I’ve read here and elsewhere about how insulating a roof deck with spray foam can help prevent ice dams, but what if you only did the lower six feet or so. Do you think that would also prevent them?
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Replies
You have to insulate the part that gets hot, not the part that gets cold. Depending on the situation the problem may be the entire roof above the attic or only the area from the top end of the overhang up 5 feet or so.
I live in Boston and my house was the only one on the block that had no ice damns. I attribute this to obsessive airsealing and and 16" of insulation. Like Dan said, the key is keeping the entire roof cool. If warm air heats the snow to the point of melting anywhere on the roof the water will run down under the snow to the eaves where the roof is still cold and freeze. If you spray foam as you describe, ice could still backup under the lower shingles, get trapped by the foam and rot the roof sheathing. Your best bet to combat ice dams is (in order of importance) air seal, insulate, ventilate, and in case you still get some, protect the lower few feet of your roof with peel and stick roof membrane under the shingles.
And there's nothing wrong with using electric heater cables if you can't manage anything else.