[Strawbale] Sand between the slab and the polyethylene

[Robert Tom]

To go one step further than Don, I would venture that a sand layer between  
the poly and the slab is a very BAD practise, both from a concrete quality  
POV as well as an indoor air quality point of view. The latter has been  
addressed in the past on one of these Lists as well as at Joe Lstiburek's  
website mentioned below by Don. I touched upon the concrete quality POV in  
a recent thread.

A technique that I've long employed (whether rightly or wrongly) is to  
sandwich the polyethylene sheet between mulitple layers of the sub-slab  
insulation.

In my own home, there are two layers of 2" rigid foam placed over  
compacted, 1.5" washed, systematically-drained, crushed stone  ...topped  
with a 2" layer of compacted 3/8" washed/crushed stone to provide a  
smooth, flat substrate that provides full support for the rigid foam.

An agricultural grade black poly sheet (ie UV-stabilised/very tough,  
intended for long-term outdoor use) with fold-locked/caulk-embedded seams  
at all joins was then placed on top of the two layers of rigid foam (with  
joints between layers of foam staggered) and then covered with a third  
layer of 2" rigid foam.

The reasoning was/is that sandwiching the poly sheet between the layers of  
rigid foam protects it from mechanical damage during the rigors of  
construction as well as long after.

The positioning of the sheet is such that it does not violate the "at  
least 2/3 of the total R-value on the cold side of the  
moisture-impermeable layer" rule. The sheet was made continuous with the  
air barrier of the rest of the building.

The intention of the above is to ensure the integrity of the poly sheet  
which, as well as being a moisture barrier, serves also the primary soil  
gas barrier (acknowledging that concrete cracks and that there will always  
be thermally-induced movement of the slab, potentially opening up cracks  
around the perimeter where it meets up with the exterior walls.


On Wed, 02 Jan 2008 10:18:24 -0500, <dfugler at cmhc-schl.gc.ca> wrote:


> 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



-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at chaffY a h o o  dot  c a >
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