First, basement walls do not have to dry to the interior, now says Dr. Joe Lstiburek, the building scientist who widely spread the notion that they did.

However, foam insulation on the interior basement wall is certainly an excellent energy savings choice. Especially if the wall is leaking, or is expected to leak. XPS is pretty much impervious to water. We use it in cottage country to keep docks buoyant.

I would just stop the foam a couple of inches above the drain.

Of course, I always waterproof from the outside to stop any damage to the walls and footings before it starts.

The best fasteners for the foam are all plastic. See photo. Or you can use 1x2s or 3s to hold a number of sheets of insulation tight to the wall.

This post was edited by worthy on Sat, Aug 23, 14 at 14:23

Thanks for the reply and providing that new link. I guess now it doesn't matter whether I put a 6mil vapor barrier behind the foam, drylock the wall w/ no vapor barrier, or just put the 2" foam against the wall.

I was leaning to power washing the wall, patching any digits, and putting xps up with no plastic, but now I guess it doesn't matter.

Where did you find that picture? What are those things called? I can't find them at the Depot.

There are a number of manufacturers. You can order on-line. Or try contacting Owens-Corning, for instance, which has a line of fasteners for attaching their foam boards to all kinds of substrates.

Locally, a building supply retailer as opposed to the Big Boxes should have these fasteners. There are even retail chains selling only fasteners.

Hi!

I am doing the same thing in my basement and had the same question about how to put the XPS on the wall given the lip of the french drain cladding. Mines about the same dimensions as yours: 3/8in thick and 4-5in off the ground.

I'm leaning toward just cutting the XPS to the top of the lip. Should I notch it instead?