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Would the following method of finishing a basement cement wall cause any mold issues?

sesjems | Posted in General Questions on

Someone suggested to lay 2×4’s flat against cement wall every 24″ and secure with adhesive and nail gun. then put 1.5″ XPS rigid foam insulation in that 24″ space against cement wall. Finally cover wall with 1/2″ regular sheetrock. What about using pressure treated 2×4’s? Would the chemicals cause more health issues?

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  1. GBA Editor
    Martin Holladay | | #1

    The concrete is likely to be cold and wet, so you don't want to have any lumber (whether treated or untreated) in contact with the concrete.

    The first step is to attach one or more continuous layers of rigid foam to the interior of your concrete wall. After you are done insulating, you can build a 2x4 wall to the interior of the foam.

    Here's an article to help you: How to Insulate a Basement Wall.

  2. Expert Member
    Dana Dorsett | | #2

    Putting wood against the foundation wall and surrounding it with foam is akin to burying it in soil- almost guaranteed to cause a mold & rot condition in most situations. The pressure-treated would limit the mold hazard but would still have a substantial thermal bridging effect.

    The thermal bridging of the 1.5" thick wood (about R1.5) is huge hit in performance too. With 3.5" every 24" you have a framing-fraction of 15% that is R1.5, which delivers a whole-assembly performance (gypsum included) of about R5, despite the center-cavity foam being R7.5. You'd do nearly as well with 3/4" foam tacked in place using 1x furring over the foam on which to hold the gypsum, and with 1" foam and 1x furring you'd do better than the 1.5" foam with the pressure treated 2x4s.

    But R5 is a pretty low insulation value for basement walls. In most US locations it's better (sometimes even code) to put R10 or better against the wall. Using 1.5" continuous EPS (R6) and a studwall with unfaced R15 rock wool or R13 fiberglass batts (R10-12, after thermal bridging) adds up to better than R15, which is pretty good, and would have low mold risk for all but the coldest US lower-48 locations. If that's too big a hit in floor area, 2.5- 3" polyiso roofing sheets held in place with 1x furring saves a couple inches of depth.

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