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Community and Q&A

Basement insulation

softball59 | Posted in GBA Pro Help on

I had an opportunity to purchase a couple of pallets of rigid foam insulation; tapered from .5 to 1″ and with a fiberglass embedded facing on both sides. I believe it is polyiso.?
I realize this is for roofing applications but am wondering if I could reverse the tapers to each other to achieve a consistent 1.5″, or more, and use it below grade to insulate the interior of a cement block basement.
Thank you!

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  1. Expert Member
    Dana Dorsett | | #1

    Fiberglass faced iso works fine for basement walls as long as they're reasonably dry. (If you have lots of efflorescence showing you may want to deal with the drainage & moisture issues first.) Just be sure to keep the edges off the slab, since polyiso can wick up groundwater moisture through slabs that don't have sufficient ground vapor retarders & drainage.

    At 1.5" you're only getting about R8-8.5 out of 2lb roofing iso, which is fine if you live in US climate zone 2, but if you're in zone 4 or higher it's worth going for R15-R20 or more, at least for the above-grade portion.

  2. softball59 | | #2

    Ok. Drain tile/sump pump were just installed around the interior perimeter of the basement.
    Initial thought was to install 1.5" against concrete wall, air space, and then frame with 2X4's.
    Perhaps I would be better off framing against the wall and filling between studs with the polyiso. If that's the case, would a vapor barrier between the concrete and framing be advisable? And, is a rigid foam adhesive the correct choice to use with this material/facing or is a construction adhesive better?
    Thank you for the quick response!!

    p.s. project is in Minnesota; zone 4, I believe.

  3. GBA Editor
    Martin Holladay | | #3

    You definitely want a continuous layer (as thick as possible) of rigid foam against the interior of your basement wall. You want to keep your studs as far as possible from the foundation wall, and you want to insert all of your insulation between the foundation wall and the back of the studs.

    It wouldn't make any sense to cut your rigid foam into strips and install the strips between the studs, because you would end up with thermal bridging through the studs. And you definitely don't want your studs to touch your basement wall, or your studs will get wet.

    You don't need to install any polyethylene against the basement wall; the rigid foam will act as a moisture barrier.

    Rigid foam can be attached to the wall with mechanical fasteners (TapCons with washers or proprietary plastic fasteners) or with foam-compatible adhesive.

    For more information, see How to Insulate a Basement Wall.

  4. Expert Member
    Dana Dorsett | | #4

    Minnesota is zones 6 & 7, not zone 4. Ideally you'd want the whole-wall R (after thermal bridging) to be at least R20, but under MN code anything over R10 is technically a violation (strange but true), and must stop at the frost line or the footing, whichever is less, over some theory relating to frost-heaves buckling the foundation. Don't sweat the MN code too much on this- it's out to lunch. As long as you have reasonable surface drainage and a reasonably dry footings you won't frost-heave the foundation any time before the next ice age. Technically ALL insulated concrete form foundations are violations of that code: (R10 is required down to the frost line, but R11 is prohibited, got that?)

    Frost heaves propogate in the direction of the heat loss- the way to frost heave the basement walls or slab is to let the basement drop to below the ground temp when the ground is already freezing. As long as the house is heated, the direction of force on the frost heave is in the other direction (and small, if insulated.) Frost heaving the foundation is indeed an issue with unheated barns and outbuildings, where during cold snaps the direction of heat flow is from the 30F ground into the 10F barn. There's no real science behind the MN prohibitions against R10+ walls, or the requirement that the R10 stop at the design frost line. You can get a variance from the local inspector, or just ignore it- your house won't fall down. Run this by the code enforcers if you get any push-back:

    With 1.5" of polyiso trapped to the wall with a batt-insulated 2x4 wall 24" o.c. with unfaced R13s or R15 rock wool would get you to about R20, and would be sufficient foam- R to not need any sort of interior side vapor retarder beyond latex paint.

    As Martin correctly points out do NOT put the framing against the wall- that puts the cold stud edges at maximum risk from both ground moisture AND wintertime adsorption from interior air. With the R8-ish foam between the studs and CMU it'll keep them sufficiently warm to avoid wintertime issues, and put the vapor retardency of the facers between the studs and ground moisture, keeping the studs at the nearly the same moisture levels of the basement air averages.

    Put an inch of EPS (not iso) between the bottom plate of the studwall and the slab, anchored in place with TapCons or similar. The cool edge of the bottom plate can otherwise wick and store quite a bit of moisture out of the slab, increasing the mold risk.

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