Does sub-slab insulation need to be secured and where does plastic go when concrete is the finish material?
I am using two 1″ layers of reclaimed xps insulation under my slab, and it does not lay really flat. Is there a way to secure it to the gravel layer, and does it need to be? The concrete slab will have radiant heat in it.
Secondly, the poured concrete basement floor will be the finished floor with a burnished power trowel finish. Does the polyethylene go under or on top of the insulation. I ask as one of the concrete contractors bidding the job, felt that with the plastic on top, then the water in the concrete all dries upward and can cause what he called “flashing” and can affect the finish floor look negatively.
Any advice would be greatly appreciated.
Thank you!
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Polyethylene goes between the insulation and concrete.
"Notice that the sheet polyethylene “vapor barrier” is located on the top of the rigid insulation between the rigid insulation and the bottom surface of the concrete – in direct contact with the concrete. Do not, and I repeat, do not locate the polyethylene under the insulation as it will keep the insulation wet. Trust me on this – or go back and read “BSI-003: Concrete Floor Problems.”
http://buildingscience.com/documents/insights/bsi-059-slab-happy
Steve,
Many concrete contractors, including yours, have outdated ideas on this topic. I urge you to read our article on the issue: Polyethylene Under Concrete Slabs. You may also want to read the comments posted on that page.
Ordinarily, the rigid foam is not secured in any way. Once the concrete is placed, the weight of the concrete holds everything down. It sounds like you may be using rigid foam sheets that are warped or curled, however, and that might be adding problems in your case.
2" of foam under a heated slab is substantially less than what is financially rational in most areas even if it were virgin-stock foam at virgin stock pricing. With reclaimed foam 4" would be cheaper than 2" of virgin-stock, and would likely still have a financial rationale. The exception might be if you're in US climate zone 2.
Is it too late to go thicker?
Martin, yes the XPS is not totally flat, which is why I ask for any suggestions on keeping it down or more flat.
Dana, It may not be too late to add some, depending on what happens with floor height. I have access to really inexpensive new Polyiso, (brown paper/fiberglass faced) but I know it should not go against the gravel. Can it be used between the XPS and plastic, then concrete on top?
Thanks for the help :-)
Nope- polyiso can't go under the slab, even if it's the middle of an XPS sandwich.