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Community and Q&A

Wide gap between mudsill and foundation

user-7039735 | Posted in General Questions on

Hi all,

I am in the process of air sealing and insulating the rim joist area in my crawl space. I’ve already caulked in each of the bays and am about it install XPS and seal around with canned spray foam.

While crawling around I noticed that the mudsill is anywhere between 3/4″ to 1 1/4″ off of the top of the foundation wall. There are shims spaced out all around, some of which are degrading.

The main section of the house (with a full basement) was built in 1976 and the addition with the crawl space I am in was probably done sometime in the 80s. We moved in about 3 years ago.

Is this normal? Is there a need to fix this and add some structural support? Or is it OK to just use canned spray foam between them?

Photos here:

Thank you very much!

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  1. Expert Member
    Michael Maines | | #1

    While not exactly unusual, a gap between the foundation and mudsill is not a good thing either. It can be hard to pour a foundation perfectly level on top, and tools available in 1976 were less sophisticated than the lasers we use today.

    For structure, you don't really want your house resting on degrading shims. Although certain types of foam can be used in certain structural applications, this is not one of them.

    Once sealed in the shims may be even more vulnerable to rot. The best product for the job is non-shrinking grout--just pack it tightly into the space. If you don't see it on the shelf, hydraulic cement is the same thing. If possible it would be a good idea to slip some galvanized steel flashing under the mudsill, to stop capillary water movement up from the concrete.

    If you haven't done it already, I would suggest using a rigid foam other than XPS. The blowing agents in XPS are potent, long-lasting carbon polluters--a new term for benign-sounding "global warming agents."--and should not be left exposed to potential sources of ignition. Dow's Thermax is the only rigid foam rated for exposure, and has slightly higher R-values than XPS as well.

  2. Expert Member
    Dana Dorsett | | #2

    Michael has it right- the shims will be more susceptible to rot once it's sealed up and insulated. Using a grout (or even pre-mixed concrete for the really big gaps) to add additional support is warranted.

    Where possible use EPDM flashing tape on the underside of the sill where it's going to be grouted rather than metal flashing. EPDM is orders of magnitude less thermally conductive than sheet metal.

  3. user-7039735 | | #3

    Thanks all! This is very helpful.

    Would this EPDM tape be sufficient and is this what I should expect for price?

    Should I replace the existing wood shims with composite shims?

    As for the XPS, unfortunately it's already cut and sitting in the basement. Could I cover the XPS with some Roxul?

    Thank you,

  4. Expert Member
    Dana Dorsett | | #4

    Yes, that tape is fine. It might be hard to install using the adhesive where the gap is less than 3/4" (unless you have very slender fingers, but even with the backing paper still attached it's still a good capillary break.

    Where are you located, and how thick is the XPS?

  5. Expert Member
    Michael Maines | | #5

    User-7039735 (do you have a real name you want to share?),

    Dana's idea to use EPDM is good. It might be easy or hard to get it all the way across the top of the foundation. I think it's more important to stop moisture flow than it is to reduce thermal bridging at that location, but it's up for debate.

    The tape you linked to is uncured rubber, comparable to Vycor, and will be pretty floppy. You may have a hard time getting it across the top of the wall. Ideally you'd have access to offcuts from 0.060 EPDM membrane, the type used for roofs--it's thick and fairly stiff.

    If you can pack the cavity full of non-shrink grout, you don't need any shims. But you could install some composite shims while the grout sets up, and leave them in place if you want.

    The 2015 IRC code allows sprayed foam at the rim joist without a covering, if certain conditions are met, but there is no exception for rigid foam. In crawlspaces entered only for repairs or maintenance, you can skip a thermal barrier and just use an ignition barrier. For that, Rockwool (Roxul's new name) in 1 1/2" thickness qualifies, as do a number of other products. If the crawlspace is entered more regularly it should have a thermal barrier of 1/2" drywall, 3/4" plywood or similar.

    For more info, read section R316 here:

  6. user-7039735 | | #6

    I did update my First/Last Name and Screen name in my profile before I posted. Is there another spot where I can change my forum username?

    Sorry I should have mentioned my location in the first post. I am in Chicago suburbs, so 5A.

    I'd only be entering the crawl space for repairs/maintenance. So 2" of XPS would be R-10 plus R-14 w/ the Roxul would give me an R-24. Do I have that right?

    Unfortunately, I wouldn't have access to 0.060 EPDM membrane anywhere (at least that I can find). Is there any other alternative? I'd like it to be done right and if $150 for the self-adhering EPDM tape is what it takes, I'll do it. But if there is a cheaper alternative that works just as good, I'm all ears.

    Thank you all for your help.

  7. Expert Member
    Michael Maines | | #7

    Unfortunately the software is buggy, so new users just state their name somewhere. I believe they are working on a fix for the problem.

    XPS is tested and marketed at R-5/in, though it offgasses and loses R-value over time. 2" would be R-10, officially, and more like R-8.5 eventually. Rockwool is R-4/in in Comfortboard (semi-rigid) and R-4.2/in (batt) form. They sell their 3 1/2" batt as R-15; is that what you're proposing to cover 2" of XPS?

    As for the capillary break, there are a wide range of products that will provide a capillary break, so it's a matter of what will work best in your situation. You can find 9" x 75' rolls of Vycor in any decent lumberyard or box store, for less than $50. For less cost per square foot, and stiffer material (at least in cool temperatures) you could use Grace Ice + Water Shield. Buy a 3' x 75' roll , and make your own 8-9" strips.

    Although the vast majority of homes have no capillary break, or one that doesn't perform well, the lack of one becomes more of a problem as the rest of the house gets tighter. This is your only chance to address the issue.

  8. user-7039735 | | #8

    Good to know! My name is Ed Kerstetter. Nice to meet all of you!

    Looks like Lowe's carries the Grace Ice + Water Shield at a responsible price. $79 for 3'x36'. I think I'll only need about 90 linear feet max. So I should have plenty to spare.

    From what I can find, Home Depot carries the Rockwool in batts @ R-15 (for 2x4 w/ 16oc) or R-22 (for 2x6 w/ 16oc). I could do the R-22 just to account for some of the XPS off-gassing. Or would the Comfortboard be better in the crawl space? I worked with the Batt stuff before, never the board.

    Just to note. I do get some mice in the basement. Is there any issue with Rockwool attracting Rodents or providing nesting material?

    And I totally agree with you, this is the chance to do it right!

    Thanks again!

  9. Andrew_C | | #9

    @ Ed, re mice
    One of the benefits of doing a good job air sealing a house is that all the access holes for undesirables get smaller. If you do a decent job of air sealing, you will eliminate mice. If you do a really good job of air sealing, you'll also eliminate a lot of bugs and spiders. I think this is particularly true wrt sealing crawlspaces or basements. Air sealing and insulating the rim joist is right up there with eliminating recessed canned lights in the ceiling plane in terms of bang for the buck/effort, IMO.

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