Okay, cold climate, full basement, 4″ slab over 2″ 2lb High Density XPS, not taping the seams, over 12″ compacted gravel with subslab drains to daylight.
Do I need poly?
If I do, does the poly go over or under the XPS and do I tape the poly seams???
I have searched the forums and buildingscience and have gotten multiple answers, can anyone who has done this in the past, and had good results answer this?
Thanks
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Replies
I don't know either, but poly is really cheap...
edit: (and I meant to say I don't think it matters top or bottom, but I'd put mine below the xps)
Edited 9/28/2005 8:37 pm ET by Brian
I have been advised to put it above the insulation just as if it was an air/vapour barrier. This seems pretty senseless when you consider the air barrier qualities offered by 4 inches of machine trowelled concrete.
I have also been advised to put it below the insulation in order to keep moisture from rising through the crushed stone and wicking up through the concrete.
My own opinion is that the poly is there to keep water from rising and you can't really go wrong by putting it either above or below the insulation.
Ron
You will be stopping the moisture either way, so is there any additional insulation value in dry vs. wet xps? To me that would be the only difference.
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I vote for under, I hate walking on poly.
The XPS isn't supposed to soak up water. I vote to put the poly over the insulation mainly because it seems less likely to be perforated by stones during the pour.
Ron
Any slab that is going to be inside needs poly under it. Standard practice here is to put the poly over the foam. I don't think it is necessary to tape the seams, but make sure they are well lapped. Probably at least 6".
Edited 9/29/2005 6:24 am ET by Matt
From what I have read on the Building Science web site, the concrete acts as the air barrier in the system, the poly acts as the moisture barrier. The question of whether the poly goes over or under the XPS I think comes down to the question of does moisture affect XPS? Since it is fairly impermeable to moisture, I think you're OK putting the poly in either location; pick whichever is most convenient.
What thickness poly do you plan on using. i know may contractors don't like it directly below the slab because it can cause concrete curling. I however don't know if XPS has the same effect on concrete. I am doing mine below the XPS. I am trying to find a on-line source for 10 mil poly, anyone know of one?
Aaron
Aaron,"Congrete curling"? Please say a little more about this. Thanks.Bill
Let me first state that i am in the environmental business not contracting buisness, but we often spec vapor barriers for building designs and many contractors have complained that it causes curling of the concrete along the edges. This may be more of a contractor just doesn't like it complaint than a real issue, as i have not seen the curling effect.
I'm not an authority, but in my judgement I'd put the poly on top. Its purpose is to act as a capillary break to keep moisture from the ground from wicking into the slab. In that respect it should work either way.
But if it is below the poly you'll create a cavity between the slab and the poly (formed by the spaces between the insulation sheet and the uneven surface below). If any moisture did get in there it would be trapped like water in a balloon. It would be better, I'd think, to have the poly on top so the concrete conforms exactly to it with no cavity which can trap standing water.
Many contractors order a mix that is too wet because it is easier to place, but that leads to increased shrinkage, a weak surface subject to dusting and spalling, long wait before the slab can be finished, and could possibly lead to the alleged curling. I'll bet if the mix were the proper stiffness and cured under plastic rather than using a chemical curing agent there would be no curling at all.
Edited to correct misspelling of "its" and insert missing punctuation.
Edited 9/29/2005 6:41 pm ET by WayneL5
I usually see the pointy ends of remesh just punctue the poly anyway. I'd put it under the insul. Spheramid Enterprises Architectural Woodworks