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Community and Q&A

Insulating a Rim Joist With No Mudsill

thegiz | Posted in General Questions on

So hopefully I’m learning from what I’m reading here or may be clueless. I want to insulate a rim joist in an old house that has no capillary break or mudsill. The joists sit directly on the stone wall on the west and east side of the house. The front of the house you can’t access the rim joist all I see is a concrete ceiling. I think it was a converted front porch. The back side of the house has joists embedded in concrete. The rim joist all around except for the front of the house is above grade. I guess my questions would be:

1. On the east and west side of house can I put 2in eps directly against rim joist with 1in rigid board mineral wool over that for a fire barrier. The top where mudsill would be I leave uninsulated.

2. I have some plumbing that is extremely close to the rim joist in 1 or 2 cavities. Could I just leave those uninsulated so I can see the pipes at all times and I don’t freeze them behind insulation?

3. On wall that has embedded joist, I want to just leave it be and not deal with any potential problems. Yes there would be heat loss but I’m insulating other areas of the rim joist.

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  1. GBA Editor
    Kiley Jacques | | #1

    Take a look at this article Insulating Rim Joists. Martin makes the case for the use of spray in complicated scenarios like yours. He also states: “If your basement is dry, and the exterior grade meets modern expectations, you probably don’t have to worry about insulating rim joists that lack sill seal.”

  2. Expert Member
    BILL WICHERS | | #2

    For #1, yes you can, and that's exactly what I did at my own house. I used EPS to allow for a little bit of drying ability, since EPS is more vapor open than the other options.

    For #2, yes you can leave those areas uninsulated if you have to. If you insulate half of your rim joist, you've still made a 50% improvement, so you still have a benefit. I'd try to get at least some insulation on the rim joist behind those pipes and cables though -- maybe try some thinner EPS here?

    Regarding #3, it's not an "all or nothing" project to add insulation. Every little bit you add helps. If there is a spot or two that you can't reach, or that have some reason to be concerned about problems, you can skip over those areas and insulate the rest. You'll still have a benefit from the insulation you do add.


  3. thegiz | | #3

    In the same situation if I use rigid foam on walls and if there are areas I can't get behind water lines, what is the risk of just drywalling over that area with the rest of insulated wall, making sure not to insulate in front of the pipes but on the sides. This is not the majority of the wall only one water line that is connected to the wall but it it is coming from main water line.

    1. Expert Member
      BILL WICHERS | | #4

      I would use a high R per inch insulation in any areas where you can't get full depth, this lets you maximize the amount of insulation you can get in the space you have available. Mineral wool is great for this, since it can be bent and squished into place, and still has reasonably high R value.

      I would put pipe insulation on the pipes to minimize condensation risk. I would also put a smart vapor retarder on the inside of at least the minimally insulated areas too (I'd probably do the entire wall though). You'll find those sections of the wall you couldn't insulate as well will be a bit colder in the winter, and they'll be very obvious with an IR camera, but otherwise they should be OK.


  4. Expert Member
    PETER G ENGLE PE | | #5

    Don't ignore the possibility of relocating water lines. Plumbers are expensive, but not that expensive, and relocating a few water lines is super easy. Probably worth it to get a couple inches of insulation behind them. If you're doing a lot of work yourself, Sharkbite fittings make DIY plumbing a breeze.

    1. Expert Member
      NICK KEENAN | | #6

      Copper pipes can often just be moved by hand a little ways. Take the straps off, slip foam behind the pipe and then put the straps back on with longer screws.

  5. thegiz | | #7

    I attached a picture of the rim joist in question. I also have 3 large windows that I would like to insulate picture attached as well. I’m thinking for window glue 2in rigid foam to inside of window and build a 1x4 box on front edge of window. Tap con that in, drywall it all and that attach pvc trim to cover up front of window edges. Insulating the 3 windows and the rim joist should be considerable help over having nothing as far as insulation goes. Hopefully it will be enough of an improvement in winter I can put a complete insulation drywall project on back burner

    1. Expert Member
      BILL WICHERS | | #8

      That looks like PEX. PEX is pretty easy to work with. I'd have a go at relocating that run that is clamped to the rim joist out to where the other one is located so that you can get your insulation in there. It's probably less than an hour worth of work, and maybe a few bucks of materials, if that.


  6. thegiz | | #9

    Just going back to the original question what is the risk vs reward insulating a rim joist in an old house without a mudsill. It looks dry and it doesn't flood but that means nothing for possible moisture in the air or from outside, faulty old siding, etc. How much energy savings am I getting compared to the wall? If it is little it is not worth any extra risk. Also if I do insulate the rim joist, once I insulate against the rim joist do I simply lay foam board on top of the foundation wall since there is no mudsill.

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