Why do rafters telegraph through the snow on your roof?

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I got an email this morning from a homeowner wanting to know why she could see the rafters marked by snow on her roof.  Snow can help us much like a crude thermal imaging device–we use higher tech infrared cameras.  But the idea is similar.  Both indicate areas warmer areas where heat is escaping faster.  I’ll have to dig through the library to find a couple photos highlighting this.  Meanwhile, a couple of possibilities. 

First, if the rafters are telegraphing through with areas of snow/frost melt, this means that the insulation is working better the the wood in the rafters to keep heat in your home.  While the “thermal bridging” the the rafters isn’t ideal, at least you know the insualtion is working to some extent.

Alternatively, if the rafters are indicated by lines of snow/frost that haven’t melted, while the snow/frost has melt is the larger areas between the rafter, it may mean that the rafter are doing a better job stopping heat flow than whatever is between them.  This could mean no insulation.  Are it could mean poorly installed installation, especially with a lot of air moving through it.

See the previous discussions of icicles, ice damming, and attic insulation and air-sealing for more info.

Thanks,
Mike

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