice buildup.

Typical soffit and ridge vent hardware systems are designed to have matching flow rates; i.e., the manufactures plan on one foot of ridge or shed vent (outlet) for one foot of soffit vent (inlet) in the same framing bay. Therefore, the soffit inlets on a first floor on the North side El should not be counted as inlets to balance ridge vent outlets on the third floor South side cathedral slope that dies in a valley. Don’t count soffit vents that service dead-end vent chutes as inlet area – best practice is to provide both inlet and outlet in each vented roof slope bay, or use an unvented roof design.

a. Vent chutes are typically used to prevent the soffit from being filled with insulation and to maintain air flow from the soffit into the attic or up along insulated slopes to the ridge

The use of mechanical inlet ventilation in this project incorporated a false chimney designed to mimic the bathroom and fireplace chimneys already in use in this structure.



vent. The space the chutes create should have at least the same net free area as the soffit and ridge vents.

b. If the slopes are very long, they may need to be larger than the inlet and outlet areas to avoid reduced flows from friction (pressure drop) in the air passages. If the soffit is a continuous box rather than a succession of bays defined by the extension of the rafters into the overhang, intermittent large openings can successfully serve as the inlet for multiple rafter bays. If the soffit volume at the end of each rafter bay is isolated (serves as the inlet for just one bay), inlets need to be appropriately sized and installed in each bay.

c. Note that covering the opening in the sheathing at the ridge with tar paper, shingles, or crushing the ridge vent when nailing the shingles over them are common ways to reduce or eliminate ventilation product effectiveness. This also assumes the opening in the sheathing is of adequate size to begin with. The opening should be at least the sum of the net free area required on both sides of the roof.

d. I often see poorly distributed inlet and outlet areas (at hips and valleys, etc.) and no planning for snow-cover at the outlet vents. Our real-world roof ventilation testing confirms that virtually all venting is cut off during a snow event, and depending on subsequent weather (freezing rain, quick freeze-thaw temperature swings, etc.), may remain sealed for extended periods.

Keep in mind that maintaining adequate roof and attic inlet and outlet ventilation provisions without an adequate air barrier system can actually promote roof warming, moisture accumulation, snow melt, and icing. This occurs when exterior (wind) and interior building pressures (from stack effect and/or mechanical systems) tend to force interior conditioned (warm, moist) air into the attic or cathedral slope vent chutes. For increased ventilation to be an effective ice dam or moisture remediation strategy, air sealing must be completed first.

Impediments to balanced flow: Dormers, skylights, chimneys, and other obstructions can prevent uniform distribution of ventilation in cathedral slopes. Inlet and outlet areas on either side of these obstructions should be increased in size and cross-connected to provide adequate flow from the inlet below them to the outlet above and below them. Obstructions that are low on roof slopes are generally easier to design bypass ventilation for than obstructions high on the roof slopes. Unvented roof designs are easier to plan and implement in these locations as well.

Clearly, the opportunities for matching the sizes of the inlet and the outlet venting in this commercial building are very limited.



7. Unvented attics:

Unvented attics are essentially another level of cathedral slopes on a building. 8. Unvented cathedral roof slope strategy:

An alternate strategy to ventilation in cathedral slopes is to increase the R-value and eliminate venting altogether. Remember that the roof insulation values that are necessary to prevent melt vary significantly with climate zone, local snow depths, roof configurations, pitch, wind/drifting, and solar exposure. Lee sides of roofs, transition walls, and dormers pile up snow to depths that may require as much as R=80 or more to avoid melt completely. Low-pitch roofs hold more snow per square foot than steep slopes. Snow tends to stay on low-pitch shingled roofs longer than steep metal roofs which may allow snow to slide off the roof in straight roof configurations (except in valleys or above dormers and skylights). The deeper the snow cover, the more R-value the snow has, and the higher the roof insulation R-value has to be to keep temperatures in the snow pack at the roof-to-snow interface below freezing. The National Weather Service has information on the typical design snow depth for each weather station area in the U.S.

The U.S. Cold Regions Research Environmental Laboratories has studied the R-value required to prevent melt for a given snow pack depth and documents the means of calculating the given R-value required for a given design snow load. They present two R-values for any given snow depth – one for unvented roofs and one for vented roofs. The R-value for unvented roofs is always higher than for vented roofs as the ventilation can remove some of the heat that escapes due to lower R-values. Of course, higher R-values mean less wasted energy. Unvented roofs are also relatively immune to warming caused by air leakage as there is no easy path (the vent chutes) for air to escape through. For this reason, unvented roofs are generally also immune to external heat source warming. If an effective air barrier is not possible due to access limitations, increasing the higher R-value and eliminating the vents can be a successful approach for remediation projects. Of course this is always an option for the original design of the roofs in new buildings.

