Summer is the Time to Head Off Ice Damscomments (2) August 14th, 2011 in Blogs
Winter seems like a bad dream, doesn't it? But guess what? It will be back, and with it will come cold temperatures, snow, and, unfortunately, ice dams. Few problems plague homeowners in snow country more relentlessly than the telltale accumulation of icicles along roof eaves, and the water damage that all too often accompanies it.
Summertime repairs to roofs damaged over the winter won't do much good unless the underlying causes of ice dams are identified and corrected.
There's not much mystery in why ice dams form (Paul Fisette's article in FHB provides a good summary of the problem). Snow and ice melting on a warm roof deck run down to the edge of the roof where it refreezes. Icicles form, and over time a thick layer of ice collects at the eave. As more water melts, it accumulates behind this dam, enough so that water can work its way underneath the roof cladding and into the house.
A Crash Course in Roof Venting
Water can stain ceilings, soak into insulation and render it useless, make oatmeal out of drywall and allow rot in both sheathing and structural framing. In short, it makes a mess.
While builders, designers and building scientists seem to agree on the general causes of ice dams , there's still some debate on the exact construction details that will prevent them. Known fixes include installing an adequate amount of insulation for the climate, and creating an effective air barrier that prevents warm air from the house from filtering into the roof system.
Where builders sometimes part company is over the importance of roof ventilation. Some insist that continuous ventilation beneath the roof deck from eave to ridge, what's called a "cold roof," is an essential part of keeping roofs ice free. Others have no qualms about a "hot roof," one that is sealed to outside air, providing it's assembled correctly.
It's not the roof itself: Air seal
One thing that doesn't have much to do with the problem is the roof cladding itself. GreenBuildingAdvisor senior editor Martin Holladay disposes of this red herring in an essay in which he recalled an interview he'd conducted some years earlier with the late Tony Woods, a Canadian building scientist.
Woods told Holladay about a woman who had called three different roofers in as many years in an effort to stop a persistent ice damming problem. The first talked her into replacing the roof, and the following winter the ice dams were back. She called in the second roofer, who said the first one hadn't ventilated the roof properly. So he built a second roof over the first one. The problem was still there. Finally, she called in a third roofer who, with Woods, took a closer look at what they were dealing with.
There were pot lights galore," Woods told Holladay. "...When we said that the problem is not the roof, the lady burst into tears.”
No, the problem wasn't the roof. It was air leakage around the recessed lights that allowed warm air from the house to keep the bottom of the roof deck correspondingly warm, and melt the snow that collected there over the winter. Hence, ice dams.
That led Holladay to the first rule for preventing ice dams: seal all air leaks. Recessed lighting is but one possible cause. Wiring and plumbing chases and gaps around the chimney or air ducts where they enter the attic also are likely culprits. Any gaps, cracks or other defects in the air barrier should be sealed with polyurethane foam, caulk or another reliable sealant. All ventilation fans must be ducted to the outside of the building, preferably through a gable end and not into the soffits.
posted in: Blogs, insulation, weatherizing, water and moisture control
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