Preventing Ice Dams
Margy Wilshire| Posted inGeneral Questionson
I live in Winnipeg, MB which is above North Dakota. I have a 1,000 sq. ft. bungalow built in 1960. The vaulted ceilings were recently spray foamed with a commercial grade polyurethane foam from the inside with no ventilation and now I would like to put a new roof on. The pitch is very, very low and I need some suggestions on how to prevent ice dams next winter. We get ALOT of snow here!
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Replies
Margy,
The answer to your question depends on:
1. Whether the spray foam is closed-cell (with a density of about 2 pounds per cubic foot) or open-cell (with a density of about 1/2 pound per cubic foot).
2. Whether the spray foam was installed thick enough to prevent heat loss that melts snow.
You really don't want to sandwich your roof sheathing between two impermeable layers. If you installed closed-cell spray foam on the underside of your roof sheathing, it's hard to install more insulation on top of your roof sheathing without creating the dreaded foam sandwich. That means you are locked into adding insulation only on the underside of your roof, not the top side.
You also need to be careful about installing membrane roofing (that is, roofing that doesn't "breathe") on a roof insulated on the underside with closed-cell foam. Of course, it's always possible to install ventilation channels on top of your existing roof sheathing -- an approach worth considering.