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Community and Q&A

Drainboard or XPS on basement walls? (Tying to existing drainboard)

user-5254480 | Posted in General Questions on

I am finishing a 1400 sq/ft basement on a remodel for a home built in 1948. Poured concrete foundation walls on beefy footers with a poured concrete slab (no insulation or vapor barrier under the slab).

The foundation walls are in excellent shape with just a few/thin cracks. There is no water intrusion on the walls. We just had one of the worst months of total rain fall. The walls are dry. All good.

There was a internal gravity driven french drain added for extra insurance as there had been previous signs of water getting up under the footers and trapped under the slab. Again – after all of the recent rain – all is dry.

When installing the french drain – (see picture) drainboard was started a few inches above the slab height and then rolled down to footer area and tucked next to the drain. The idea was that if there was any water intrusion lower on the wall it would just travel down to the drain.

Now that we are finishing the basement we want to get some insulation on the walls and floor. I really like the approach outlined here:

It was recommended that I first run drain board all the way down the wall and tuck it behind the existing drainboard. Then build my inner wall and 1″ from the wall to provide an air gap. Then insulate stud bays. This doesn’t make sense to me.

Given that we don’t have an existing water problem on the walls I’d prefer to go with an approach of 2″ build out of foam from the walls and then build by wall directly to the foam.

1) Is applying foam directly to the wall a smarter approach vs. drainboard+airgap+wall?

2) If I go with foam on the wall – how should I tie the bottom of the foam to the top of the existing drainboard?

3) I was thinking about sealing the top of the drainboard with some low expansion spray foam and sealing tape. My thought was that since we don’t have a water problem higher on the wall and if we go with XPS on the walls we’d avoid condensation – we don’t need a reason to channel water off of the wall. My bigger thought here was that sealing the top of the existing drainboard helps keep vapors from coming up from the slab. Thoughts?


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  1. GBA Editor
    Martin Holladay | | #1

    My advice is, don't overthink this. Either approach will work. There are arguments in favor of either approach.

    A third possibility is to choose one of the rigid insulation products with integral drainage channels; one example is Foamular Pink-Drain.

  2. user-5254480 | | #2

    Thanks Martin. Once again - you rock.

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