Cool attic ends melt-freeze cycle that causes ice dams

Home Work

March 09, 1996|By Karol V. Menzie and Randy Johnson

Every now and then nature conspires to rivet homeowners' attention on a particular maintenance problem. Heavy rains direct concern to wet basements, and this year, heavy snows, followed by thaws, followed by more snow, and so on, have people everywhere worrying about ice dams.

When the syndrome was at its worst, we wrote about retrofit solutions for ice damming -- rubberized shields that cover the roof edges, heating coils that keep the ice melting. However, there's another solution to what's become a big problem in the region, said Rudy Fischer, president of Homes by Fischer, in Glen Arm.

"It's something about this winter," Mr. Fischer said. "I've been in this business 30 years, and I've never seen so many problems with ice damming."

Ice damming occurs when snow on a roof melts when in contact with warm shingles. Freeze and thaw cycles mean ice can form along the edge of the roof, causing icicles and sometimes forcing water back up under the shingles, where it can damage gutters or get inside and cause leaks.

Mr. Fischer says the problem is that people are keeping their attics too warm. "You have to try to keep the air in the attic about the same temperature as the outside air," he said. "People think if they're keeping the attic warm, they're helping to keep the house warm. It's the exact opposite. If the attic is as cold as outside, you'll never have ice damming."

Keeping the attic cold is a matter of insulation and ventilation, Mr. Fischer said. There should be a minimum of R-30 insulation between the living space and the unheated attic space, with plastic foam baffles to keep the insulation from touching the roof sheathing. In addition, the attic needs continuous ventilation, ideally from gable-end louvers or ridge venting plus some kind of soffit vents. The air should keep moving, with cold air continuously moving in.

A good ventilation system will help with air-conditioning efficiency too, and keep summer cooling costs down.

New-house construction almost always employs these techniques. A house Randy built last year had soffit vents, a ridge peak vent, and two 2-foot-by-3-foot louvered redwood vents in the gable ends. He also sealed the tops of all the chases (drywall bumpouts to encase flues and pipes) so warm air from the house couldn't blow up into the attic. Standing on a ladder outside one of the gable end vents was like standing in front of a hot fan, he said. When he went by this winter to see how the system was working, the house was one of the rare ones in the neighborhood that wasn't festooned with icicles.

Keeping the attic as cold as the air outdoors is a simple concept, Mr. Fischer said, but people have an ingrained notion that any heat inside helps heat the house. "I was in one person's house and they had put insulation between the rafters," he said. Trapping warm air and moisture in the attic can result in condensation, or rain, in the space, and, Mr. Fischer said, it will also rot the plywood roof sheathing.

The insulation-ventilation system works best in houses that have attics. It's harder to get air moving properly in houses with flat roofs and little space between ceiling rafters and roof rafters, mostly because there's not much room for soffit vents or louvered vents. The three decorative pierced panels that traditionally adorn Baltimore rowhouse fronts just under the cornices were designed to provide ventilation, but it's rarely enough.

Mr. Fischer said the one way to get air moving in such a situation is to install a mechanical ventilation device -- like those silver stacks with the spinning balls on top. But, he conceded, "They're pretty ugly."

Pub Date: 3/09/96

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