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Community and Q&A

Rim joist insulation: will it jeopardize the sill plate?

Sam P | Posted in Energy Efficiency and Durability on


I live in East Central Illinois, climate zone 5. Our home is 1941 on a poured concrete foundation. The 2×6 sill plate seems mostly dry and solid though I’ve found a few soft spots on the exterior side (likely caused by outside water.) There is no capillary break between the concrete and the wood. The amount of above grade foundation exposed to the exterior ranges from as little as 10″ to as much as 22″ but most of it is 18″ to 20″ above ground. The interior basement walls have at least 2 coats of mystery paint and I have no way of telling if the exterior has ever had an application of sealer or not. Generally the basement seems quite dry, although there are a couple hairline cracks in the foundation through which I’ve observed a small amount of moisture weeping through.

My question is whether or not it will be safe to spray foam the rim joist. I’m worried things won’t be able to dry sufficiently and worried about sill plate rot from capillary wicking. As far as I can tell the sill plate is not fastened to the foundation at all and along some of walls the sill plate has “kicked up” under the weight of the rim joist leaving a fairly sizable gap under the plate. Would it be prudent to try to slip some type of capillary break beneath the plate in the areas where I can? I’m not really up to the task of jacking up the house, even though I’ve read it’s not as hard as one might think. Would closed cell spray foam fill the gap in those areas and serve as a capillary break? Something about encapsulating that area with foam makes me nervous, although it’s pretty standard practice around here. Then again so are a number of things which as I learn more about this stuff don’t seem to be best practice. I also have concerns about the environmental impact of the foam.

I’ve thought of asking them to keep the foam in the “box,” not totally cover the sill plate and then I can seal the gap beneath the plate using a different strategy. I’ve also thought of asking them to keep it thinner in general.

It looks like my family will likely be eligible for the federal weatherization grant and I’m sure that area will be one thing they want to address and probably with spray foam. Beggars can’t be choosers, right? But if there’s any concern about rot I was thinking maybe I should fill/caulk gaps and try to do the rigid foam (polyiso) cut & cobble method myself before they come do the energy audit. This way, also, the money could possibly go toward things that I really can’t afford and don’t feel as comfortable doing myself.

Many thanks for all of the awesome information on this forum.


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  1. Sam P | | #1

    Another important factor: at a later time I would like to insulate the walls as well...

  2. GBA Editor
    Martin Holladay | | #2

    In a basement like the one you have described, the risks associated with insulating the rim joist on the interior with spray foam are quite low. I would go ahead and have the work done if I were you.

    If there is a significant gap between the top of your foundation wall and your sill, it's perfectly OK to fill that gap with canned spray foam or caulk. Go right ahead and do that work any time you want.

  3. Expert Member
    Dana Dorsett | | #3

    If the mystery paint isn't flaking off with efflorescence & spalling the foundation wall is pretty dry, and you don't have anything to worry about when insulation & air sealing the band joist & sill. If the walls were wicking major moisture it won't hold paint, and you'd have bands (usually lower down from the slab up to maybe knee or waist height) where it was heavily deteriorated.

    Even in those cases you'd still be fine if you stripped the paint off the bottom foot or so of wall to let it dry toward the interior, and you could still insulate down to that point without any increased risk.

  4. Sam P | | #4

    The two people I was hoping would respond :) I've learned a lot from reading your responses here on the forum. Thanks for the input and for being generous with your knowledge.

  5. Chaubenee | | #5

    Samuel, I have one question in mind since you are quite above grade with that area- have you considered an open cell foam that could dry to the interior should it get damp or wet? What do the good people here think about the distinction between the open cell and the closed cell foam in this area since I tend to favor open cell where it CAN be used?

  6. Sam P | | #6


    That thought had crossed my mind, as had using a vapor permeable rigid foam on the walls when the time comes...but I'm just trying to cross one bridge at a time. Hard to know what's best; I'm learning a lot from perusing these forums. Seems like closed cell is favored for the rim joist application. I think one concern might be that being vapor permeable the open cell foam could allow condensation on the rim joist/sill plate via moist interior air contacting the cold rim, especially if its adhesion against the joist eventually failed (which might be expedited by the passage of vapor?). Still trying to wrap my head around the complexity of insulation vs. moisture and vapor vs. environmental responsibility of product choice. I assume you favor the open cell because the production process is more benign for the environment? The spray foam in general makes me a little uneasy. It's rather permanent once you install it. Also, it's relatively new and and we already know that in either form (closed or open cell) it's not exactly great for health or the environment but neither are fossil fuels so the trade off in efficiency seems to justify it. But it's only been used in construction since the 70s and who's to say that in another 40 years we won't discover that the breaking down of that material causes major human health/environmental issues just like asbestos and lead paint. I'm probably going to go ahead and spray it but as with many things you often have to wonder if the cure could be worse than the original ailment.

  7. GBA Editor
    Martin Holladay | | #7

    In most climate zones, I think that open-cell spray foam would work fine on the interior of a rim joist. I'd probably insist on closed-cell spray foam (or even better, exterior rigid foam) for Climate Zones 7 and 8, though.

  8. Chaubenee | | #8

    In zone 5 which I too am in, I like the open cell there because it can dry out. I also plan on using 2.5" eps as outsulation with firring strips to hold my siding so I think in my case there isn't a good drying path outwards, but in your case you may have no plan to put exterior foam. I have been advised not to create the "foam sandwich" so I feel better with open cell that can dry inwards. Also I was thinking I might tape the top of the concrete with leftover Vycor just in front of the mudsill wood so that the open cell isn't in direct contact with the concrete. Kind of like a capillary break I guess. I am placing R20 foam with foil faced on the inside on the basement concrete walls just below that so I figure I will tape TO the foam in that spot. Are you planning foam insulation on the basement walls?

  9. Sam P | | #9

    I'll definitely consider open cell for the rim joist as well for it's drying capacity to the interior. At this point installing exterior foam is cost prohibitive for me and also on my house would be a little daunting because my house has no eves or soffit so it would bring the siding out too far past the roof line and would require some new framing. Maybe in the future if I reside. Currently it's siding, felt, board sheething, mostly fiberglass, lath, plaster. I'm hopeful that through the weatherization grant I might end up with some dense pack cellulose in the walls.

    When I can afford to I would like to insulate the basement walls with foam. I had wondered if EPS would be good here for the same ability to dry to the interior. As Dana suggested I'll probably leave the bottom foot or so uncovered just to be sure. I think code will allow polyiso without sheet rock so I had also considered that, although if funds allow I'll probably plan to frame finished walls in front of the instulation. You are using foil faced to the interior as a vapor barrier to keep warm inside air from passing through and condensating? What is the concern about foam in direct contact with the concrete? Will open cell foam wick water?

  10. Chaubenee | | #10

    Samuel, it has been pointed out many times on this forum that foam board such as XPS, EPS OR POLYISO whether foiled or not, all work nicely in basements in direct contact with the interior concrete walls of a basement. The reason I am using the foil faced is that I have no desire to have a finished basement whatsoever. So therefore the foil will be the finished surface (I plan on getting the ok on this from the inspector when he comes for the framing inspection.) but as far as the vapor issue there, I think all of those foam types are vapor barriers to one extent or another, and I am simply not concerned that water will condense between the foam and the concrete. But rather than have open cell in direct contact with the concrete, the little area of concrete at the top of the foundation that touches the wooden mud sill will be coated with flashing tape (which I have a few rolls left over from my window flashing job.) ...maybe it is overkill but it is Vycor and I bet if it sits around in my workshop for a few years it might lose its stickiness so I might as well use it there.

  11. Expert Member
    Dana Dorsett | | #11

    Unfaced EPS is about 2x as vapor open as XPS of equal density, but anything over 2.5" of Type-II (1.5lb nominal density) is edging into Class-II vapor retardency.

    In US zone 5 you could fairly safely hit IRC 2015 code-min performance with 1" of Type-II EPS , and a 2x4/R15-batt studwall 16" o.c.. The vapor permeance of 1" Type-II EPS is over 2 perms, but less than 3, making it quite a bit less vapor permable than interior latex paint- any ground moisture coming through the foam will be coming it at a rate low enough that it won't be backed up in the stud wall. At only R4.2 the above-grade section would be ever so slightly susceptible to wintertime condensation events at the foam/fiber boundary, but the wicking of the fiber will distribute the moisture into the warmer part of the cavity where it re-vaporizes- the is no real accumulation, even though there will be condensing events, and even though the average mid winter temp at the foam/fiber boundary is a degree or so lower than the dew point of the room air.

    If that's deemed too risky, at 1.5" (R6.3) there is plenty of dew point margin at the above grade section, and could get even more margin if you dropped back to R13s in the studwall, but the vapor permeance will drop to the 1.5-2 perm range. That's still decent drying capacity for the foundation- much higher drying rates than you'd get through the alkyd or epoxy paints that are probably on the wall, but lower than with most latex.

    With any stud wall (even partition walls) in the basement it's worth putting an inch of EPS under the bottom plate as a capillary & thermal break. The slab can be colder than the summertime ventilation air's dew point, which can draw moisture from the air into the cold bottom plate (and concrete), and if there isn't a vapor barrier under the slab there is a ground moisture drive to be concerned with as well. R4.2 of non-wicking foam is more than enough to fix both problems.

    Open cell foam in contact with the ledge at the top of the foundation is rarely a problem but open cell foam doesn't always fare very well in contact with concrete below grade (from both ground moisture drives and summertime dew point issues.)

  12. willgoodwin | | #12

    I have seen serious sill rot rot on a house with a similar situation. If you are ambitious you could replace the plates with pressure treated and install a capillary break before you spray.

  13. JMSTP | | #13

    Hello, I know this is an old post, but hopefully some of you are still watching!

    I own a house in Toronto, Canada built in 1890.

    It's a brick veneer house sitting on a brick foundation without a capillary break beneath the sill plate. The sill plate is dry and in very good condition.

    Currently in the process of installing some interior water mitigation (dimpled membrane and weepers), but I'm struggling with how to insulate the joist cavity (8"x16") -- particularly in the context of managing the "rising damp".

    From everything I've read, open cell is the way to go to ensure the sill plate remains dry while blocking airflow.

    The wall assembly I'm planning is:

    Brick->Dimpled Membrane->2" EPS->Framing/Rockwool->Drywall (no polyfilm)

    For the joist cavity, I'm struggling between the following (note: no exterior sheathing on cavity area):
    1. Fill with open cell foam lapping on to EPS of interior wall assembly
    2. Place a piece of rockwool on sill plate (partially filling joist cavity; allows moisture to evap from sill plate) and spray remaining cavity with CLOSED cell, lapping on to EPS
    3. Place a piece of rockwool on sill plate (partially filling joist cavity; allows moisture to evap from sill plate) and spray remaining cavity with OPEN cell, lapping on to EPS

    Thanks in advance!

    1. Expert Member
      BILL WICHERS | | #14

      You may also want to consider adding a capillary break between the top of the masonry wall and the sill plate. It's not particularly difficult to jack up the floor a small amount and slide something in, working your way down the wall in stages. I like HDPE sheet for this, Dana often recommends EPDM. HDPE is relatively stiff and slick, so it's easy to slide in, EPDM is more flexible and grippy, but easier to get at building supply places. Both work for the purpose.


      1. JMSTP | | #15

        Agreed, your solution would definitely address the problem! Unfortunately, do not have the time to make this happen. Also, I suspect extricating the sill plate after sitting on brick under significant pressure for 130 years might prove unexpectedly challenging.

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