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Insulating walk-out basement

Geoff_Frood | Posted in Green Building Techniques on

I’m insulating a walk-out basement in a new construction. The house is of a double wall design, and will be filled with dense pack cellulose. I understand that for the foundation walls that I can not apply cellulose directly, however I had thought that if I apply a layer of closed-cell either sprayed or laid in in sheets then I should be able to fill the rest of the cavity with cellulose, assuming that there is no concrete exposed. My question is how many inches do I need to use to assure I have no problems with the cellulose?

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Replies

  1. charlie_sullivan | | #1

    Yes, that can work, if the foam is thick enough. The guidelines for exterior foam thickness ( https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing ) can be a reasonable guideline, even though the scenario is different as you get below grade. What climate are you in?

    Personally, I prefer to avoid cellulose below grade, just as extra insurance, in case, for example, a future resident allows high humidity in the summer in the basement, or in case of a flood.

    Green builders avoid closed cell spray foam and XPS because of the high global warming impact of the blowing agents used in them, which leaves you with EPS and polyiso as the options for below-grade walls. You also have the option of building the foundation wall with ICF which puts EPS on the inside and outside as part of the process of making the wall.

  2. Geoff_Frood | | #2

    I'm in quebec, so zone 6 bordering on 7. I'm not concerned about flooding as its a walkout above grade. I was looking to use this method for simplicitys sake as I'm already blowing cellulose elsewhere in the house. Am I just over thinking this?

  3. GBA Editor
    Martin Holladay | | #3

    Geoff,
    You are not overthinking it. Your walk-out basement is at least partially below grade, however, so Charlie's conservative approach -- one that assumes that a future flooding event or moisture seeping through the foundation might cause problems -- is worth heeding.

    My advice has always been to use only foam insulation (spray foam or rigid foam) on the interior side of below-grade concrete walls. Others, however, have used cellulose as you propose. The decision is yours.

    If you go ahead with your plan to install cellulose, the thickness of the rigid foam layer depends not only on your climate zone, but also on the R-value of the cellulose you plan to install. If you have a double-stud wall with 10 or 12 inches of cellulose, you need a correspondingly thick layer of rigid foam to stay out of trouble. To be conservative, I'd advise that at least 36% of your wall's R-value should come from rigid foam or closed-cell spray foam, with the remaining 64% coming from cellulose.

    Here are links to two articles to guide you in your work:

    How to Insulate a Basement Wall

    Combining Exterior Rigid Foam With Fluffy Insulation

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