A long run of relatively mild winters ended this past season with record snowfalls in several cities, including warm weather locales such as Dallas, Atlanta, and Raleigh, N.C.
In addition to snarling travel plans, the snow and fluctuating temperatures combined with poorly sealed or incorrectly insulated attic spaces to cause ice dams, something many homeowners had not confronted in years, if ever.
While the formation of an ice dam is obvious, its potential to wreak havoc on a house is not always easily or immediately apparent. Water that pools behind an ice dam at the eaves of a roof can take several paths into the structure of a house. If that water is allowed to remain or continue unabated over several snow seasons, the latent damage it causes can range from mold and drywall damage to structural catastrophe.
The good news is that the potential for future ice dam formation on an existing roof can be mitigated fairly easily, while slight changes in building practices and materials in the construction of a new roof can prevent them from happening.
The key is not to wait for a call. By the time latent defects caused by moisture in the roof and wall cavities becomes apparent, it’s far too late to address and remedy the problem short of an intrusive teardown of the affected areas.
Cause for Concern
Ice dams occur when melting snow on a roof reaches the eaves or an otherwise unvented or uninsulated portion of the roof that is colder than the attic space that is incidentally heated from the conditioned areas below it. That dynamic causes subsequent snowmelt to pool up and seep under the shingles and/or down the perimeter wall cavity. And that can result in a host of moisture-related problems, most of them latent, long-term, and costly.
For existing homes, insulate the attic floor to at least R-38 and seal all penetrations and framing joints, insulate intruding light and vent housings (if possible/allowed) and mechanical runs that extend into the attic or through the roof, and weatherstrip the attic hatch, which reduces heat loss from the conditioned spaces below that can warm the underside of the roof deck. Consider creating a soffit that facilitates ventilation with existing gable vents and/or installing a continuous ridge vent. If you replace the roof shingles, add a one-piece polymer membrane between the deck and the felt to block moisture penetration.
For a new home, consider using raised-heel roof trusses, which enable full-depth floor cavity insulation to the eave (an added energy-saving benefit) and more easily allow soffit-to-ridge ventilation that balances the roof’s temperature across the entire structure. For the most effective cure, cover and seal the uninsulated roof cavities with plywood, drywall, or rigid foam panels to contain the cold airflow to the underside of the roof deck. Avoid placing penetrations in or through the space to mitigate heat loss from below.
Illustrations by Harry Whitver