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In Md. I have a vapor barrier and I want to insulate and finish basement

blgoodw | Posted in GBA Pro Help on

What is the best way to finish the basement. I already paid for drainage system; vapor barrier is up on walls. What is the best type of installation…? Basement is framed 24′ vs 16. This is so frustrating for me, my brother gave me R19 that he had left over but I don’t think I can use it. Someone told me not to use paperback fiberglass near vapor barrier…Help!

Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    B.L.,
    First, we need a better description of what exists. There are four issues which are unclear.

    1. You wrote, "I have a vapor barrier" and "vapor barrier is up on walls." What does that mean? Do you mean (a) Your basement wall has exterior asphaltic dampproofing, or (b) Someone installed polyethylene on the interior side of the concrete wall, or (c) Someone installed polyethylene on the interior side of a stud wall that was framed near the concrete wall, or (d) Something else?

    2. You wrote, "I already paid for drainage system." Do you mean (a) There is an exterior footing drain leading to daylight, to a sump, or to a drywell, or (b) There is an interior French drain, or (c) Something else?

    3. You wrote, "Basement is framed 24' vs 16." Do you mean that there is a stud wall that was framed with studs 24 inches on center? If so, we need to know whether there is a gap between the studs and the concrete wall, or whether the studs are touching the concrete.

    4. Finally, we need to know whether you have already installed fiberglass batts between the studs.

    After you provide us the missing information, you should read this article: How to Insulate a Basement Wall.

  2. User avatar Expert Member
    Dana Dorsett | | #2

    Most of MD is in US climate zone 4A, where ideally there would be enough space for 1/2" of foil faced rigid polyiso foam board between the studwall and the foundation, but in your climate it's not necessarily a disaster if it isn't, as long as there is a true vapor barrier between the foundation and susceptible wood, and the fiberglass won't be in direct contact with the concrete (where it can wick ground moisture out of the concrete toward the interior.) With R2.5 or better between the fiberglass & foundation on the above-grade portion any winter condensation events would be fleeting and there would be minimal wintertime moisture accumulation in the wall cavity. This would be true even in zone 4C, with it's much longer and cooler shoulder seasons:

    https://up.codes/viewer/utah/int_residential_code_2015/chapter/7/wall-covering#R702.7.1

    R19s when compressed to 3.5" in a 2x4 cavity perform at R13. (That's right- an R19 is just a fluffed-up R13, it's same weight per square foot!)

    The thermal performance of 2x4/R13 would meet IRC 2015 code minimum performance for zone 4A.

    https://up.codes/viewer/utah/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.2

    Kraft facers are not true vapor barriers, and will become more vapor open than standard latex paint when the humidity in the cavity rises to a level that would support mold. If you have kraft faced R19s it's fine to install them with the facers next to the wallboard side.

    If the basement has any history of flooding it's worth trimming them to stop the batt at the high water mark, since both the fiberglass and facer would wick water higher up in the wall if the bottom edge gets wet. With low & mid dentisy batts it's worth installing an air barrier at the bottom edge of a trimmed batt to limit performance losses to convection. (With R15s that wouldn't be necessary.) Cutting some 1" foam board to 3.5" widths and tacking them to the studs a bit above the high water mark, and air-sealing them with polyurethane caulk or foam board construction adhesive would keep them in place for the long haul (A 4" steel wallboard taping knife sharpened along the edges, using a 2x4 as both a width gauge and straight edge makes cutting 1" foam board to width dead-easy.)

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