On frosty mornings, many houses exhibit the same interesting phenomenon described by Allison Bailes in his most recent Energy Vanguard post. "Ghosting" is when the outline of the rafters is clearly visible on the roof because of the lighter frost pattern on the shingles directly above the rafters.

The conventional wisdom is that this type of ghosting is caused by thermal bridging, where there’s more heat loss from the attic through the wooden rafters than there is through space between the rafters. But as Bailes explains, not only are the rafter bays fully insulated with spray foam, but so are the rafters themselves. So, if not caused by thermal bridging, why the ghosting? Bailes has an answer that might not occur to even those who have a good understanding of building science.

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