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Community and Q&A

Small rim joist gap (Updated)

timmyzaid | Posted in Energy Efficiency and Durability on

Update: I drilled 2.5″ holes in the joist every 1.5-2′ this weekend and then spray foamed in the cavity. The holes didn’t help me foam at all cause they didn’t allow for coverage everywhere, however they did help me see into the cavity better so I can’t say they were completely useless. The gap ended up being just enough so that I could angle the gun with the wide spray nozzle such that it pretty evenly coated from top to bottom. It’s not the prettiest foam job ever, but I think it’ll work.

Original Post: I’m in the process of insulating my crawl space, including sealing and insulating the rim joists with rigid foam. However, on each end of the space there is only about a 1.5″ gap to work between (see attached picture). Does anyone have any recommendations for dealing with it? I’ve thought about trying to spray foam in there but that seems like it would also be quite difficult.

Thanks, Zaid

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  1. MattJF | | #1

    I have a similar setup in my basement. I can't actually get my hand in there.

    There is an injectable DIY kit, but I couldn't justify the cost/energy savings for my situation.

    I also thought about cutting away just enough of the joist to get something in there and the scabbing a new joist to it.

    I have considered foaming whatever I can, dense packing the cavity with extra fiberglass I have and air sealing at the access able joist in the basement. This leaves an incomplete seal to the first floor, but is quick and cheap.

    I haven't actually addressed mine, so staying tuned to see if anyone has any brilliant ideas.

  2. Expert Member
    BILL WICHERS | | #2

    You could probably get in there with one of the spray foam kits. If the void isn’t too big, the spray foam will tend to expand and seal any small voids where you didn’t apply quite enough material. You could use an inspection mirror or a borescope to inspect your job when you’re done and fill any spots you may have missed.

    I would NOT cut away any of that joist. Cutting out part of a joist is asking for trouble.


  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Zaid.

    I'm not sure what the best way to handle this situation is. It's an unfortunate one, but it is also common. I have seen people stuff fibrous insulation through the gap and them attempt to air seal to that first joist with blocking and caulk. I also have heard of people blocking the gap, drilling holes in the joist, and spraying foam in. It would be great to know what you decide to do and how it goes.

  4. timmyzaid | | #4

    I *think* I'm going to get and try to spray it as well as I can. I'll report back and try to take some pictures as well, though it's hard to get good ones obviously!


  5. joshdurston | | #5

    I would probably use a small hole saw to cut some 2-3" diameter holes every 2' in the middle of the joist, and then spray foam with a kit like the one you linked too. I had a similar case where I tried to do it thru the gap and you end up with a poor angle for covering the far side, better to spray from the middle height of the joist bay to get full coverage of the bottom plate as well.

  6. MattJF | | #6

    Ha, I shouldn't suggest cutting structural members so casually. Get an okay from the inspector first. In my case I could install a new sistered joist first that runs support to support and is still less than 16" on center, so everything meets code. Then ripping a 1.5" 45deg along the bottom edge of the original joist would give just enough room to get a spray gun in there.

    That is a lot of work for not a ton of insulation, so I haven't actually tackled this yet.

  7. joshdurston | | #7

    Don't rip a 45, that would seriously compromise the strength.

    The top and bottom 2" of a joist are carrying most of the strain. A hole in the middle won't affect the strength if it's small in diameter and not in the top or bottom 2" of the joist height.

    1. MattJF | | #8

      I think you missed the part where I would install a new joist first. I could rip out the entire original joist if I wanted.

      If you have 2x8's you are limited to 2 3/8" holes, which is pretty limiting. You can do 3 1/8" in a 2x10, which might be workable.

  8. user-6184358 | | #9

    I have no basis for this - why not manually stuff it with cellulose. Get the bags from the big box and jam it in there with a stick.

  9. Andy6572 | | #10

    I had the same issue at a parallel rim joist with the cut n cobble method, and it was too cost prohibitive to buy a spray foam kit for such a small area in my case, so I just air-sealed with caulk and stuck some more mineral wool in that area (I had that stuff anyway for thermal barrier on all the other rims). Condensation could be an issue with air permeable insualtion, but if your rims were dry beforehand I imagine the risk is low.

  10. timmyzaid | | #11

    Brief update, I drilled a few 2.5" holes in the joist to see if I could get a better look and I'm glad I did. I still need to clean it out more and I could also see daylight coming through so it isn't even close to being sealed.


    1. Expert Member
      BILL WICHERS | | #12

      You can chamfer the edge of your holes to be able to get a better angle with the spray foam gun. They did this in medieval times with slit “windows” to give the archers a better field of fire without compromising the defensive capabilities of the castle.

      Don’t go overboard with your chamfer though or you’ll start to get into the “compromising the joist” range.


  11. Expert Member
    BILL WICHERS | | #13

    You might be able to verify you got good coverage by using an IR camera. Any missed spots should show up pretty well on an IR camera.

    BTW, it probably would be better to comment back on your original posting rather than modifying it. It was a bit confusing seeing an "updated" question with no new comments...


    1. timmyzaid | | #14

      Will do. I did it more for posterity anyway in case someone runs across in the future.

  12. cdvcj8 | | #15

    I have a similar situation in zone 4. I cut and cobbled rigid foam, for most of the basement. However, I cannot get even hand into the gap for 2/3 of one wall. I have HVAC linesets and massive wire runs interfering me from putting another joist in so I can notch the original one. For the 1/3 that's accessible I used can spray foam with a flexible hose to add successive layers to cover the complete area. I can do this for the tight section, but I won't be able to reliably get to the top section. Instead of drilling larger holes every 1.5 to 2 feet, what about drilling 3/16" holes (the size of the straw for the can foam) every 5" about 1 1/2" from the top of the joist. My thinking is they are so small they won't affect the structural strength. I would then use the Big Gap foam to get maximum coverage. Would this work?

    Chris V.

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