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Ice Dam Aftermath: The Facts about Mold

Many people had ice dams this winter and are now repairing the resulting damage. At the Boston Building Resources annual meeting on June 4, a number of members expressed worries about mold. Here is how some of their questions were addressed by panelists Brian Butler and Flemming Lund.

If I had water coming in from ice dams, will it cause mold?

Most likely not, Flemming said. “The sheetrock may be damaged from the one-time ice dam, but that’s not your big cause of mold growth. It has to be a continuous feeding of moisture.” Brian pointed out that the winter weather, with its very low humidity, is unfavorable to mold growth. “If you get wetting, especially in an old house with lots of air leakage, it dries almost instantly. There could be exceptions, but 99% of the time, and everything I’ve seen experientially, the moisture dries very quickly.”

Ice dams caused water to leak through the clapboard siding and brown stains formed. Am I at risk for mold?

An infrared scan would tell you more, such as whether there are spots in the walls or ceilings where there is no air barrier, said Flemming. One contractor in the audience cited the example of a house with brown “coffee” stains on the siding. The walls consisted of multiple layers, from the inside out: lathe/plaster, cellulose insulation, sheathing, tar paper, clapboards. Water from the ice dams had gone between the tar paper and clapboards and never went into the wall. When the wall was opened up, everything had dried out by that time. The “coffee” stains came from the tar paper. There may not always be evidence of water ingress on both sides of the wall, but brown staining on the siding is sufficient evidence to implicate the ice dams.

Mold from inadequate ventilation

If people try to build energy efficient homes, but do not ventilate properly, mold can result, Flemming said. “All the moisture that’s trapped by cooking, showering, breathing, maybe off-gassing from the basement, stays in the building.” Unvented gas fireplaces are a particularly significant offender, bringing in “buckets of water” every day. The vapor is invisible at 68 or 70 degrees, but it will condense when the moist air enters lower temperature areas, such as the basement, inside the walls, or in the attic when hot air rises. “Mechanical ventilation in a home that is energy efficient is one way of reducing mold,” Flemming said. “Dehumidification in your basement is another way. Any leakage or pooling of water should be fixed.”

The best approach is to start from data and testing rather than guessing at the source of the problem. A wise approach is to have an energy audit done by a certified HERS (Home Energy Rating System) rater. In our region, conducting a radon test is also recommended. Radon test kits cost about $30 and are found to be very accurate when used as directed. With these data points and knowledge, you can make informed decisions about air sealing and ventilation.

Mechanical systems in the attic

“Builders should be hung by their toes” for putting mechanical systems in the attic, said Brian. While current code requirements for insulation levels in the roof are R38, a cabinet around an air handler might provide a level of R4 at best. (R value is a measure of a material’s ability to resist heat traveling through it. The higher the R value, the more heat loss will be stopped by the insulation.)

Compounding the problem, ducts wending through the cold attic often leak while trying to deliver hot air in the winter. In the summer, the situation is reversed, as cool, conditioned air leaks into the hot attic. “When the attic is hot, and you have moist heated air at 93% humidity up there, and you are delivering 72 degree air to the rooms, what do you get? You’ve all seen the soda bottle with sweat—condensation,” Brian explained. “The attic is an especially bad place for condensation to happen because you run the risk of mold.”

Having ductwork in the attic is not a good situation, but you may need to make the best of it, Flemming advises. “It’s not a good solution if the attic is outside the thermal envelope. But if the ducts are up there, they need to be as tightly air-sealed as possible”

Mold on storm windows

If your primary wooden windows are getting older, they may be leaking more, and warm air is escaping through the primary window and condensing on the inside of the storms. The moisture on the storm windows allows mold to grow, said Flemming.

Brian Butler is a contractor with Savilonis Construction who specializes in deep energy retrofits and sustainable contracting and design.

Flemming Lund is owner of Infrared Diagnostic LLC, where he uses infrared imaging to perform energy audits on residential and commercial buildings. He also worked as a home inspector for 14 years.