Replies

1. GBA Editor
   Martin Holladay | Jun 05, 2014 07:44pm | #1

   Matthew,
   The best approach is to put the polyethylene on top of the rigid foam (directly under the concrete). That way they poly won't hold "bleed water" or rain water that sits between the foam cracks or trickles down to form puddles under the foam.

   For more information on this topic, see Concrete Floor Problems.

   In your case, the solution is fairly simple. Get a 4-foot length of rebar and use it to poke about 50 holes in the poly by jamming it between the seams of the rigid foam. Then install new poly on top of the rigid foam. Remember, polyethylene is cheap.

2. 
   spearrock | Sep 11, 2022 12:02pm | #2

   So, if I’m using closed cell (formulae 250- I know I’m bad and should be using #2 EPS) do I need the polyethlene?

   Thanks,

   PS, hope I am not co-opting this thread, sorry OP.

1. Expert Member
   Michael Maines | Sep 11, 2022 06:36pm | #3

   It's an eight year old thread, I think you're safe ;-) In theory you could be ok with taped XPS but the risk of something going wrong is pretty high. If you're in an area with high radon levels or high water table, or if your sub-slab prep does not allow the foam to lay flat, I wouldn't risk skipping the poly sheeting.

3. 
   spearrock | Sep 11, 2022 07:03pm | #4

   Thanks, after I posted this I read one of formular’s pamphlets and it looks like 2” of formular is 100x as vapor permeable as polyethylene. I did my best to chip the spilled messy excess concrete that ended up on our interior fittings but there are two spots about 1/3 “ to high, where the foam is a little rocky. I have the plastic on site so I’ll put it down. Better safe…