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

Rim joist insulation: No wood sill plate – influence on rigid foam

sfillery | Posted in Green Products and Materials on

Good evening,

I have been reading posts on this forum for the last year and greatly appreciate the thoughts offered. I own a 75 year old stone/brick colonial in SE Pennsylvania (region 4a). The basement is unfinished and the sill boxes are not insulated. I would like to utilize rigid foam (foil faced PolyIso) with spray foam to insulate the sill boxes.

However, the joists sit directly on top of the concrete wall, there does not seem to be a wood sill plate (see attached photo illustrating a representative sill box, note the joist (right) sits on the concrete wall). The diagrams for insulating rim joists indicate that the rigid foam should sit on the wood sill plate, which I presume limits a direct water uptake path from concrete to foam insulation.

Given the lack of a sill plate to rest the insulation on top of, what do I need to be aware of? Can I continue to utilize rigid foam directly on top of the concrete, do I need to place a ‘capillary barrier’ under the insulation and if so what would be the most appropriate material?

Thank you in advance,

Scott Fillery

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. charlie_sullivan | | #1

    I don't recognize all of what I'm looking at. Probably someone else who does would give a better answer, but in hopes of jump-starting the process, I'll ask about what I don't understand. First, the joist on the left looks dark for the bottom 40% and light for the top 60%. What's going on there? Second, it looks like there is a white layer of something on top of of the foundation, under the mortar. Is that a plastic layer?

  2. sfillery | | #2

    Hi Charlie,

    I will hopefully offer a better explanation the second time. The picture in the above post and an improved picture below are of one sill box. The previous picture included a sister joist which I don't believe is critical to the question - so I have taken another picture of a neighboring sill box. The far wall is brick that sits directly on top of the concrete basement wall (no sill plate). The floor joists sit on / are embedded into the top of the concrete basement wall. The concrete ( block maybe) wall looks to have parged and sealed with 'dry-lock', hence the white look. At the back of the box, the bottom of the brick masonry has mortar buildup due to the joist embedding.

    I would like to insulate these sill boxes to limit air leakage. My questions are as follows:

    1. With no sill plate, is resting rigid foam on the exposed concrete face going to pose a problem.

    2. I can't directly rest rigid foam next the brick facing at the back of the sill box - the mortar prevents this. So I would move the rigid foam forwards, thus leaving a pocket of space between the brick face and foam. However, this could achieve two undesirables: (a) I might have an air pocket that could trap condensation (??). (b) a portion of the floor joists would be behind the rigid foam layer, open to rot from any condensation up from the concrete or from the air leakage into the air pocket.

    What options are best? (i) do nothing and accept the air leakage. (ii) utilize the rigid foam technique but not at the brick face. (iii) utilize spray foam onto the brick face, but realize that there could be a change in moisture in the brick leading to more rapid brick deterioration.

  3. sfillery | | #3

    Note the iPhone rotates the picture 90degrees for some unknown reason. The left side is the concrete basement wall, the right side is the subfloor.

  4. user-4524083 | | #4

    Scott - How many inches deep is the space, from concrete foundation to brick? Is the foundation below the joists above grade? Since the space looks so rough you have a big challenge. There are also wires running through there that may need access to in the future. If this were mine and I was committed to insulating it, I would fill the space with mineral wool, then place 3/4" wood strips on the floor joists and subfloor in plane with the concrete foundation. I would then fix water resistant drywall to the wood strips and foundation using construction adhesive or polyurethane caulk, and screws into the wood. After attaching, re-caulk the edges of the drywall.The drywall would allow some degree of drying potential to the inside from any possible condensation on the brick,while preventing warm air into the space. I'm concerned that the foundation wall, especially above grade needs to be insulated as well, probably just as badly, and this might change the approach. Some people would spray- foam this space, and be done, but it's quite unforgiving if repairs/maintenance are required down the road. Others more experienced with this situation will hopefully chime in. Good luck!

  5. sfillery | | #5

    Thank you for you're advice.

    The answer to you're questions are as follows:
    1. Sill box depth - 10" (from brick face to edge of basement foundation wall).
    2. The top of concrete foundation wall is approximately 8-10" below current grade. Hence, the top of the sill box is right at grade.

    As you mentioned I am very interested in hearing everyone's opinions. I am interested in the right way (after accounting for pros/cons) regardless of effort.

