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Community and Q&A

Insulating at the sill when joists are embedded in concrete?

Steve Smith | Posted in Green Building Techniques on


I have a 70-year-old house in Ontario. The floor joists are fully embedded in a poured concrete foundation (the top of the joist is flush with the top of the foundation).

I’ve read several comments on GBA suggesting that I avoid putting rigid insulation between the joists because it’ll make the joist ends colder and more prone to rot (if moisture is somehow getting in there). However I’ve read contrary statements in NRCan’s “Keeping the Heat In”, section 6.2.6. The diagrams there show that it’s okay to put in 1″ or less of rigid foam, and caulk the gaps.

Who’s right?

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  1. Steve Smith | | #1

    I should clarify my intent is to place 10"x12" rectangles of 1" rigid foam in between the joists on the interior of the foundation.

  2. Expert Member
    Dana Dorsett | | #2

    Can you point to the page where they are suggesting it isn't a problem?

    Figure 6-18 B & C on p.90 shows rigid foam insulation on the exterior of embedded joists, which would be a safer way to go, far safer than Figure 6-19 B & C on p.91

    Figure 6-19 also shows exterior rigid foam below the joists, with no interior insulation below the joists, so there is a severe thermal bridge through the concrete keeping the joists warm(er) , despite the interior insulation between the joists (which is doing next to nothing for thermally breaking that thermal bridge.)

    You can't cheat the physics- keeping the heat in means that the outer part is colder, and colder wood will have a higher moisture content. If the foundation is dry and well drained it will probably be OK, but it's always safer to insulate it from the exterior.

  3. GBA Editor
    Martin Holladay | | #3

    If your floor joists are embedded in the concrete foundation wall, then the best way to insulate the area is with exterior insulation (either rigid foam or mineral wool). The insulation needs to be on the exterior side of your foundation wall, and should extend from the top of the foundation wall down to several feet below grade. The insulation that is above grade needs to be protected with metal flashing or stucco to prevent physical abuse.

    If you insulate this wall on the interior, it is, indeed, true that you raise the chance that the ends of the joists will rot. Interior insulation will make the beam pockets colder and damper.

  4. Steve Smith | | #4

    Dana: Figure 6-19 C) on page 91 is the detail I was looking at, as this is what my house looks like (except I'm pretty sure I don't that rim joist embedded on the outer edge of the foundation wall). I was also looking at the text on page 92: "If it is not possible to extend the exterior foundation insulation to cover the full header joist area, the space will have to be insulated and sealed from the inside. " and also "For fully-embedded joists, do not exceed a maximum of 25 mm (1 in.) of foam board insulation as the concrete below the floor may make the floor above uncomfortably cold and prone to damage."

    You mention that there is a thermal bridge through the concrete that keeps the joists warm(er), and that interior insulation won't really affect that bridge. Isn't this kind of a good thing? The joists stay warm because of thermal bridge, the foam/caulking cuts down on the amount of cold coming in via air from outside and reduces my heating bill? Will putting up 30 of these little foam squares make the concrete wall that much colder (chilling the joist ends), even given the fact that the joists themselves act as thermal bridges and heat will transfer from the warm part of the joist inside the house to the embedded ends?

    The top of the foundation wall is about 3' above grade, if that helps.

    Thank you for your reply!

    Steve - thanks for your reply! Exterior insulation that extends below grade isn't an option right now, unfortunately. I should mention when I bought the place last fall, there's already maybe an inch of rigid insulation on the interior of the basement wall, terminating at the underside of the joist. I think this may have been done about 10 years ago. Hopefully the joist ends aren't rotting away in the beam pockets!!

  5. Expert Member
    Dana Dorsett | | #5

    Yes, the thermal bridge keeping the joist-end warm is a good thing, for the joist, but not for the heating bill.

    But since you already have rigid foam board on the rest of the interior of the wall you don't have that thermal bridge to keep the joist ends warm, which may have been why they stopped there rather than insulating between the joists. With the concrete between the joists exposed to the conditioned space air, that's the thermal bridge keeping the joist ends warm. In 6-19C it's the exposed interior concrete below the joists that's the thermal bridge, a thermal bridge you don't have.

    If it's possible to insulate on the exterior (at least for the above grade portion) of the foundation that would be better. In most retrofits you have to slip in some Z-flashing to direct bulk water running on the upper wall's drain-plane to the exterior side of the foundation insulation.

    To figure out the safest way forward we'd need to know:

    What type of rigid foam is on the interior?

    Any vapor barriers?

    How deep are the roof overhangs?

  6. Steve Smith | | #6

    Hi Dana! Thanks again for the reply.

    To answer your questions: The rigid foam is covered up with some fake wall panelling (glued with a ridiculous amount of PL, so I can't really pry any of it back), and I can only see bits and pieces. I think it's 1.5" EPS. It's a greyish colored board, and the only thing I can see on it says "QPL 55, 158-L" and somewhere else it clearly says "mousse de polystyrene".

    +No vapor barriers that I'm aware of in the basement. Surprisingly I think there's a vapour barrier on the the main/2nd floor of the house, or at least I'm occasionally coming across some kind of plastic sheeting when I drill holes in the walls.

