Attic Air Sealing, Insulating, and Ventilating for Ice Dam Prevention

Description

In cold climate zones where heavy snow fall is expected (Figure 1), it is important to design attic and roof assemblies to minimize the likelihood of ice dam formation. An ice dam is an accumulation of ice that forms at the eave of a roof and prevents melted snow from draining off the roof. Some snow melt may overflow the dam and create icicles (Figure 2). These icicles are often a sign that more liquid water is building up behind the dam where it can be drawn up beneath the shingles by capillary action and cause roof leaks. If enough water and ice accumulates on the roof, it could overwhelm the roof structure and cause a roof collapse (University of Minnesota 2019).

Design Snow Loads Map for the United States, adapted from ASCE 7-10, Figure 7-1
Figure 1. Design Snow Loads Map for the United States, adapted from ASCE 7-10, Figure 7-1 (Source: Wilkerson).

 

The icicles and the bare spot along the left roof ridge indicate that heat from the second-story room may be heating the underside of the roof deck, melting the snow and likely leading to ice dam formation
Figure 2. The icicles and the bare spot along the left roof ridge indicate that heat from the second-story room may be heating the underside of the roof deck, melting the snow and likely leading to ice dam formation (Source: Straube 2006).

 

Ice dam formation requires three things to occur: snow on the roof; a poorly air sealed and/or poorly insulated attic; and freezing temperatures (UMass Amherst - Department of Environmental Conservation 2019). Ice dams can form with as little as 2 inches of snow accumulation on a roof. When heated air from the house escapes into the attic, it can warm the underside of the roof deck, melting the snow on the roof above. This snowmelt water runs down the roof until it reaches the cold eaves where it refreezes as shown in Figure 3 (UMass Amherst - Department of Environmental Conservation 2019). As this melting and freezing action continues, the ice ridge grows larger, forming a dam that blocks the flow of water behind it so that a puddle of liquid water forms on the roof above the eave.

Heat loss through the roof of a home in a cold climate zone leads to snow melting to form ice dams
Figure 3. Heat loss through the roof of a home in a cold climate zone leads to snow melting to form ice dams (Source: University of Minnesota Extension 2019).

 

The three best ways builders can combat ice dam formation are to

  1. Fully air seal the ceiling plane.
  2. Thoroughly insulate the attic.
  3. Ventilate the roof.

The most important step is to seal all of the air leaks from the conditioned space into the attic space to keep warm air from entering the attic in the first place (Lstiburek 2018). Common sources of air leakage into the attic include gaps around electrical wiring, light fixtures especially recessed can lights, plumbing stacks, furnace flues and chimneys, attic hatches and stairs, dropped ceilings, open soffits, exhaust fan housings, top plates, ceiling drywall seams, and attic kneewalls (Lstiburek 2014). Figure 4 shows examples of air sealing of several potential air leakage spots on the ceiling including around exhaust ducts, wiring holes, top plates, and drywall seams. Many of these air sealing measures are described in the Building America Solution Center guides listed at the end of this guide.

 

Spray foam air seals gaps around holes and drywall-to-top plate seams
Figure 4. Spray foam air seals gaps around holes and drywall-to-top plate seams (Source: Mantell-Hecathorn Builders).

 

Adequate attic insulation also needs to be installed to prevent heat from transferring to the roof decking. This insulation can be installed on the ceiling lid in a vented attic, or on top of the roof deck (using layers of rigid foam), or on the underside of the roof deck of an unvented attic (using closed-cell spray foam). The air barrier under the insulation has to be continuous. In a vented attic, the air barrier is usually the ceiling drywall, with every penetration air sealed. In an unvented attic with rigid foam above the roof decking, the air barrier can be the roof decking with taped seams or a full self-adhered membrane or a fluid-applied air barrier product. In high snow-load zones (> 60 lbs/ft2) the most successful approach to addressing ice dams with unvented compact roof assemblies is to construct a vented “over-roof” over the unvented “under-roof” (Lstiburek 2020). In Climates Zones 6 or higher, a vented attic also needs at least a Class II vapor retarder (kraft-faced fiberglass batts, “smart" membrane vapor barrier, or vapor barrier paint) between the air barrier (the ceiling drywall) and the insulation (Lstiburek 2020). Raised-heel energy trusses  can allow the full depth of insulation over the top plates to keep the roof deck area above the eaves cooler.

It’s critical to vent the roof assembly in cold climates, under the roof deck with a vented attic or over the roof deck with an unvented attic, to keep the roof cool to minimize the snow melt that leads to ice dam formation, as shown in Figure 5 (FEMA 2014). Baffles and insulation dams should be installed to keep insulation out of the soffit vents and to direct cooling air flow from the soffit vents up along the underside of the roof deck to the ridge vents (Figure 6). An air gap of at least 2 inches should be provided between the top of the insulation and the underside of the roof deck (Lstiburek 2018).

