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Community and Q&A

Rim Joist Area

fall50 | Posted in Green Building Techniques on

Zone 6 Mpls

Fairly versed on what needs to be done to insulate this area. In my last 1940’s house the rim joist area was a cavity in which spray foam or foam board could be placed inside as it had 4-5 inches of depth. Having bought a new house around same age, I begun the beginning stages or remodeling the basement and this was on the list.

The ceiling was sheetrocked however we needed to remove some of it which also exposed the rim joist area. What I uncovered was 1×10 wood blocking between the joists. This blocking is flush with the face of the cinder block. foundation wall (in other words their is no depth to the rim joist cavity), 1st picture

Being a student of these forums I decided to see if I could remove this wood blocking and expose the rim joist cavity in preparation to spray it with foam or XPS squares foaming them in place. Instead after I popped the 1×10 off, the cavity instead of a hollow cavity it was filled with concrete. The depth of the cavity is literally .75” relative to the cinder block as the wood blocking is flush with cinder block foundation. 2nd picture (note as you can I didnt fully remove the wood blocking)

Can I just leave the wood blocking in place for the other cavities and attach my XPS foam board directly to it and spray foam the seams? Or do I should be removing the 1×10 wood blocking and attach the foam board directly to the face of the brick that sits between the joists . This blocking is toe nailed between the joists and just assume leave it place attach my foam to the face of the 1×10 blocking as it will be fair amount of work to remove this in all the cavities, unless their is a real value add to removing it as to attach the XPS to the brick instead.

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  1. NormanWB | | #1

    Are you planning to insulate the basement wall? If so, then just extend whatever foam board you are putting on the wall up into the cavity and seal around the joists. If not, I would just put foam board over the 1x10 and seal with spray foam. In that case, though, you will need to insulate the ceiling.

  2. user-2310254 | | #2

    This issue comes up a lot on GBA. Here is how Martin responded to a similar scenario in 2011.

    The problem raised by several readers -- that any interior insulation makes the ends of the joists colder, and therefore wetter, and therefore more likely to rot -- is a real one. Moreover, concrete is a conductor, so it acts as a thermal bridge. You can't just insulate the rim joist area if you want the insulation to be effective; you need to bring the insulation down to the footings.

    Really, in this situation the best solution is to insulate the foundation and rim joist area from the exterior.

    The problem of embedded joists and beams was discussed in Insulating Old Brick Buildings:

    “If the building is insulated on the interior, the ends of these embedded beams get colder — and therefore wetter. Moreover, less energy is available to help them dry out.

    “ ‘Embedded wood timbers can rot,’ says [John] Straube. ‘... There are a number of techniques to address embedded beams. You can inject the wood with borate salts to preserve the wood. You can insert metal wedges to conduct heat to the end of the beam. You can install hot water pipes to heat the end of the beam. Finally, there’s the practical Yankee solution: you build a load-bearing wood wall to support the beam, and then you fire up your chainsaw and cut off the end of the beam.’

    “Almost all of the solutions to the embedded beam problem have drawbacks except the chainsaw solution.”

    Read more:
    Follow us: @gbadvisor on Twitter | GreenBuildingAdvisor on Facebook

  3. fall50 | | #3

    Thank you both for answering. To answer a few questions, yes I was planning on extending the insulation down to the floor using 2" of XPS as detailed here

    I did read the thread Steve linked to which is interesting. Unfortunately none of those solutions are plausible. What if I simply used 1" of XPS vs say 2". One inch would allow more drying? Keep in mind this a retrofit situation and I would imagine their is a fair amount of houses in Mpls and Saint Paul that have this detail. (where concrete has been poured between the joists). In my last house it had the same detail, however in that case instead of a nicely formed square of concrete they just piled in the concrete so it was very irregular in terms of form , which capped off the hollow cores of the cinder block, and is why I used 2 part foam.

    I am no expert which is why I cam here, but if the joists that were in contact with this concrete were prone to rot, I see no evidence of this. Placing a minimal amount of 1" XPS in this cavity (over the wood blocking) seems like it would still allow for some drying while at the same time providing insulation and air barrier benefits.

    Hopefully Dana will come along as he has had some great perspective for a few other projects I have done.

  4. Expert Member
    Dana Dorsett | | #4

    What's on the other side of the concrete fill? A brick veneer? Plywood/plank sheathing & siding?

    Are the roof overhangs deep enough to avoid major splashback or direct wetting of the foundation just below the other side of those joist ends?

    How much above-grade exposure do have on the exterior of the CMU wall?

    Are the CMU cores fully grouted, or are they hollow?

    CMU isn't likely to wick as much ground moisture as poured concrete, and if the cores are hollow rather than grouted it'll be even less so. But it'll still wick some. The more above grade exposure to dry toward the exterior the better.

    Also Minneapolis is still a lot warmer than Manitoba, and the average temp of your joist ends will be warmer than in that other thread. I suspect the bigger threat for you is from ground moisture.

    If it's a reasonably dry foundation with a foot or more of above grade exposure you can probably just go ahead and insulate to R15 (IRC code minimum for foundations) and call it a day. In this instance air tightness is a bit more critical, and it may be better to do the rim-joist area with 2" of HFO blown 2lb foam (~R14), even if you're using rigid foam for insulating the rest of the foundation. The foundation wall insulation should be installed first so that the joist-blocking area foam will seal over the top of the wall foam.

