Closed cell spray foam has a tendency to crack in colder climates in roof assemblies due to the large temperature fluctuations from thermal cycling and the presence of an impermeable roof membrane. Despite closed cell spray foam serving as a vapor retarder, small amounts of vapor can still pass through, and this moisture gets stored in the sheathing and top of the wood framing during the colder months, causing the wood to expand (this can also occur if the moisture content of the framing is higher during the installation of the foam). Solar radiation drives that moisture back to the interior, causing the wood to contract. This rapid expansion and contraction of the foam is what causes the spray foam to crack, as closed cell foam is quite rigid - negating its benefits as an air barrier. Moisture-laden interior air can then leak through and condense on the underside of the cold roof sheathing, rotting it out over time, as moisture won't be able to dry out easily.

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0:00 Why Spray Foam Cracks
1:12 Preventing Spray Foam From Cracking
1:40 Spray Foam Offgassing
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