  6. user-4524083 | | #6

    Scott - Since we now know that the floor joists are below grade, my advice above needs to be withdrawn, as there is zero potential to dry to the outside, and those inner floor joists would be more susceptible to moisture accumulation. I'm not sure that anything can safely be done without putting the structure at risk.Your efforts may be better spent elsewhere, insulating and air sealing above grade. It's hard for me to imagine a system for your situation that would allow this structure to last another 75 years. I hope that others will have some better advice for you.

  7. user-4524083 | | #7

    Scott - Read Martin's blog at this site, "Insulating Old Brick Buildings". While talking about above grade walls, the concepts covered have similar bearing to your situation.

  8. GBA Editor
    Martin Holladay | | #8

    Here is a link to the article that Kevin is talking about: Insulating Old Brick Buildings.

    I have to agree with Kevin that your situation is difficult to resolve. There are several problems here:

    1. It's never a good idea for a foundation to be backfilled above the top of the foundation wall.

    2. It's never a good idea for floor joists to be below grade.

    3. It's never a good idea to embed floor joists in masonry.

    4. Insulating this area will definitely make the bricks colder (and therefore wetter). This could cause the ends of your joists to rot.

    There is no easy fix. I could imagine a fix -- but it wouldn't be cheap. It would probably involve lowering your grade to 8 inches below the top of your foundation wall -- and that might be impossible. At best, it might cause aesthetic issues, and require new steps to your porches or entries. At worst, your landscaping might make this approach impossible.

    I can also imagine the "chainsaw solution": cutting off the ends of the embedded joists and supporting the cut ends with new bearing walls. Also not cheap.

    The easiest solutions are either (a) Do nothing, and keep an eye on everything, or (b) Sell the house.

  9. charlie_sullivan | | #9

    How about exterior insulation for that region, perhaps mineral wool boards? Dig down 1.5 feet by hand to install?

  10. GBA Editor
    Martin Holladay | | #10

    Your suggestion is a good one. That approach would solve some, but not all, of this home's problems.

    Of course, the above-grade portion of exterior mineral wool would need to be protected from physical abuse on the exterior with some type of material like metal flashing or pressure-treated plywood, and the insulation would also need horizontal Z-flashing at the top of the insulation -- flashed into a horizontal mortar joint in the exterior brick.

  11. user-4524083 | | #11

    Charlie - In addition, while behind the exterior mineral wool the bricks would stay warmer, the bricks above the insulation would stay cold and conduct the heat from the warm bricks, a super thermal break that seems like it would negate a lot of the benefit.

  12. sfillery | | #12

    Martin, Kevin and Charlie

    Thank you for help. I was aware of this potential after I cleaned away the debri. The cost benefit does not outweigh the potential impact. Regarding re-grading, I have the ability to regrade half of the sill boxes, down to a couple inches below the foundation wall, but not 8". Is 8" a hard number?

    One further question, the foundation walls parallel to the joist structure feature a joist on top of the wall, with a small sill plate. These areas remain below grade by approximately 3". Can I caulk the seams of the wood? I have read that these areas are not high air leakage areas, so again I wonder about the cost benefit.

    Many thanks, Scott

  13. GBA Editor
    Martin Holladay | | #13

    If you can lower the exterior grade so that the grade is 2 inches below the top of the foundation wall, that is a big improvement over the current situation. If this were my house, I would do that.

    I would also make sure that the roof has functioning gutters, and that conductor pipes convey the roof water to a location far from the house.

    It's always a good idea to caulk any obvious air leaks. Caulking air leaks won't cause any moisture problems.

  14. sfillery | | #14


    Aided by some reasonable weather, most (2/3) of the foundation below grade has been re- graded so that the soil is 2-3 in. below the top of the foundation wall (current grade features a min. 6" drop over 10ft). I will wait and observe the sill boxes, but I would like to seal at a later date. Given this change in grade, as well as gutter extensions I fixed last year, can I take up Kevin's suggestion?

    Many thanks for you're advice, I feel better having acted on the grading issue before a major landscape planting.


  15. GBA Editor
    Martin Holladay | | #15

    Q. "Can I take up Kevin's suggestion?"

    A. You didn't specify which of Kevin's suggestions you're thinking about, but I'm guessing that you are talking about the idea of insulating the rim joist on the interior with mineral wool.

    To me, that's a risky approach (because air-permeable insulation allows humid indoor air to contact the cold rim joist.) Carefully installing airtight rectangles of drywall on the interior side of the mineral wool may help, but it's not a foolproof solution.

    Only you can decide whether to try that approach. The level of risk depends in part on how humid your basement is. The colder the climate, the riskier it is to use mineral wool in this location -- and this fact argues in your favor, because Climate Zone 4 is relatively warm.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |