# Is this foamboard a vapor barrier?



## burnt03 (Sep 20, 2009)

I have this foamboard on my concrete walls in my crawlspace. Just wondering if it's considered a vapor barrier (like the pink or blue polystyrene) or if I should extend the plastic up to the sillplate?











Thanks!


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## Windows on Wash (Aug 30, 2011)

EPS is the most vapor open of the foam but you don't need to run the plastic up to the sill plate.

Run the poly about 12" up the wall and you should be fine.


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## burnt03 (Sep 20, 2009)

Great, thanks!

One more related question.... in the crawl space, one of the concrete walls has the garage slab on the other side. It doesn't have the foam board on it. Does it need it?


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## Windows on Wash (Aug 30, 2011)

burnt03 said:


> Great, thanks!
> 
> One more related question.... in the crawl space, one of the concrete walls has the garage slab on the other side. It doesn't have the foam board on it. Does it need it?


Ideally, yes.

Garage floors, although they may not be exposed via an outside wall, do conduct a bunch of energy and concrete, by itself, is not a good insulator.

If you are going to be down there, do it right and do it once.

:thumbsup:


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## burnt03 (Sep 20, 2009)

On a related note, I ended up putting a white foamboard on this concrete wall in our storage room (wanted to go with the pink XPS but they didn't have any). Planned on sealing it up then building a 2x4 wall in front of it.










The local building inspector mentioned that it has to have less than 60 ng/(Pa.s.m2) for it to work as a vapor barrier, and this stuff has 200 ng/(Pa.s.m2).

So could I just tape the plastic to the foam and to the existing vapor barrier on the framed wall above? Would I have to join the plastic to the floor with sealant or is it ok just to the bottom part of the foamboard?

Hope that question's clear, any suggestions?

Thanks!


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## Windows on Wash (Aug 30, 2011)

Poly over top would work as a vapor barrier at that point.

Just seal all the seams and lap them properly.


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