[Strawbale] Sand between the slab and the polyethylene

[dfugler at cmhc-schl.gc.ca]

Hello again,

The recommendation to put sand between the poly and the slab is 
counterproductive when the poly is there as a moisture barrier. CMHC 
testing in the nineties showed that concrete slabs poured on poly were 
more airtight and moisture-tight than slabs poured over gravel. If you 
want those effects, the slab has to be poured directly on the poly so that 
the poly spans the eventual cracks in the concrete. If you put a sand 
layer in between, it makes the concrete finishing somewhat faster and 
easier, but it reduces the effectiveness of the poly. With the sand, any 
air/soil gas/ moisture leak in the poly can communicate with any crack in 
the concrete through the sand, so you might as well not have the poly 
there.

For the same reason, poly over the foam boards is better than poly under. 
There are some reasonable arguments made on buildingscience.com for 
pouring the slab directly on the foam boards, without the polyethylene. I 
would prefer to have the added advantage of the slab poured directly on 
the poly for the air and moisture resistance created by this assembly.

Don Fugler
CMHC Policy and Research