    XPS is about the least green foam material out there (maybe 3lb polyurethane comes close), primarily due to the high global warming potential of the HFC blowing agents used. Using EPS or polyiso is much more benign, as is HFO or water blown closed cell polyurethane.

    At 2" even the labeled R of XPS isn't up to code minimum, and as it loses it's climate-damaging blowing agents it's performance drops off to R8.4, not R10, and well below a code-min R15. Using 2.5" of DOW Thermax (which apparently doesn't need to be derated for temperature) would do it, or 3" of reclaimed roofing polyiso gets you there. With polyiso stop a half inch above the slab to avoid the risk of wicking ground moisture from the concrete.

    If you're the type to be nervous, after installing the wall foam install a 2x8 ledger board through-screwed to the CMU with a couple of masonry screws directly below each joist. That way even if the ends of some joists get compromised they'll still be supported by the ledger. (If you're completely nervous, have an engineer specify the ledger, screws & spacing.)

    FWIW: A guy in my office (climate zone 5, Boston 'burbs) has a 1920s vintage bungalow with a similar CMU foundation with that sort of filled end embedded joist. His above grade exposure is about 30-36" everywhere (yes it's cold down there in winter!), and ~2' roof overhangs. He's planning to hit it with 2" of HFO-blown foam before the end of this month, including over the concrete infill right up to the subfloor. It's warmer at his house than yours and given the other factors his risk is really pretty low.

  5. fall50 | | #5

    Dana thanks as always for the detailed response. The CMU cores are hollow and the grade around the house is sufficient. The siding is brick on this side of the basement where as on the east/west sides have apprx 2.' feet of exposure. I am not really concerned about the joists being wet from ground moisture. In terms of concerns regarding the interior insulation keeping the joist colder/wetter, could I simply just go with a bit less insulation say R-10 in the rim joist area?. Yes I lose some R-Value but that's better than rotting out joists! Not that I am that concerned with that happening, but I am trying to find some middle ground so why not just put 2'' foam R-10..

    My other two questions are the following
    1) I am going to use foam board in the rim joist area as opposed to the 2 component foam kits. Do I need to remove the 1x blocking they placed over the face of the concrete cores (1st picture) so that I can attach the foam board directly to the concrete OR can I simply attach the foam board right over the wood blocking they placed between the joists? (2nd picture) I wasn't clear if this mattered or not? It will be a pain to remove that wood between the joists, but will do whats needed if the foam needs to be attached directly to the concrete thats embedded between the joists.

    2) Finding Thermax seems to be difficult these days. (I came across similar threads saying same thing as they dont sell it in the box stores) So I am looking at 2" of EPS and then unfaced FG, this will get me around R-18. However I have seen a few posts by Martin indicating he isn't a fan of any FG in basements even if its behind foam board. Considering we have interior drain tile, gutters and the proper slope, flooding risk is low. Any concerns with the 2" of EPS and unfaced FG. Any risks this detail brings if flooding isn't a concern vs going with 2 layers of foam? (problem with 2 layers of foam I loose valuable space)

  6. Expert Member
    Dana Dorsett | | #6

    If there is a history of flooding, stop the fiber above the high tide mark.

    I'm not sure if there is anything to be gained by removing the 1x blocking. But a 2- part 1.5-2lb foam such as FrothPak is going to have better adhesion and fewer shrinkage issues than can-foam.

  7. fall50 | | #7


    I have spent a fair amount of time on the forums over the last few days. Your concerns around XPS due to high GWP are well documented and resonable. I did want to ask about the following comment I came across in which you stated "XPS outgasses it's blowing agents (a mix of HFCs, predominantly HFC134a) over time, and loses performance in the process"

    Is the "outgassing" of the blowing agent a concern if used in below grade interior settings like basement insulation, where it would be dissipating into the building envelope (as opposed to say when used on exterior siding of a building )? I presume not, as this would be mentioned on the boards , but nonetheless wanted to inquire.

    Lastly I am bit concerned about using something as vapor permeable as EPS for insulating the interior of my basement, the XPS seems to be a compromise between the EPS (high perm rating and something like Johns Manville AP poly iso which has a low perm rating of .05 at 1" Assuming all things equal from and their were no GWP concerns between the three, it seems XPS gives you the middle ground for permeability which allows for some drying potential and would the preferred choice of the three Perhaps my concern is misplaced?

  8. Expert Member
    Dana Dorsett | | #8

    The term "outgassing" perhaps evokes a more acute & active process than what's happening. A better term would have been "diffusing". Yes the HFC blowing agents are going to be entering the indoor air environment, the rate is quite low, essentially indetectible from an indoor air quality point of view. The volume of those blowing agents an a layer of foam is small compared to the air volume of a house, and it take several years, maybe even a decade or more for even half of it to get out via diffusion.

    The vapor permeance of Type-II EPS at 2.5-3" is on the edge of Class-II vapor retardency, whereas XPS approaches that threshold at 1". Even at 1" Type-II EPS is as least as vapor tight as a couple coats of interior latex paint over latex primer. So as a rule of thumb, if installing LESS than R4 against the interior of a foundation wall it's perhaps better to use EPS with foil or plastic facers, 3/4" XPS, or half inch foil faced polyiso. When it gets to be as thin as 1/2" XPS is about as vapor open as a couple coats of interior latex over latex primer, about the same as 1" Type-II EPS.

    If installing R4 or more, EPS will usually be fine, but it also has to be sufficient for wintertime dew point control on any interior-side fiber insulation at the above grade section of the foundation wall.

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