We're installing a perimeter french drain in our 550 square foot basement. We have a subslab radon system that will tie into the drain system. Walls uninsulated, but I plan to insulate after addressing the moisture issue. What's the best way to treat the intersection of the newly poored floor patch and the walls? One contractor proposed 15 mil polyethylene sheeting that would be sealed at the top of the wall and lie on the crushed stone of the trench. Concrete would be placed over this. This approach should provide a good air seal and moisture management, but does it hinder the placement of insulation (I'd prefer not to puncture the sheet if used in this manner)? Can I use taped and foamed sheet foam to accomplish the same air seal, and limit the polyethylene sheeting to the bottom foot of the wall? If so, what's the best way to integrate the foam sheet with the polyethylene sheet? Thanks. edited to make paragraphs and improve readability

XPS is the answer, both below the floor and for the walls, if flat.

6 mi under the XPS if you must, but 2" of XPS, taped or not, is an effective vapor retarder and has worked for several passive radon systems we have installed during our regular basement remodels, which always include radiant floor heating.

Set the foam directly on the stone, and the poly on top of that. With the poly under the foam there is a chance that water could get between the slab and the poly where it will take a long time to dry out. With the poly on top, there is less of a chance, plus the poly will be on a firm surface and more likely to stay intact.

Water under any amount of pressure tends to find every tiny hole and flow through it - in foam or PE. So I expect that a dimpled mat (aka vent gap, providing pressure relief) is the best way to make sure that any water coming through the concrete wall makes it down to the drain with no chance of coming through the interior wall. Select a non vapor permeable one and tape joints to also stop air, water vapor and radon transfer.

Posted By BadgerBoilerMN on 12 Aug 2015 11:46 AM
XPS is the answer, both below the floor and for the walls, if flat.

6 mi under the XPS if you must, but 2" of XPS, taped or not, is an effective vapor retarder and has worked for several passive radon systems we have installed during our regular basement remodels, which always include radiant floor heating.

Thanks, Badger. It sounds like you're suggesting XPS under the 12-18" of floor that will be removed for the drain, and then mating that XPS that will cover the walls and sit atop the footing (forming an L shape of XPS that will be covered with concrete). Do I have that correct? Is it necessary to tape the XPS seam under the concrete to ensure an air tight seal?

Bob I: Thanks for the detail. It sounds similar to Badger's recommendation. jonr: I tend to agree with you, and I thought the dimple mat might be the better, if more expensive, way to go. If I go that route, what's the best way to attach rigid foam in front of the dimple mat? Would Dow Perimate serve both purposes?

Perimate or any type of grooved foam extended to below the floor level makes sense to me - with similar cost and labor to flat foam. On the other hand, lots of basements are successful without the explicit capillary break.

Thanks Jon. I will talk to the waterproofer to see if he'll install insulation as you describe. If he's only comfortable installing the polyethylene sheeting on the whole wall, or dimple mat on the first 1-4 inches of the wall, what's the better way to go? The wall could be (bottom up): 1 inches of dimple mat, XPS held 4 inches off the ground and applied directly to the wall with seams taped. I could then spray foam between the floor and XPS, and the top of the XPS. It doesn't have a formal capillary break, but water going down the wall/back of the foam, would find its way into the drainage mat.