GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

More rim joist insulating questions

swee0099 | Posted in Energy Efficiency and Durability on


I know this topic has been discussed a lot, but I cannot find exactly what I am looking for.

We have a 1926 Tudor in Minneapolis, MN. I am in the process of finishing our basement and I plan to use 2″ of rigid foam on the walls and potentially between the joists to seal up the rim joist. I may use closed cell spray foam to seal the rim as well. My problem are:

1. We have concrete poured between our joist onto the foundation making it difficult to use rigid foam. The concrete is also poured very uneven. This is why I may use spray foam.

2. The joists are flush with the foundation on the two walls where the joists run parallel to the foundation, thus making a cavity. This makes it impossible to get rigid foam to the outside joist. There are also radiator pipes buried in that cavity.

My questions:

1. Besides energy loss and a bit of discomfort; is there any harm in not sealing the rim joist if I only insulated the foundation? My thought here would be if I leave it alone, the house can breath as it has for the last 90 years.

2. If I used spray foam, I would need to drill holes in the joist flush and parallel to the foundation to fill the cavity with foam. Assuming that is possible, am I risking any issues with the radiator pipes?

3. Is it worth only sealing the rim with the concrete between the joists and leave the cavity alone? Is a partial job going to lead to issues down the road?

4. Is air sealing without insulation a bad idea?

5. Can I just insulate straight up from the basement slab to the sub-floor, with the rigid foam, without going out to the outer rim joist ,thus leaving a cavity all the way around the foundation? I know this will not stop the leaks into the house, but the basement would be comfortable (I assume).

I have also included a picture for reference.

Thanks is advance,

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    This is a judgment call. It looks like the ends of your joists are embedded in your concrete foundation. Embedded joists are at risk of rot. The colder the concrete wall, the greater the chance of moisture accumulation in the joist ends, and the greater the chance of joist rot.

    Keeping the rim joist warm (by not insulating) reduces the chance of joist rot.

    The risk is lower if the rim joist is significantly above grade (on the exterior of the house), so that the rim joist is exposed to air and sun. If you have bushes close to the house, it would be a good idea to cut them down or trim them. You want to encourage drying to the exterior.

    For the best job, use the chainsaw technique. Cut off the ends of the embedded joists with a chainsaw or Sawzall, and support the joists on a new interior stud wall. Then you can go to town with spray foam.

  2. Dana1 | | #2

    What is the maximum temperature of the radiator pipes likely to be in this system?

    And the maximum possible temperature, with the existing boiler?

    The max operating temp for most polyurethane foam is on the order of 225-250F, but I'd be reluctant to use it in contact with 215F steam.

    Most rigid polystyrene has a typ- upper service temperature rating between 150-175F, which can be an issue with high-temp hydronic systems too.

    Is there stucco siding on the exterior of the band joist area, or something else?

  3. swee0099 | | #3


    Thanks for the response. I think I am going to avoid cutting the joists. They feel dry and rot free when I poke them with a screwdriver. The concrete is only about half way up the joist, so hopefully they are drying if damp. They bottom of the joist is about 18" off grade. Exterior of house is stucco and bushes are minimal.

    For budget sake I my just insulate the block wall up to the bottom of the joist and leave the system as is to avoid any unwanted changes. Besides wasting energy, what are your thoughts on that plan?

  4. swee0099 | | #4


    Thanks for the reply. We have a hot water boiler, so I am not worried about the temp of the pipes. I can touch them when they are hot. The exterior is stucco. I would worry about getting all areas sealed if I had to spray foam in that cavity where I couldn't see my work.

  5. swee0099 | | #5


    What if I did insulate from the concrete slab up to the wood subfloor, flush with the block wall, leaving the cavity around the perimeter of the house uninsulated? Do you think that would change the way the house would breath or do you think there would still be enough leakage through the subfloor, from the newly formed cavity, into the main floor of the house to help prevent joist rot? Is this not worth the hassle; would it be better just to insulate from the concrete slab to the bottom of the joist if I do not plan to seal the rim joist?

  6. GBA Editor
    Martin Holladay | | #6

    Here's another idea: Insulate your foundation wall (and the rim joist area) from the exterior. Sure, you'd have to excavate -- but you could just go down 4 feet instead of all the way to the footing if you wanted to reduce the hassle. You could cut the siding off horizontally above the rim joists, and install Z-flashing at the transition between the bottom of the siding and the top of the exterior rigid foam.

    Because of the risk of joist rot -- especially in Minneapolis, which is cold -- I'm going to stand firm with my statement that installing interior insulation is a judgment call. You have to make the judgment, not me, because you're the one who will have to make the repairs if the joists get spongy.

  7. swee0099 | | #7


    I thought about exterior insulation, but that is beyond the scope of this project. If I only insulated the block wall from slab to bottom of joist, there should be no increase of joist rot, correct? This would still increase the comfort of the basement without turning this into a huge project.


  8. GBA Editor
    Martin Holladay | | #8

    That approach sounds like a good compromise.

  9. swee0099 | | #9

    Thanks Martin,

    I very much appreciate the advise. I realize it is a compromise, and I know what should be done, but budgets are budgets.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |