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Community and Q&A

Shimming and air sealing sill plate

RobInNorCal | Posted in Green Building Techniques on

I’m getting ready to frame the garage for my project and  the foundation walls aren’t dead flat and level (are they ever?) so I need to shim the sill plate. But the sill plate is part of a conditioned crawl space system so needs to be air sealed. 

Best practices for sill plates, from everything I’ve read here include 
– make sure there’s a capillary break between the top of the foundation and the sill plate;
– make sure there isn’t an air gap between the concrete and the sill plate
– make sure the sheathing is sealed to the plate, and that the plate is air sealed to the concrete.

In the real world, how do I do this? Questions include:

– What’s the best material to use for the capillary break? I have some 10 mil poly, or I could round up some EPDM from an unused pond liner and cut strips…  or use a butyl flashing tape like Vycor. What do you use, or recommend?
– The traditional method of “shim, then grout” won’t work if the capillary break is sitting on the concrete, so it needs to be attached to the bottom of the plate. What to use for shims? 
– has anyone found a good method for reliably inserting grout into the thin (shimmed) spaces under the sill plate? Should I just run a thick bead of something that will harden (like fireblock caulk) on the top of the foundation wall as the grout, before bolting down the plate?
– and, finally – having done all this, is there still value in using something like the Conservation Technology plate gasket, or should I just use a great flashing tape to seal the plate to the concrete? 

Thanks for the advice – this green building stuff gets complicated quickly!

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  1. BrianPontolilo | | #1

    Hi Rob,

    This is a pretty critical detail to get right, both for the construction process and for energy efficiency. When it comes to getting the plate level, I've seen builders do everything from shim with wood or steel, to knocking down the high spots on the foundation wall with some sort of chisel to planing the installed wood sill (which seems pretty crazy).

    Here are some articles and a video that may be helpful:

  2. Expert Member
    BILL WICHERS | | #2

    For a capillary break, I posted a question about that a month or two ago and was discussing it with Dana. Dana has recommended pieces of EPDM roofing membrane as I recall, but my concern was that since that stuff is pretty flexible, it might be difficult to slip into a small gap without getting bound up up and folded/wrinkled. I have some of this type of work to do and plan to use strips of 1/32” thick HDPE (high density polyethylene) sheet as the capillary break. This material is fairly stiff, and very thin, so I expect to be able to slide it into the gap easily. HDPE sheet is also pretty cheap, something like $50-60 for a 4x8 foot sheet which makes a lot of strips for use as capillary breaks!

    In terms of how to install things or lift the structure, that’s difficult to say without actually seeing what you’re working with. Anytime you do this type of work it’s going to be pretty site specific.


  3. STEPHEN SHEEHY | | #3

    Is the foundation so uneven that the Conservation Technologies gasket won't fill any gap? It's a pretty substantial gasket.

    1. Expert Member
      Dana Dorsett | | #5

      For new construction EPDM sill gaskets are definitely the way to go.

  4. user-6184358 | | #4

    Use a laser level & a diamond concrete grinding wheel. Get the surface level before you ever start with wood.

  5. RobInNorCal | | #6

    Q. 1: "– What’s the best material to use for the capillary break? ...What do you use, or recommend?" No one seems to have a better idea than the Conservation Tech gasket; so that's where I'll go. I will probably also use a "belt and suspenders" approach and tape over the extra 2-1/2" of foundation wall that aren't covered by the 5-1/2" CT gasket.

    Q2: "What to use for shims?" Answer - courtesy of several threads on JLC and Contractortalk- seems to be steel or high density plastic shims around the anchor bolts, leveling in between with mortar before setting the sill plates. @Tim, in this case I think I'd rather build up than grind down, there are a couple of spots where grinding would take as much time and more effort than laying a thin nonshrink mortar bed.

    Q3: "has anyone found a good method for reliably inserting grout into the thin (shimmed) spaces under the sill plate?" Still a good question, hoping someone has a good answer. I've tried this a couple of times, and I'm confident I didn't find a good method. Pushing nonshrink grout into a 1/16" gap is a real PITA and unlikely to be done well.

    @Brian, thanks for the links; I had read the GBA articles, but the Fine Homebuilding article was exactly the kind of article that I hate: generalities, with no prescription. In fact, that article answers none of the practical "how to" questions that made me start this thread.

    1. Expert Member
      BILL WICHERS | | #7

      For Q3, if it’s a fairly even 1/16” gap, use some HDPE or a piece of sheet steel as a shim. The steel would be easy to pound in, but would be a thermal bridge. Even 4” square electrical box covers would work for a gap that size since they’re about 1/16” thick.

      I would try one of the really large bore syringes (like the kind used for glue by hobby people), but I think a 1/16” gap is too small to get the grout to ooze in very well. I’ve never tried to fill a gap smaller than maybe 3/16” with grout.


  6. user-6184358 | | #8

    A diamond wheel on green concrete can do a lot of removal quick.

  7. natesc | | #9

    Are you trying to shim out 1/16" ? That is a waste of time, your stud length will vary by more than that, same with cripple and trimmer studs, your floor joists thickness will vary by a lot more than that if you're using real wood too.

    The hardest part about building is judgment.

    1. Expert Member
      MALCOLM TAYLOR | | #10

      I agree. Gaps that small aren't worth worrying about. Cinching down your actor bolts will cause the sill plate to vary more than 1/8".

  8. RobInNorCal | | #11

    LOL wasn't worried about 1/16th per se, but when you have 1/4" at one bolt and nothing at the next one, there will be a 16th in there somewhere along the way. It's easy to shove some nonshrink grout in a 1/4" gap, but an 1/8" is about as narrow a space as I feel confident about plugging. Was hoping maybe one of you had a better method (and I am going to try using a cementitious caulk on the foundation before bolting the plate down, seems like that's the best bet).

  9. RobInNorCal | | #12

    @Tim, been rethinking your suggestion so went around the foundation with a 6' level - turns out most of the gaps will go away with a bit of grinding, so that's step one. Thanks for that advice!

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