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Dam Right!: Alert readers offer tips on ice dams

By Mag Ruffman

I wrote recently about solutions for ice damming, which happens when your roof is warmer than it should be. Snow melts on the roof, runs downward and forms mini-glaciers along the colder edge of the roof. The ice build-up then displaces shingles and gutters, sometimes causing freshly-melted water to back up under the shingles, which produces expensive ceiling leaks inside the house.

In my dam column I didn't cover the entire scope of building science behind the dam problem, because to do so I would have needed a whole dam section. Several alert readers pointed out that inadequate roof ventilation causes a lot of dam problems, and I didn't cover ventilation, so today I'd like to clear up any dam misunderstandings.

Hi Mag,
You explained ice damming very well, but you didn't mention anything about properly venting the attic. I am in the house aluminum business and we regularly add vented soffits to houses that don't have enough vents. We find all manner of reasons for the soffits to be blocked, but the end result is the same. If air cannot enter the soffit and escape through the roof vents, then the heat that radiates through the insulation becomes trapped and leads to ice damming. Unfortunately, changing the soffits, and usually the fascia and eavestrough as well is not a cheap and quick fix. But it is the proper way to fix ice damming. A properly vented attic can also prolong the life of your shingles and save on your heating and electric bills.
-Angus W.

Your roof is part of a slick building system that aims to keep the roof the same temperature as exterior air. During the cold winter months, your first line of defense against ice dams is preventing warm interior air from sneaking up into the attic and heating the roof. You can stop the upward migration of warm air by sealing air leaks between the heated part of the house and the attic. Main culprits are usually attic trap doors, plumb ing stacks, chimneys, electric wiring and even unsealed pot lights. So if you're installing pot lights on the second floor of your home, use the sealed ones. End of lecture. It may also help to add insulation to your attic. In cold climates, attics may need up to 16 inches of insulation for an R-value of 49 — now there's something to brag about at a cocktail party. But unfortunately, even if you take all of these precautions, crummy attic ventilation can still screw things up. How?

Modern homes are designed so that outside air circulates continuously through roof and attic spaces. This keeps the attic from overheating in summer and also keeps the roof surface chilled in winter. Air flows into the attic at the lowest point through vents in the eaves, travels up the rafter bays and escapes through ridge and roof vents.

A problem in many homes is that eave vents are blocked by poorly installed insulation. Too much insulation at the edges of the attic can cut off the entry point for air. You may need to pull insulation back with a garden rake.

You can also increase airflow by adding soffit vents. First, let's discuss soffit. I always thought soffit was a modern brand name for the sheathing (usually plywood, aluminum or vinyl) that runs underneath eaves. But in fact, soffit is from the Latin suffigere ("to fix underneath"). Who knew?

Anyway, you can add airflow by adding vents to the soffit. Retrofit vent styles include small round vents, rectangular vents, and &foot long strip vents. But how many do you need? The goal is to balance air intake and exhaust, so the square footage of your soffit or eave vents should match the square footage of your ridge and roof vents. Want to know the formula to figure out how much ventilation your attic needs? I thought you'd never ask.

You need at least one square foot of attic ventilation (both intake and exhaust) for every 300 feet of attic space. So a 1,200 square foot attic needs 4 square feet of ventilation, or 288 square inches at the eaves, and 288 more in ridge and/or roof vents. This is manageable, right?

Once you've got enough exterior air moving from the eaves to the rafters and escaping through the ridge vents, the roof of the house can't heat up and cause ice dams.



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