Plastic layers under basement slab
Hello:
I have a question regarding the 6 mil plastic that goes under the basement slab. In our case we will have close to 8 inches of peastone to get near level with the footings. I am also going to be installing 2 inch foam boards on top to insulate the slab. Our excavator is suggesting we put a layer of plastic down UNDER the peastone, in addition to the plastic layer that goes on top of the peastone to sandwich the peastone between the two layers of plastic. His opinion is this will ensure good drainage throughout and not allow the peastone to mix with the soil, which in our case is all clay. We do have footing drains that run through the footings and also the drain tiles that run the perimeter of the footings also.
Does this sound reasonable or could this cause any potential issues?
Thank you again for this great resource.
Chris
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Replies
Chris,
Both layers of plastic are in the wrong place.
The lower layer of polyethylene suggested by your contractor might hold puddles of rainwater that falls from the sky before the concrete is laid. You don't want that.
The upper layer of polyethylene suggested by your contractor is in the wrong place. It belongs between the rigid foam and the concrete. This article explains why: "Polyethylene Under Concrete Slabs."
Hi Martin,
Thank you for the information. I read the article and it sounds like the main concerns are from water coming from above. Fortunately our basement is framed above it, so in our case we shouldn't have any water coming from above before the concrete is poured. I was mainly wondering if there is any harm in putting the plastic below the peastone and any long term issues with that as well as having the rigid foam against the peastone. Based on that article it sounds like there should not be an issue with the foam against the peastone.
Thank you and Best Regards,
Chris
Not an expert, but I have built some things and hang out here. Martin is saying pea gravel, rigid foam, poly, concrete slab as basement floor. The extra poly below the gravel wouldn't contribute anything.
Putting the poly below the peastone is a very bad idea. When the concrete is placed and it settles down, it releases a fair amount of "bleed water." If you have a porous layer under the concrete, the bleed water flows down into that layer, filling the voids in the peastone. This water can take literally years to dry out through the top of the concrete slab.
If the plastic is right under the concrete, the bleed water is forced to rise to the surface. It then reacts with the cement and soaks back into the concrete within an hour or two, helping to complete the curing process. The result is harder and stronger concrete, and no water in the stone.
Masons have been fighting putting the plastic right under the concrete for as long as it has been a requirement - at least 3 decades or so. They are worried that if the top surface of the concrete cures/dries faster than the bottom, then the slab can warp. This does actually happen, but it just shows that it's important to keep the top of the slab damp by fogging for a few days in hot/dry weather.
Chris,
That makes three votes to zero in favor of eliminating the polyethylene below the crushed stone. You want just one layer of polyethylene, not two, and it belongs immediately under the concrete.
Thank you all for the feedback. Exactly the advice I was seeking, and I'm glad I posted the question here. Thank you very much for your time!
Chris