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How much cc in a roof in New England Post New Topic | Post Reply
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Eric Machemer
Posted: Jan 21, 2009 06:31 PM
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How much cc in a roof in New England
How much cc spray foam does everyone think is needed to stop ice damming in a Massachusetts climate (Zone 5)?I have heard a range of thicknesses and lots of talk about diminishing returns, but how much is needed to stop ice damming? |
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Roger Morrison
Posted: Jan 29, 2009 05:49 AM
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Actually, this is not an easily answered question. It depends on how cold it is outside and how much snow is on the roof. The whole object is to keep the exterior temperature of the roof surface below freezing. But snow insulates, thereby tending to warm the roof surface. If you assume that the R-value of snow is about 1 per inch, then 6 inches of snow is adding an R-6 to the exterior side of the roof. There is a formula for calculating the temperature of any layer in an assembly in the Moisture Vapor Transmission paper AY-118 published by SPFA (yet another reason to support your trade association). You will need to make educated assumptions regarding exterior temperature and snow thickness. And a working knowledge of algebra will be helpful. Roger |
