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Community and Q&A

Basement wall assembly in climate zone 3A

Jamie Royal | Posted in Energy Efficiency and Durability on

Hi folks,

I’m finishing my homes basement and have been reading on GBA and FHB around the issues and approaches for resilient and efficient assemblies. Below are some facts about my situation:

Climate Zone 3A, new house built in 2016, daylight basement with 3 poured concrete walls and one 2×6 framed, wall height 9′-6″.

As i’ve mentioned, I’ve been reading pretty extensively and think the approach I’m taking is sound, but as Martin mentioned on a recent FHB podcast, I don’t want my imperfect understanding to lead to big problems down the road. Some of the articles I’ve read: “How to Insulate a Basement Wall,” “Calculating the Minimum Thickness of Rigid Foam Sheathing,” and “Combining Exterior Rigid Foam with Fluffy Insulation.”

I’ve attached a diagram of the wall assembly I’m planning to use , but in short I’ll be insulating the interior of the concrete wall with 1″ rigid foam with taped seams, 2×4 wall tight against the foam with R15 fluffy insulation, 1/2″ drywall. My primary objectives with the walls and floor, in order of importance, are: 1. Moisture and condensation resistance, and 2. Energy efficiency.

I think if I’m reading the “Calculation the Minimum Thickness…” article, in my climate zone 1″, or even 1/2″ of rigid foam is fine. Since I’m planning to put fiber insulation in the wall cavities, I also have to consider the ratio of the rigid:fluffy insulation. The “Combining” article doesn’t have a entry for climate zone 3 (hottest zone listed is marine 4), but if I use the first entry of 16% as a conservative minimum ratio, my R5(or 4.2)/15 comes out to be 25-21%, so that looks good also.

Anything I might be missing or misinterpreting? This is a great forum and thanks to everyone (Martin especially) for sharing their knowledge.


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  1. Expert Member
    Armando Cobo | | #1

    2015 IECC requires you to install 1" R5 rigid foam minimum against the concrete wall. Any additional insulation will help.
    What I'm not clear is the insulation on the rim joist. The rim joist is part of the exterior wall assembly, and I always specify 5.5" R20 OC sprayed foam minimum, and I would make sure it covers the sill plate to avoid thermal losses. In your drawing it appears to be a break in the continuity of the thermal boundary.
    I just notice in your wall detail that you don't appear to have a 1 1/8" rim board or 7/8" minimum sheathing on the end of your TJIs. A rim board is essential for wind and lateral racking loads when using a TJI flooring system.

  2. Jamie Royal | | #2

    Thanks Armando. To your last point, the rim board is drawn that way just for illustration purposes. The floor joists are installed to specification.

    For the rim joist bays, I would hope its considered part of the basement wall. Getting the R20 or 13+5 required of an exterior framed wall would be a little difficult. I went with 2" rigid foam there because that area doesn't have earth sheltering like the poured walls.


  3. GBA Editor
    Martin Holladay | | #3

    Armando is right: you have forgotten to include a horizontal piece of rigid foam above the mudsill. This rigid foam connects the vertical rigid foam which you installed on the interior side of your concrete wall with the vertical rigid foam installed on the interior side of your rim joist.

    For all intents and purposes, your rim joist area is like an above-grade wall, and it's a good idea to insulate the rim joist with an R-value that is similar to your above-grade walls. It wouldn't hurt to increase the R-value of your rim-joist insulation.

    For more information, see Insulating rim joists.

    -- Martin Holladay

  4. Jamie Royal | | #4

    Thanks for the input Martin. I've revised the wall assembly diagram and included the horizontal foam for the mudsill.

    For the rim joist, I don't relish cutting another 2" piece of rigid foam to fit in there...its pretty labor intensive and maybe a bit demoralizing to cut the shapes out to fit inside the I-joist. I can put fluffy insulation on top of the 2" rigid foam though. That would give me a combined R-value of ~R26, which is higher than the required wall assemblies in IECC 2015. Would this be a reasonable approach?

    And I assume by the lack of comment on other details the assembly looks good otherwise? I'm mainly concerned about condensation and keeping the innermost side of foam warm enough to prevent moisture accumulation.


  5. Steve Knapp CZ 3A Georgia | | #5


    You might price out using a contractor vs. DIY foam kit to spray the rim joist.

  6. GBA Editor
    Martin Holladay | | #6

    Yes, you can put fluffy insulation on the interior side of the rigid foam in the rim joist area.

    Your horizontal rigid foam is OK (see below) -- just not quite wide enough. Why not make it wide enough to extend over the vertical rigid foam below it (doing a better job of insulating the corner)?

    -- Martin Holladay


  7. Expert Member
    Armando Cobo | | #7

    "Fluffy" insulation needs to be sealed on all six sides and it is NOT air impermeable insulation, thus still not the correct application. Open cell foam at 5.5" is air impermeable, so you don't need to install a cut-out air barrier in front, thus needing a tremendous amount of labor and sealing.
    You still have a thermal break on your new detail!

  8. GBA Editor
    Martin Holladay | | #8

    I disagree with Armando. If your 2-inch-thick rigid foam rectangles are sealed on all sides, there is no need to "seal" the fluffy insulation (which is a good thing, because fluffy insulation can't be sealed).

    -- Martin Holladay

  9. Jamie Royal | | #9

    The 2" vertical rigid foam in the joist area was the intended air barrier, I certainly wasn't intending the fiberglass/mineral wool would serve that purpose. I've thought about a DIY spray foam, but have read some horror stories about their use, and am just unsure of my skill level there. This is primarily a DIY job, I'll be contracting some items like drywall out, but framing, rough plumbing, etc I plan to tackle myself. So paying a contractor to do the spray foam right isn't first on my list. The 2" rigid foam is sealed with one-part expanding foam around its perimeter.

    Martin - for the horizontal foam, it's really just a matter of access. For the walls that are perpendicular to the floor joists, I can extend the foam out over the fire block OSB with little issue. For the walls that are parallel though, I'm thinking blocking, or the floor joist itself would prevent me from putting a wider piece down...I'll have to do some hands on and see though.

    Thanks for the feedback everyone. I feel better about the general approach I'm taking. Now on to floors (and a different post), which I'm much less sure about.


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