Is poly (VB) necessary under insulated slab?
user-1112350
| Posted in Green Building Techniques on
I am building a shallow frost-protected foundation that is entirely underlain with 6″ EPS (including footers). This is, from my understanding is a class II vapor retarder, not the Class I vapor retarder (vapor barrier) that should be used directly under a slab over just stone. I see in some cases it is omitted when foam is used, and in other cases, it is recommended to make sure it is on top of the foam in direct contact with the slab. I am in zone 6, btw. I am more interested in it’s use to help to restrict air (and potential radon) infiltration. Thanks
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Yes and Yes... You do need poly below the concrete and above the rigid foam; the moisture retained by the poly will let your concrete cure better. If you are in an area where Radon is present a passive vent is very inexpensive to install now vs. a costly remedy later on. See a BSC attached document.
Thanks- I was definitely going to put in a passive vent pipe for radon. Is that the only reason for poly- to allow the concrete to cure properly? It seems that moisture will not migrate that quickly through 6" of foam vs poly, especially compared to air or stone
Apparently, ALL foam will let some water get through, eventually. Water has nothing to do all day but migrate, and time is meaningless, so I'd stop it w/ poly. Concrete will do NOTHING to stop radon, as far as the "experts" here have told me. (The fact that concrete is a good air barrier does not mean it is a radon gas barrier, I guess. Smaller molecules? Meaner ones? Dunno.) If there is radon, vent.
Poly is a vapor barrier. If you are going to install it between concrete and rigid foam, a 6 mil. will do fine; however, if you are going to install it between concrete and aggregate, I would use a 10+ mil. to minimize punctures.
Klingel, cracks in concrete. Not through the concrete.
AJ: Ah so. Thanks.
Just out of curiosity- Why does Stego call their 10 mil a vapor retarder and the 15 mil a vapor barrier. Marketing?