9. Deciding when to ventilate and when not to: Ventilation strategies are not without risk, both in vented and unvented scenarios. Pros and cons are as follows:

a. Vented attics: i. Require a more rigorous air barrier system

ii. Must have access ways that do not compromise the air barrier system iii. Should have the minimum thermal bridging necessary for the structure and/or a

thermal break at the attic floor iv. Must have adequate ventilation devices in each ventilation zone v. Must have inlet and outlet ventilation devices that are the right size for the

required ventilation flow rate in each ventilation zone vi. Must have balanced inlet and outlet ventilation in each ventilation zone

vii. Should not have uninsulated chimneys and other heat sources in them viii. Must have a strategy for maintaining the required ventilation rate when snow

covers the ridge vents ix. Should not have mechanical system components in them that are not adequately

air sealed and insulated



x. Should be tested during the construction to verify that the air barrier system is complete

b. Vented cathedral roof slope assemblies: i. Require a more rigorous air barrier system

ii. Should be sealed through the depth of the roof assembly around penetrations iii. Should have the minimum thermal bridging necessary for the structure and/or a

thermal break in the roof slopes iv. Must have a strategy for maintaining the required ventilation rate in valleys, hip

roofs, and above and below dormers and skylights v. Must have a strategy for maintaining the required ventilation rate when snow

covers the ridge vents vi. Require less R-value, but may require more space in the framing depth to include

the ventilation space vii. Can avoid ice dams with less R-value, but require larger mechanical systems and

use more energy viii. Require baffles and vent chute devices to prevent wind washing and R-value drift

ix. Should be tested during the construction to verify that the air barrier system is complete

c. Unvented cathedral roof slopes: i. Should have the minimum R-value required by the code or the US CRREL

recommendations, whichever is higher ii. Should have the minimum thermal bridging necessary for the structure and/or a

thermal break in the roof slopes Designers must consider all of the issues in a. or b. when deciding whether to use vented attics

and cathedral roof systems; however, unvented cathedral roof designs only require selecting the proper R-value for the design snow load for the building’s location.

a. Given the likelihood that all of these considerations will never be completely addressed on most projects, unvented designs are more reliable. They are also easier to design and don’t require the comprehensive documentation necessary to inform the installers of all of the “details” that must go with a vented roof system design. Successful vented roofs are possible, but well-defined specifications and air barrier testing with performance standards are required to make installations using this approach work.

b. Complex roof configurations are more difficult to vent properly. c. Inlet vents usually allow wind-driven rain and snow infiltration, and aren’t blocked to

prevent “wind washing” at the eave insulation. d. Experience indicates that properly sized and distributed “rule-of-thumb” or established

ratios of net free ventilation area to roof or attic area are likely to be adequate to provide the necessary cooling and drying, but only if “connections” between the interior space and the attic and/or insulated roof slope vent spaces are eliminated and insulation values are adequate. Most ice dam failures occur where rules of thumb are not observed or air leakage into the vent spaces is prevalent. Ventilation cannot overcome air leakage as adding attic or roof slope ventilation generally increases the rate of air leakage. The second most common cause is inadequate R-values. When air leakage-specific melting is



not evident in localized areas of the roof, ice dams are usually due to melting that occurs more or less uniformly over the entire roof surface due to inadequate R-values. Ventilation is basically a Band-Aid for either excessive vapor diffusion or heat loss due to inadequate insulation.

e. Bottom line, if the roof geometries are conducive to adequate and evenly distributed ventilation and air leakage can be eliminated, vented roofs are appropriate and will be successful against ice dam formations.

f. Given that both vented and unvented strategies can work, and that a builder may or may not understand ventilation, owners should require a written guarantee against “problematic” ice dams.

g. Ice dams usually indicate problems that are occurring in the roof above where the ice actually forms. Ice forms where the roof is cold, melt occurs where the roof is warm. Melt water runs downhill. Roof surfaces at the overhanging soffits are generally cold enough to refreeze the melt water which accumulates in the snow pack along the edge-of-roof frontier.

This infrared image shows melt water flowing down a roof slope under a light layer of snow. The melt here is caused by an air barrier

breach around the chimney flue. Ice dams initially form at the bottom of the stream, and then spread in both directions, as melt

water refreezes at the edge of the overhanging soffit



By: Henri Fennell, CSI/CDT

PROP Jericho report - Efficiency Vermont...ATTIC AND ROOF SLOPE VENTILATION DESIGN Table of Contents...

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