January, 2008

How to Prevent Ice Dams

Proper insulation and good building techniques will help you avoid this common winter problem.

By Steve Easley

I'm often asked to explain what causes ice dams and how they can be prevented.

Ice dams can be very destructive and often create serious moisture problems that are expensive to repair in attics and walls. To understand how to prevent ice dams, it’s important to understand what causes them. Ice dams on sloped roofs result from two primary problems:

1] Heat loss through attic ceilings due to air leaks or insulation defects
2] Poor attic ventilation at the eave areas.

Ice dams are caused when snow on the lower areas of a sloping roof melts. The melted snow flows down the roof onto the
colder soffit area at the eave, where it eventually freezes. As the ice builds up, it begins to dam the water and prevent it from running off the roof. Over time, this ice builds until it backs up over the living spaces, where it melts and flows under the shingles. This creates water leaks into the attic and exterior wall.

Ice damming occurs when a roof is too warm because of heat loss from the home into the attic. The process starts when warm, conditioned air from living spaces exfiltrates into the attic, where it warms the underside of the roof deck. This warmer area is what starts the melting of the snow on the roof.

Inadequate, missing, or poorly installed insulation is the cause of the heat loss, which is the root cause of ice damming.

Insulation works by its ability to trap air. If the insulation is compressed, it will lose much of its ability to resist the flow of heat.

Often insulation gets very compressed at the eave areas because there is not enough space for a full height of the insulation.

I recommend using raised heel trusses in cold climates. The additional cost is minimal and it allows for the full thickness of the insulation all the way to the outside of an exterior wall. In cold climates, I recommend R-38 as a minimum. (Always check local code minimums.)

It is also important to use insulation baffles to prevent the blown or batt insulation from being blown or pushed into the soffit areas, where it would clog the soffit vents and stop the ventilation. These baffles also prevent wind intrusion. I like plastic baffles since the cardboard ones can be a food source for mold.

It is also important to pay attention to attic bypasses so you prevent warm air from living spaces from exfiltrating into
the attic, where it can warm the roof deck. Attic bypasses are basically holes in the attic. Examples include:

– Can lights
– Duct chases
– Plumbing and electrical penetrations
– Framing areas that have no ceiling sheet drywall
– Poorly sealed skylights
– Kitchen soffits
– Openings around combustion appliance flues

It is critical to properly seal these bypasses, and closed-cell foam works best for most areas. Be sure to use code-approved, fire-rated materials to seal around any combustion appliance flues.

Lack of proper attic ventilation also contributes to the problem of ice dams. The best way to ventilate an attic is to have
air enter at the soffit vents and flow up and out at the ridge of the roof. The flow of this outdoor air up the roof deck scavenges away heat and moisture, keeping the roof deck cold and dry. It’s best to have continuous soffit vents and continuous ridge vents to ensure adequate air flow.

To Avoid Ice Dam Problems:
1] Make sure insulation is adequate and that it is properly installed to the outside edge of the exterior wall at the upper plate.
2] Use raised heel trusses to provide enough space at the eaves for the full thickness of the insulation.
3] Eliminate attic bypasses.
4] Use continuous soffit vents and ridge vents.
5] Consider innovative products like DuPont’s AtticWrap, which is an air barrier system that essentially creates an airtight, ventilated attic. It also has a reflective barrier that helps keep heat in in the winter and out in the summer.

STEVE EASLEY is president of Steve Easley & Associates, which consults and trains on building science issues. For more info. Visit www.codecollegenetwork.com.