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Drying Time for Basement Walls

dritter2 | Posted in General Questions on

I purchased a new construction home and the builder did a ResCheck instead of following minimum recommendations. The result was that most of my basement was not insulated .   🙁

The builder applied some kind of black barrier below grade to the exterior of the foundation – I believe this is tar from what I have heard?

I am basically looking for someone to tell me I’m not a complete idiot here. My objective is to reduce the probability of moisture, mold, etc.  Based on the research I have done, I have come up with the following:

I live in climate zone 5. My plan is to have a contractor install 2in EPS on all the interior foundation walls, frame the walls with 2x4s against the EPS, and then install mineral wool batts (drywall later).

I don’t notice any moisture coming in anywhere in the walls so far (I’ve lived here about 2 months).  I have heard, however, that concrete can take a while to dry ( a year plus).  Would there be any issues with installing the EPS at this point in time? Would that trap moisture in the concrete, and if so, is that an issue, especially with the tar (I think) on the outside?

Bonus question – Is there any reason (besides added R value) to do EPS or would just mineral wool against the concrete be sufficient in most cases against mold/rot issues?

Thanks so much!

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  1. Expert Member
    BILL WICHERS | | #1

    The black coating was probably asphaltic water proofing. It's basically tar, and it dries to a thick, hard layer that looks a little like tar with some sand in it. It's been around forever, and it lasts pretty much forever too. There is nothing wrong with this -- it's the traditional way to waterproof a foundation wall. As long as the coating was applied evenly and thick enough, you should have no problems.

    Concrete cures, it doesn't dry to set up. Curing is a chemical reaction where the water gets incorporated into the chemical structure of the finished concrete. Curing goes on for a long time, decades even, but concrete is usually defined to reach it's full yield strength after 28 days. After 28 days, the concrete has reached 99+% of it's full strength. Some residual moisture may be released gradually for a while after that, but if the initial concrete mix was correct, it shouldn't be excessively wet so you shouldn't have to worry about it. If you're worried, do the "plastic square" test -- tape a piece of clear plastic a foot or so square to the concrete wall. Watch for signs of moisture on the inner surface of the plastic for several days to a week or so. If it stays dry, you're good to go.

    I don't see a problem putting up 2" EPS now. Note that in CZ5, current code is R15 for basement walls, 2" EPS is only about R8 or a bit better. If you use a double layer of 2" EPS with seams staggered, you can mount the studs flat (instead of the usual way on edge) against the EPS and end up with the same overall wall thickness, but a more robust assembly. You can use 1.5" deep 4" square electrical boxes for your electrical devices, and mud rings to get through the drywall. This is a good way to build, and it's a very robust assembly giving a pretty moisture proof R16+ or so.

    You can use 2" EPS against the concrete and then batts in a studwall too. You do NOT want to put batts directly against the masonry though -- that's asking for mold problems.


  2. jadziedzic | | #2

    I'd be inclined to wait through the spring rainy season before doing any work, just to be sure there aren't any areas where ground water might find its way into the basement through the foundation walls and/or slab. Once the walls have been covered with insulation it will be hard to track down and address any problematic areas.

    Another option you might consider is to have the 2x4 walls built about an inch from the foundation walls and have an insulation contractor spray 1-1/2" of closed-cell spray foam on the walls, followed up with your mineral wool insulation. The labor to install the EPS AND properly seal all the edges and seams MAY offset the (possibly) higher cost of the spray foam.

    If you go with spray foam you can also have the installer spray 3" or so of spray foam on the inside of the rim joist, continuing over the top of the wall to fully air seal that area.

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