Why are we so confident in CC spray foam as a long-term vapour barrier?
I’ve been reading a lot about spray foam roof assemblies lately and the general underlying assumption is that when properly applied, closed-cell spray foam acts as an adequate vapour barrier.
But it seems to be accepted fact that foams dry out and become brittle over time. That foams shrink over time. This is why we have to throw away our bike helmets after 7-10 years. This is why we have to throw away children’s car seats after 7-10 years.
So why does it seem to be accepted fact that 7-10 (or even 10-15 years) after application, that well-installed spray foam is still functioning as a vapour barrier?
Wouldn’t it be better to use a belt-and-suspenders approach and apply some type of fluid-applied vapour barrier first, let that cure, and then follow up with a spray foam application underneath that? So even if the foam shrinks or cracks over time, the fluid-applied vapour barrier is still acting as the vapour barrier, and the foam is still doing it’s job as a thermal barrier and everyone is happy.
What am I missing?
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Replies
Maxwell,
A few thoughts - which don't directly address your central question about longevity.
- I think many of the suggestions here to use closed-cell spray foam aren't thinking about its longevity, but rather in comparison with the common alternative of open-cell, which is fairly vapour-open by design.
- If the degradation is limited to shrinkage and gaps it don't matter much to its function as a vapour-barrier, just if it is also being used as an air-barrier.
- Adding a more robust vapour-barrier, like a fluid applied one, to the sheathing before applying the foam wouldn't help much if it's the foam failing that you are worried about. The point of using closed-cell foam is to stop interior moisture being able to move though it so it doesn't accumulate in the foam or get cold enough to hit the dew point. Vapour-barriers or retarders need to be located close enough to the interior of walls or roofs to stop that happening. Exterior ones don't work.