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.
Full disclosure - I worked for a spray/injection foam contractor ...
Is there something specific about a proper field application of closed cell foam that would be different from say a polyurethan SIPS panel in terms of this type degradation that you specifically described (dry out and become brittle over time)?
Of course in a SIP factory setting you have optimum conditions, proper QA testing, & the foam chemically bonded to 2 large surfaces ...
So, I am not sure I'm in agreement with your statement about an "accepted fact" without direct exposure to UV light
https://blog.synthesia.com/en/what-does-the-durability-of-polyurethane-depend-on
You are thinking that all foams are the same. They're not. Bike helmets I've seen use a very dense EPS material, which is polystyrene. Car seats are usually ABS I think, which is a different material.
Spray foam is polyurethane. Polyurethane is very stable over time and tends not to degrade.
UV exposure will damage all of these materials.
BTW, the factory manufacturer foams are, by comparison to anything mixed in the field, made "perfectly", since ALL conditions are well controlled. Those materials are very stable over time (aside from UV exposure issues). I have, as an example, some polyiso here from 1977 or 1978. It's a bit yellowed on the edges from past exposure, but if you cut into it, the exposed stuff in the new cut looks like new.
What makes you think a fluid applied vapor barrier is going to hold up any better over time than the foam?
Bill
Closed-cell polyurethane spray foam has been used as an insulation in homes for several decades already. If there was a problem with it degrading over time, we'd know about it by now.
A little tip for anyone reading this and new to the subject. Closed cell spray foams (CCSPF) can be a very useful PART in the solution for your air control layer in your house. But they aren't a complete solution in and of themselves. It's really important to plan the entire solution from the start.
Also, pay for quality work. Use a contractor with proven training and experience. When it comes to CCSPF, a bad installation can be a horrid mess. And a good one can hugely benefit your house. Some of the bigger foam manufacturers, like Carlisle, certify contractors who go through their training. It's worth paying the price to use a certified installer and make sure the work is done right the first time.