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Community and Q&A

Insulating Rim Joist Beyond Foundation?

BobTheWeekendWarrior | Posted in Green Building Techniques on

Completely below grade basement in Zone 5.
Poured concrete foundation
Water barrier/proofing sprayed on foundation.
R-5 rigid foam applied to foundation (on top of water barrier)
Rim joist insulated with roughly r-7 rigid foam foil faced on both sides. Over the insulation (interior side) is a 1/2″ piece of OSB. The floor joists butt into this OSB. The joints of the OSB were caulked.

I am finishing my basement and have been planning to have 1″-2″ of spray foam applied to the rim joist for more r-value and most importantly air sealing. However doing so will increase the r-value of the rim joist to more than double that of the foundation. Should I be concerned about having a rim joist with a much higher r-value?

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should I be concerned about having a rim joist with a much higher R-value than the basement wall?"

    A. No. But you might consider improving the R-value of your basement wall when you get a chance, since the basement wall has only 1/3 of the insulation required by most building codes. (The 2012 International codes call for R-15 minimum for basement walls in Zone 5.)

  2. Expert Member
    Dana Dorsett | | #2

    Adding a 2x4 fiber insulated wall tight to the R5 wall foam would bring performance up to IRC code-minimum with low wintertime moisture accumulation risk at the above-grade portion. That can be risky if the basement has a history of flooding though.

    Adding another 2" of continuous rigid polyiso foam to the foundation walls also works. Strapping it to the wall with 1x furring through-screwed to the foundation with masonry screws gives you something to hang the wallboard on. If using 2" reclaimed roofing polyiso it's even cheaper than a 2x4 fiber-insulated wall.

  3. BobTheWeekendWarrior | | #3

    Home was build by a national builder in 2014. Ryland, who is now CalAtlantic, who is now merging with another national builder.

    I considered adding 2" of continues rigid foam on the interior, but was concerned about moisture between the foundation and the foam growing mold. What I read online and here was that the "solution" was to ensure the foam is sealed to the foundation to prevent mold from coming out. I do not see why I would build a solution that will lead to mold and the saving grace being, "well just seal the mold away". Energy saving seem secondary if I need to rip out walls to deal with mold in 5-10 years.

    Another contributing factor to my decision is that my basement stays pretty warm already with no open heating vents. This design aligns with what the builder did and neighbors who have their basement finished like this also agree their basements stay plenty warm without intervention (no I have not confirmed their energy bills).

    All that being said, the walls are not framed yet so I could still change it. City building department did sign off on no additional insulation being added.

    Bad decision to forgo additional insulation? I feel like I would be spending money for a pat on my back that may not actually result in savings.

  4. user-2310254 | | #4


    Here is a key takeaway from one of Martin's articles.

    "Performing this work will lower your energy bills, and will also provide an important side benefit: insulated walls are less susceptible to condensation and mold."

    If you haven't already, consider reading the article for more detail on this topic:
    Follow us: @gbadvisor on Twitter | GreenBuildingAdvisor on Facebook

  5. GBA Editor
    Martin Holladay | | #5

    Building code minimum R-values are generally set at cost-effective levels. Since your basement wall doesn't meet code, it make sense to add more R-value, especially if you are planning to finish the basement. You will make back the cost of the insulation in lower energy bills in a reasonable amount of time -- and your basement will be more comfortable.

    Of course it's possible to keep your basement warm with below-code levels of insulation. All you need is a furnace or a boiler. The point isn't whether you can keep it warm -- the point is whether you want to keep your energy bills low.

    I've heard about people worrying whether there is any mold growing behind continuous insulation in their basements. The brief answer is, "Probably not -- but so what if there is?" If you seal the seams in the rigid foam, there isn't much air behind there, and no air exchange with your home. For what it's worth, the soil adjacent to your foundation is full of mold. But who cares? It's outdoors. And for all intents and purposes, the mold (if there is any) behind the continuous insulation on your basement wall is outdoors, too.

    There are other things to worry about.

  6. Expert Member
    Dana Dorsett | | #6

    My below-grade basement stayed pretty warm before insulating the (bare concrete) foundation too, primarily from the losses from the uninsulated ducts (installed in 1923) to the basement.

    After installing 3" of fiber-faced polyiso on the foundation walls it's a few degrees warmer down there, DESPITE the fact that the ducts are no longer being used for space heating, and the house is using 15-25% less heating fuel per heating degree day than previously.

    With R5 on the walls your fuel consumption improvement won't be as dramatic as mine was, but it will still be pretty easy to measure.

    A few years after installing the insulation I had to remove some of it on the cool north side of the house to accomodate changes to the plumbing drains. There was no hint of mold forming there at that time, not that it would matter much. On the other side of the foundation concrete there is all sorts of stuff you wouldn't want inside your house, but there's no path for it to get in. The same is true with exterior side of rigid foam on your wall- any mold that forms there is isolated from the living space.

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