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Old 12-02-2015, 04:43 PM   #1
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Vapor barrier needed with spray foam?

Does one need to install a plastic or otherwise vapor barrier when using closed cell spray foam?
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Old 12-02-2015, 09:45 PM   #2
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I've heard that it's unnecessary with closed-cell spray foam as it actually performs as a vapor barrier itself (some building codes don't even require one when using it). I am not a professional and this advice should be taken with a grain of salt. Regardless, I won't be doing a vapor barrier when I do my insulation here in the next couple of months.
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Old 12-02-2015, 11:21 PM   #3
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No idea in this case, but I have noted that some spray foams at Home Depot talk about being able to block moisture (as opposed to those that just create glorified sponges).
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Old 12-03-2015, 12:17 PM   #4
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Some foams are listed as air barriers; some are listed as vapor retarders; some as vapor barriers. It seems to vary from one product to another. Often achievement of the "barrier" threshold requires some minimum thickness, too. I believe there exist foams that can serve as a vapor barrier but I couldn't cite one as an example. (I looked briefly at Dow's Froth Pak just now, but didn't find any mention of vapor barrier in the materials I skimmed through.) You'll probably have to choose a foam based on availability, price, etc, and then learn about its characteristics.
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Old 12-03-2015, 03:41 PM   #5
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...oh, and make sure it is compatible with metal. Not all foams are (like "Great Stuff" for example).
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Old 12-05-2015, 03:06 PM   #6
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don't put plastic up.

thats the beauty of spray foam, it IS the vapor barrier. the other thing thats awesome about it is that it adds lots of rigidity. if you put a vapor barrier up, the foam would bond to the plastic instead of your sheet metal/studs, and then it would be floating/moving around. no bueno.

spray foam = air seal

air seal = no condensation

spray foam is by far the best option for insulating a skoolie.
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