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Community and Q&A

Best way to air seal and insulate floor and wall assemblies

Spbreslin | Posted in GBA Pro Help on

I’m remodeling a house in climate zone 4 outside Seattle. The existing exterior walls are 2×4 with 1×8 board sheathing. I want to achieve the required R21 insulation value without having to fur our the wall to achieve a 51/2″ wall cavity while also sealing the 1×8 sheathing. The subfloors are 2×6 T&G and I want to seal the floor assembly and achieve the minimum R30 insulation value. What would be the best strategy to achieve these goals?

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  1. Reid Baldwin | | #1

    You are not going to get much improvement in whole wall R value unless you are willing to make the wall thicker. Although you could increase the center of cavity R value with closed cell spray foam, that would be a waste. Are you willing to add thickness toward the outside? What are your reasons for not wanting to fur out the wall?

  2. GBA Editor
    Martin Holladay | | #2

    To air-seal the walls of an old house with board sheathing, there are limited options. The two best approaches are probably to insulate between the studs with open-cell spray foam, or to use the Airtight Drywall Approach (ADA). For more on ADA, see Airtight Drywall.

    To advise you on your floor assembly, we need more information. What is under the floor? Does your house have a basement or crawl space? Or is this a house on piers?

  3. Spbreslin | | #3

    Answer 1: I am trying to find the most cost effective way to achieve both air sealing and insulation R-value. Furring out the walls has two drawbacks from my perspective - it reduces the floor area of the rooms affected but more to the point it is an extra expense in labor and materials that I would prefer to avoid if possible. Can you tell me why you believe that using closed cell foam would be a waste?

    Answer 2: I will look into the ADA approach. To answer your question about the under-floor conditions about 2/3 of the house is above an unconditioned basement and the rest over a crawl space. The basement and crawl space are open to each other.

  4. GBA Editor
    Martin Holladay | | #4

    Closed-cell spray foam will work. It has two major disadvantages compared to open-cell spray foam, however: (1) It is much more expensive, and (2) It is shunned by green builders, since almost all brands of closed-cell spray foam are made with a blowing agent that has a high global warming potential. (The exception is Lapolla 4G, a brand of closed-cell spray foam made with a benign blowing agent.)

    If you have a house with a basement and crawl space, most experts advise that you shouldn't insulate the floor assembly. Instead, you should insulate your basement walls and crawl space walls, so that you bring your basement and crawl space into your conditioned space. This approach has many advantages. Here are links to two articles to explain what you need to do:

    How to Insulate a Basement Wall

    Building an Unvented Crawl Space

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