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Community and Q&A

Air sealing anchor bolt hole in sill plate?

kurtgranroth | Posted in Green Building Techniques on

I’m going to eventually seal all around my exterior walls — both inside and outside — with either tape or liquid flashing, but as a belt-and-suspenders approach, I also want to seal the holes in the sill plate that the anchor bolts come through.  My thinking is that IF air was to somehow get past my primary attempts, then it could infiltrate my exterior wall by slipping under the sill plate and up through those holes.

Yes, I do have a sill sealer rolled sheet under the sill plate but I’m honestly treating that more as a capillary break than as any air sealing barrier.

Okay, so I have some SC 175 acoustical caulk and some low expanding (“window/door”) foam.  Which of those would be better in this case?

a) the acoustical caulk is definitely better
b) the low expanding foam is definitely better
c) they are both roughly equivalent so it doesn’t matter which you use
d) don’t use either since it’s a waste of time!


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  1. Expert Member
    BILL WICHERS | | #1

    If the bolt is tight, the wood surface should act like a gasket and you should have essentially no air leakage around the bolt head.

    If you really want to seal something like this, I would suggest using duct mastic instead of any of the sealants you listed. Duct mastic can be easily applied like a very thick paint, and it’s intended for air sealing things. Any of those other sealants will be a lot more work to apply.

    I would absolutely NOT use acoustical caulk in a place like this. Since it’s going to be awkward place to work, you’re likely to get more of the acoustical caulk on yourself than you will get on the bolt. It’s messy stuff.


    1. kurtgranroth | | #5

      I'm curious how the wood surface would act like a gasket. I can sort of see how the metal on the washers might press down on the wood with enough pressure to essentially create an air-tight seal. But the bolt + nut and nut + washer are both metal to metal connections and is it even possible to make those connections air-tight short of welding them?

      I'm also very curious what you are picturing with acoustical caulk! I've used duct mastic before and it's incredibly messy and imprecise stuff. Acoustical caulk, on the other hand, is applied like any other caulk and it washes off skin easily with water. In my experience, at least, caulking off all sorts is leagues less messy than mastic. But maybe I'm thinking of this in the wrong way?

      1. kurtgranroth | | #7

        To answer one of my own questions -- the acoustical caulk in question is almost certain Temco. I've never used that brand but I've seen it referenced a number of times with relation to passive homes and apparently it is super nasty to work with.

        When I think of acoustical caulk, I think of it from the traditional dedicated home theater and studio perspective. The acoustical caulk used in that realm tends to be very similar visually and texture to "normal" latex caulk. The only difference is that it is supposed to not fully cure and so will remain flexible long-term. I use SC 175 since I can get it in bulk for notably cheaper than Green Glue and the like.

        1. Expert Member
          MALCOLM TAYLOR | | #8


          On the scale of messiness, Tremco acoustical sealant is in-between roofing-goo, and vinyl-flooring glue. If you can come home without getting some on the seats of your truck, you are a better builder than me.

        2. Expert Member
          BILL WICHERS | | #9

          Note that “acoustical sealant” and “green glue” are very different things.

          Acoustical sealant is basically caulk that never hardens. It makes a liquid(ish) gasket that stays sealed even if things move a little over time.

          Green glue DOES set up into an elastomeric compound, basically. It is intended to form a rubbery layer between two sheets of drywall to help absorb sound energy.

          You’re not the first to confuse the two a little :-)

          I second Malcolm’s statement that acoustical sealants are nasty to work with. Most of them are pretty smelly too. I only use them when absolutely necassary, and your application isn’t one of those times. Use something like duct mastic or a bit of spray foam. My guess is spray foam will actually be harder to use for this though.


          1. kurtgranroth | | #11


            Yeah, I used quite a bit of Green Glue Compound when building my home theater. But in this case, I'm not referring to that but rather the Green Glue Acoustical Caulk -- a different product from the same company.


            As an aside, when I built my theater, I don't remember Green Glue offering up a caulk/sealant and instead the products bandied about the most were SilenSeal and QuietSeal. I don't see any references to SilenSeal anymore, though, so I wonder if Green Glue or QuietRock bought them out....

      2. maine_tyler | | #10

        While threads may not reach fully gas tight without the help of slippery tape, if tightened well, I bet they aren't bad. I've made fittings on compressors work for a time without tape. The thread presents a fair bit of length for surface contact, and its thin enough metal to deform a bit. All Teflon tape does is reduce friction; the threads make the seal.
        My guess is the weakest point would be the nut to washer, unless the wood had some substantial wonk to it not taken up by the washer.

        You mentioned sealing the wall, exterior and interior. You also mentioned having sill gasket but not relying on it for air seal. I'm curious then, what is the air-sealing plan for the sill plate? A tape suitable for concrete/ some sort of sealant?

        1. kurtgranroth | | #12

          I haven't committed to a particular air sealing product quite yet. In general, it will be either a tape that can adhere to both concrete and OSB (and concrete and polyiso) or a liquid flashing. Siga, at least, makes a tape that would work and Prosoco and Huber both make liquid flashing products. I haven't yet dived into pricing and benefits/limitations to make a choice. At this stage, it's mostly just watching Matt Risinger and the like and absorbing the general idea.

  2. user-6184358 | | #2

    e) rubber gasket under the washer

    If you snug the nut prior to closing the wall, when the wood is down to equilibrium moisture then it should not leak air.
    You should wait to apply any sealing effort until the wood is dry.

  3. Expert Member


    It's simply not worth worrying about.

  4. maine_tyler | | #4

    Yeah, sounds more like a belt, suspenders, and helium-balloon-tied-to-pants approach.
    I would just spent the time/money to seal the planes of the wall, including the sill plate to foundation from the exterior, interior, or both.

  5. jberks | | #6

    Hey, we all get OCD... Sometimes more so than others.

    I agree with others that it's not worth the effort, but if you really want, take 30mins and hit them with a squirt of spray foam and call it a day. You'll be able to sleep better at night and at least with spray foam you'd be able to break it off to inspect the tightness of the nuts later!

    I personally get OCD with the sill to floor gap. I have spent time at the interior side with a polyurethane hybrid sealant, gunning in and towelling the gap to block out bugs. These days I don't bother with the roll out foam sill sealer, it's garbage, I just gun a bed of sealant on the floor before putting the bottom plate down or raising the wall. Saves me time and I feel better about it.

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