Ice Dams-Why They Form and How to Reduce their Risk

There is a lot of confusion as to why ice dams form on roofs in cold climates.  This is understandable, some years we experience heavy ice dams, other years have little to no issues.  Changes in weather from year to year can have a major impact on their formation.  I’ve seen new homes with ice dams that are completely related to winter freeze/thaw cycles.  That being said, most ice dam issues are directly caused by certain characteristics of the home, namely insulation levels and air tightness.

Ice dams form from heat that migrates from the living space of the home into the attic.  When outside temperatures are right, typically 20F or warmer, snow that has accumulated on the roof begins to melt from this escaping heat.  (We will get into how heat escapes a home a little later in this article.)  As the snow changes from a solid form of water to a liquid, it runs down the roof until it contacts the roof eave (the part of the roof that overhangs the exterior wall).  Because this portion of the roof is not over the heated, interior portion of the home, the temperature at the eave is colder.  This causes the liquid water to re-freeze.  Over time, ice builds up.  The resulting ice dam can lead to liquid water flowing under the shingles and other roofing materials where it can enter the home.

Insulation Levels

New homes built today have a roof design that typically includes a raised heal truss.  This roof truss design allows higher levels of insulation to be installed where the pitched roof meets the exterior wall.  The higher the R-value, the slower heat moves from inside the home to outside during the heating season, resulting in a cooler roof.  Often, older homes do not have a raised heal truss, insulation levels are less for several feet into the home.

Another cause is overall lower insulation levels across the entire attic area.  Current building codes require a minimum of R-49 for attic insulation.  Depending on the insulation type (fiberglass, cellulose, spray foam, etc…), the thickness of the insulation will vary to meet the R-value requirements.  In some cases, more than 15 inches of insulation may be needed.

Air Sealing

Conditioned air that leaks from inside the home and into the attic is another cause for ice dam formation.  Much like building code requirements for minimum insulation levels, current codes also require a minimum air tightness level for all new homes.  Every home in a heating dominated climate experiences something called stack effect.  This is a phenomenon where air that is heated becomes buoyant and rises inside the home, putting pressure on the ceiling.  Any holes between inside the home and the attic will allow the heated air to leak into the attic.  As this air escapes the building, outside air will be drawn into the home, typically at the lowest level of the home.  This outside air is then heated, which rises and escapes.  The cycle continues all winter.  By air sealing an attic, we can greatly reduce stack effect, possibly eliminating ice dams and lowering energy bills.

We check the air tightness of a home using a tool called a blower door.  This test places the home under either a negative or positive pressure (about the equivalent of a 20 mile per hour wind blowing on all sides of the home at the same time).  We can use thermal imaging or theatrical fog to identify air leak locations.  In some homes, air leaking into an attic is a bigger contributor to ice dams than low insulation values.

What About Attic Ventilation

Most attics are designed to move outside air from the eave, into the attic, then exit the peak or roof ridge.  Attic ventilation was created to address our past inability to construct a roof that is well air sealed and insulated.  If no warm air leaks into an attic from the conditioned space inside the home, and we effectively reduce heat transfer from inside to outside by sufficiently insulating the attic, attic ventilation is not needed.  We still install attic ventilation in new homes as an insurance policy.

Some builders believe a solution to ice dams is to increase attic ventilation by installing powered or whirly bird venting solutions.  This can exacerbate an ice dam issue by causing negative pressure inside the attic.  This negative pressure increases the rate at which warm air is pulled from the conditioned space of the home, creating more ice dam issues.  Increase ventilation is usually not cure for ice dam issues.

Roof Design

Simple roofs are easier to eliminate ice dam issues than complex roofs.  Cathedral and vaulted ceilings often do not allow for high levels of insulation and are difficult to air seal.  Traditional dormers, changes in a roof’s direction (roof valleys and hips), and roofs interrupted by a wall to accommodate an additional level or interior height change all create more challenges in insulating and air sealing.  I have personally been involved in several building science investigations related to ice dams and water leakage in a truss design called an attic truss.  This design is a popular option to add living space without the need for a second story.  Other roof problems that can contribute to ice dams are roof penetrations.  Skylights and chimneys for fireplaces add their own challenges.

The easiest roof design to prevent ice dam problems is to use a simple ranch style roof design with a flat interior ceiling.  Though these roof designs add little visual interest, they are the easiest to construct and maintain.

New shingles or other roofing materials do little to reduce or eliminate ice dams.  Other measures are often needed, these changes can substantially improve the durability and operational costs of the home.  Be sure to have a conversation with your roofing contractor about investigating and/or providing building science solutions for ice dam issues.