    +Dismal roof overhangs! All the post-war homes in this neighborhood have like 2" overhangs on the gable ends, and my home in particular has no overhang on the gable side whatsoever because the exterior of the house (not including foundation) was also covered in rigid foam at some point. The exterior assembly is currently gpyrock / vapor barrier / 2x4 framing (I think - I doubt it's 2x6) / plank sheathing/ some kind of weird stucco from the 40s / 1.5" rigid foam / vinyl siding. I have absolutely no idea where the drainage plane would be in this assembly. I also can't tell if if anyone put an air barrier in there when they added the foam, but there might be tyvek all over the house for all I know. The other 2 sides of the house have maybe 8" overhangs.

    I could possibly insulate on the exterior above grade. The vinyl siding sticks out about 2-3" past the exterior foundation wall. I don't think there's any flashing anywhere (I can't confirm right now because there's too much snow everywhere). I could probably put in some Z-flashing at the bottom (as close to the foundation wall as possible). If you have any examples of details please post them. I'll look into how to attach the foam to the wall and how to parge over it and all that. But I would be happy to receive any advice you might have!!

    Also, in light of your recent comments, would you recommend I take down the interior rigid insulation in the basement?

  7. GBA Editor
    Martin Holladay | | #7

    For more information on installing rigid foam on the exterior of your foundation, see this article: How to Insulate a Basement Wall.

  8. Expert Member
    Dana Dorsett | | #8

    The safest/best thing do do would be to leave the interior foundation EPS insulation in place, and insulate the above-grade exterior down to at least 2' below grade (wherever that's easy to do) with 2-3" of EPS, while leaving the space between the joists open.

    The original drain plane of the stucco wall is probably behind the stucco, but with 1.5" of foam in front of the stucco you can probably treat the stucco layer itself as if it were a drain plane (which it mostly is) in terms of how to lap the Z-flashing. Does the window flashing extend to the exterior of the stucco? If yes you can insert the Z-flashing between the stucco layer and the wall sheathing it should be "good enough.

    With minimal roof overhangs the direct wetting of the above grade foundation is a given. By insulating over it with EPS you're probably reducing the average moisture content of the concrete, as long as the flashing is directing bulk water to the exterior of the foundation EPS.

  9. Steve Smith | | #9

    Quick update - all the houses in this neighborhood have embedded joists, and I found a neighbor who sprayfoamed the spaces in between his joists. He did this approximately 10 years ago and has to take some of the foam off this summer. I'm hoping to get in there with a moisture meter to compare the ends of his joists to mine to see if his are significantly wetter as a result of being colder. Will report back if I can get the data. Thanks!

    1. WinnipegSteph | | #14

      Hi Steve, I was wondering what you found out from your neighbors home? We are looking at installing spray foam in our basement here in Winnipeg this fall, but are unsure how to tackle the joist space since the joists are embedded in the concrete. Some contractors have recommended spraying just 1 inch, whereas others are recommending we fill the entire void all the way up to the flooring. Any advice would be appreciated.

      1. jstanko | | #15

        Hi WinnipegSteph,

        Did you find out an answer? I find myself in the same situation. Good thing I did some research. I didn't realize this was an issue. I have a home build in the 60s in Selkirk that has the joist encased in concrete. I'm going to use 1' rigid board, 2 x 4 framing, roxul and vapour barrier to do the basement walls. I didn't know what to do with the rim joists and had never seen that before. I was just going to have them spray foamed. Any advice would be appreciated as well!

  10. Dan Kolbert | | #10

    I'll be very interested in hearing what you find. This issue is like others (cold sheathing, for instance) where the model shows problems but there are not enough case studies to know if real-world conditions bear the modeling out.

    The other, also complicated, option is to support the joists at the interior of the foundation wall, with a ledger or similar.

  11. Expert Member
    Malcolm Taylor | | #11

    It's another of those things that is probably very climate specific. I've never dealt with embedded joists, but the end of beams in beam pockets here in the PNW turn to mush fairly quickly.

  12. user-6759891 | | #12

    I'm a little late to the party but I too have joists in the concrete so trying to work out the best solution. hile I thought it was odd, I never once thought there could be an issue until I came across a few old threads on the subject today.

    My home is a 1974 year old bungalow in Edmonton, Alberta (climate zone 7a) so we have much different conditions that does the OP, Steve, in Ontario. I'm hoping the colder and drier conditions here don't make it as much of an issue. I've read our conditions here don't give us near the moisture issues in buildings than occurs in most other cities.

    I plan on putting 2" of type 2 EPS on interior of foundation with ~R10 + 2x4 wood studs with 4" Roxul with ~R17.2. I'm not averse to putting an external layer of EPS on the outside of the foundation, as some have recommended but only at beginning of researching this topic.

    Steve, if you're still on GBA, did you ever do that MC test? Anyone hear of or learn anything new in the past year?


  13. GBA Editor
    Martin Holladay | | #13

    I notice that you are raising these questions on two different Q&A threads. I have answered your questions on the other thread; here is the link: GBA blog dated 2015: "How to Insulate a Foundation".

    -- Martin Holladay

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