Attic ventilation can reduce the likelihood of ice dam formation by cooling the roof deck
Figure 5. Attic ventilation can reduce the likelihood of ice dam formation by cooling the roof deck (Source: FEMA P-957 2014, Snow Load Safety Guide).

 

The soffit dam and baffle allow air to flow through the vents without disturbing the insulation covering the top plates
Figure 6. The soffit dam and baffle allow air to flow through the vents without disturbing the insulation covering the top plates (Source: PNNL).

 

See the following guides for instructions on specific areas to air seal to prevent the leakage of heated air from the home through the ceiling and into the attic.

These Building America Solution Center guides provide more information on attic ventilation:

If any ducts are located in a vented attic (not recommended), they can be air sealed and insulated by encapsulating in spray foam and then covering with attic insulation (Figure 7), after first air sealing by spray foaming around all of the duct boot penetrations on the ceiling deck (Figure 8). A better option if HVAC equipment must remain in the attic is to move the air barrier and insulation above the HVAC by locating it at the roof line of the attic as shown in Figure 9 where the roof joists have been netted and filled with blown insulation, then more rigid foam has been installed above the roof deck and a vented over-roof has been installed above that.

 

Buried ducts are laid on the floor of a vented attic then covered with spray foam and blown attic floor insulation
Figure 7. Buried ducts are laid on the floor of a vented attic then covered with spray foam and blown attic floor insulation (Source: Steven Winter Associates). 

 

Duct boots and other ceiling penetrations are air-sealed using spray foam
Figure 8. Duct boots and other ceiling penetrations are air-sealed using spray foam (Source: Steven Winter Associates).

 

Retrofit an existing roof by installing rigid foam above the roof deck with a ventilation space between the rigid foam and the new roof sheathing plus new moisture and air control layers and cavity insulation in the roof rafters.
Figure 9. Retrofit an existing roof by installing rigid foam above the roof deck with a ventilation space between the rigid foam and the new roof sheathing plus new moisture and air control layers and cavity insulation in the roof rafters (Source: Building Science Corporation). 

 

The following guides provide information on adding more insulation to an existing attic and on converting a vented attic to an unvented attic by adding insulation above or along the underside of the roof deck.

More Info

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

References and Resources*
Author(s)
Federal Emergency Management Agency
Organization(s)
FEMA
Publication Date
Description
Report informing building stakeholders about the risks a snow event poses to their buildings, preventative measures to take before the snow season, and actions that should be taken before, during, and after a snow event.
Author(s)
University of Minnesota Extension
Organization(s)
University of Minnesota
Publication Date
Description
Webpage explaining what causes the formation of ice dams on roofs and the effects they have.
Author(s)
Paul Fisette
Organization(s)
UMass Amherst Department of Environmental Conservation
Publication Date
Description
Article describing how ice dams form on residential roofs, how they damage homes, and solutions to prevent future dams.
Author(s)
Minnesota Department of Commerce Energy Information Center
Organization(s)
Minnesota Department of Commerce
Publication Date
Description
Article describing the causes that form ice dams, as well as solutions for preventing future ice dams on roofs, focusing on air sealing to prevent hot air leakage.
Author(s)
Straube
Organization(s)
Building Science Corporation,
BSC
Publication Date
Description
Article describing causes of ice dam formation on roofs and potential solutions.
Author(s)
Lstiburek
Organization(s)
Building Science Corporation,
BSC
Publication Date
Description
Article describing strategies for reducing the risk of ice dam formation on roofs in winter.
Author(s)
Straube
Organization(s)
Building Science Corporation,
BSC
Publication Date
Description
Article outlining the casuses and solutions for ice dam problems.
Author(s)
Chris Dorsi
Organization(s)
Montana State University Extension
Publication Date
Description
Article from Montana State University Extension explaining how proper insulation techniques can greatly reduce the risks of ice dams on roofs in cold climate zones.
Author(s)
Lstiburek
Organization(s)
Building Science Corporation
Publication Date
Description
Report that provides information and specifications to anyone that is attempting to air seal existing attics.
Author(s)
Lstiburek
Organization(s)
Building Science Corporation,
BSC
Publication Date
Description
Document providing background and approach for the prep work necessary prior to adding attic insulation - focusing on combustion safety, ventilation for indoor air quality, and attic ventilation for durability.
Author(s)
Joseph Lstiburek
Organization(s)
Building Science Corporation
Publication Date
Description
Article describing the importance of air sealing, insulation, and roof ventilation in reducing the likelihood of ice dam formation on roofs in cold snowy climates.
*For non-dated media, such as websites, the date listed is the date accessed.
Last Updated

Mobile Field Kit

The Building America Field Kit allows you to save items to your profile for review or use on-site.

Sign Up  or  Log In

Did you find this information helpful?