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Space and cost effective basement insulation

CTSNicholas | Posted in General Questions on

I’m working on insulating a basement bedroom.  The room is in a corner of the basement with 8′ poured concrete walls and no insulation currently.  It’s located in Zone 5b.

Originally I started off with the idea of 2″ XPS or PolyISO glued to the concrete and then running 1×4 firring strips both horizontal then vertical for 1.) place for wires to run 2.) place for drywall to screw on 3) a little added depth for electrical boxes.

Soon I realized the thickness of this wall is dang near that of a 3.5″ 2×4 stud wall. 2″ foam + 3/4″ horizontal 1×4 + 3/4″ vertical 1×4 = 3.5″ thick.

Now, growing up reading GBA I know that the alternative, 2×4 stud wall with fiberglass in between studs will potentially yield me R-13 vs R9 or R10 but with the thermal bridge of a 2×4 that R-13 is closer to R10 anyway AND I am worried about moisture issues.

So, GBA, what about building 2×4 stud walls, spacing them out roughly 1/2″ from the concrete, and then adding the very cheap insulation (paper faced fiberglass) between studs, and just calling it a day? The wall is never perfectly plumb it seems, so I may have 5/8″ gap between concrete and stud and maybe even 1/4″ in other places since I can now plumb the 2×4 stud wall.  I lose maybe 1/2″ but I save $$$ and the environmentalists are a little happier.  As long as my fiberglass doesn’t touch the concrete wall, is there anything to worry about? The rim joist is still going to be insulated with a rigid foam but that’s above this wall.  Is the key to not use paper faced insulation in this instance? I hate working with that stuff since it’s much harder to install and get to stay in place and be a clean install.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nicholas,
    Q. "As long as my fiberglass doesn’t touch the concrete wall, is there anything to worry about?"

    A. Yes, there is plenty to worry about. You'll get condensation on the interior side of the cold concrete walls, and puddles on the floor.

    Follow the advice given here: "How to Insulate a Basement Wall."

    1. CTSNicholas | | #3

      Martin, long time no see. Thanks - My GBA knowledge has slipped these past three years since I originally was involved in building. I remember doing a poly-iso build out in a few rooms in another home.

      If I use 2" of XPS to get R10, then 1x4 firring strips I do not get the recommended R-15. I suppose the best and most cost effective method is 1" of XPS foam then a 2x4 wall with unfaced fiberglass between the studs? Or would 2" be required to help the condensation point. Either way that concrete should have the foam as a vapor barrier to prevent that air from touching the cold surface, and then the R5 1" XPS + R13 stud wall should give me about R15 real insulation value.

  2. Expert Member
    Dana Dorsett | | #2

    Batts need a full air barrier on both sides to hit their numbers, and there's a realistic chance the studs will develop mold, though the risk is lower with the air gap.

    With an all-foam solution there is no need to run 2 layers of furring- one is enough, and it can even be installed in a dado cut into the foam if depth space is that valuable. It's not tough to make channels for the wiring with a router, and can-foam or pack the channels with rock wool or fiberglass. With 3" of cheap reclaimed roofing polyiso (~R16.5-R17, and usually cheaper per R per square foot than batts) and let-in furring in a dado with half inch gypsum tight to the foam it would even hit IRC code minimum on a U-factor basis despite the thermal bridging of the let-in furring. That would be code-compliant, and only 3.5" from concrete to interior paint.

  3. user-5574861 | | #4

    I recently used Insofast 2.5" EPS panels to insulate a new construction basement. They have electrical raceways and plastic studs 16" OC. You glue them to the concrete with Loctite PL. You can then attach additional insulation with a couple of screws and prong washers to hold it place before adding the drywall. Sounds like you would need 1" of polyiso to hit code in your area. No stud wall needed but at 4" from concrete to paint, a little deeper than Dana's suggestion. Personally, I prefer gluing things to concrete than trying to use mechanical fasteners which is why I went this route.

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