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A. Paul Malko, an engineer with Foard Panel in West Chesterfield, N.H., responds: The frost lines (called frost ghosting or snow ghosting) you’re seeing at the panel joints can have a few different causes, but I agree that air leakage isn’t likely. Typically frost ghosting is caused by the presence of splines at the panel joints. You didn’t say whether the exterior skins of the SIPs were splined together, but I’ll assume they were, since that’s most common. Most of the roof surface has a single layer of OSB (the top of the SIP), tar paper, and shingles. At the panel joints, however, there are two layers of OSB — the panel skin and a connecting spline — in addition to the tar paper and shingles (see illustration). Because of the extra layer of OSB, the thermal capacitance (or thermal mass, as it’s often called) of the roof assembly is greater at the edges of the panels than in the middle. That means the roofing over a splined joint changes temperature more slowly than over the rest of the SIP. Frost ghosting is usually seen in the morning when frost or light snow is beginning to melt off the roof. The frost or snow melts first at the center of the panels, where thermal capacitance is lowest (meaning that particular section warms the fastest in the sun). Because the thermal capacitance at the splined joints is greater, it takes longer for the sun to warm the roofing over them, which is why you see lines of frost there. The same process happens in reverse in the evening: As the air cools, the center of the panel cools first, allowing frost to accumulate more quickly there than at the joints.
Frost ghosting isn’t uncommon on SIPs with directly applied roofing, and I’ve never known it to reduce the durability of the roof. Frost ghosting also occurs on other types of wooden roof structures, but because structural insulated panels typically lose heat more slowly than conventional framed roofs, the ghosting may last longer.