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Advice for insulating a new build unfinished basement?

Regin8r | Posted in Building Code Questions on

We are planning on tearing down and rebuilding our home which will be a 4500 sq ft bungalow with an attached quad garage. Finishing the basement is not possible immediately, so what is the best choice to economically insulate the basement walls without having to drywall up? Would thick rigid foam be the only option to get R20? Is it allowed to just put up rigid foam with nothing else on the foundation wall?

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  1. GBA Editor
    Martin Holladay | | #1

    Presumably, you have already read this article: How to Insulate a Basement Wall.

    Check with your local code inspector (because code interpretations differ from jurisdiction to jurisdiction), but in many locations you can install Thermax, a brand of polyiso that has passed fire-safety tests, on the interior side of your basement wall and leave it exposed.

  2. Regin8r | | #2

    Yes, I've read it and I will check with the city if that is allowed. The 2 inch is R13 only though, to get close to an R20, would you layer a 2 inch and 1 inch on top?

  3. GBA Editor
    Martin Holladay | | #3

    Yes, it's fine to install two layers of rigid foam. Stagger the seams and tape the seams of the outermost layer of foam.

    If you end up installing two layers of rigid foam, you can probably use a less expensive brand of polyiso for the first layer, and then use Thermax for the second layer. However, make sure that you run this plan past your building inspector before you go shopping for foam.

  4. Expert Member
    Dana Dorsett | | #4

    If the basement is dry, with no history of flooding, you can get R20 or better performance with 2" of fire-rated Thermax trapped to the wall with a 2x4 studwall insulated with unfaced rock wool batts, with no risk of moisture accumulation in most US climate zones (are you in central Alaska?) If the slab is not insulated it's worth putting an inch of EPS between the bottom plates of the studs and the slab (painted with intumescent paint to mitigate the fire hazard, if the inspectors aren't happy with exposed EPS.

  5. Regin8r | | #5

    Located in Pickering, ON, Canada. I left a message with a city inspector but was told the person who would likely review the plans is on training and would probably return my call tomorrow. We are planning to put an insulated slab down.

  6. Expert Member
    Dana Dorsett | | #6

    In the Toronto area even 1" of polyiso would be (barely) adequate dew point control on R15 rock wool in a studwall, but if you're not putting up the gyprock 2" would be better, and would hit your performance goal of R20 continous insulation type performance.

  7. Regin8r | | #7

    Planning on 2 layers, 2" polyisocyanurate and 1" Thermax with staggered seams and taped. Just have to verify it's OK with the inspector. So that R13 and R6.5 should give R19.5 theoretically.

  8. Dana1 | | #8

    Sounds like a good plan then!

    Be sure to keep the cut edges of the polyiso off the slab, since ground moisture or minor leakage can wick up into the polyiso/Thermax. Even 5mm would be a sufficient capillary break to prevent that from happening. In basements with a flood history it's better to insulate up to the high tide mark with EPS rather than polyiso, since EPS is not similarly hygroscopic.

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