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Community and Q&A

Sill Plate Sealant Recommendations

conor_mc | Posted in General Questions on

I’m working on a basement re-insulating and air sealing project.  What’s the recommended product for air sealing a sill plate?

I’ve seen people recommend spray foam, silicone caulk, and polyurethane caulk!  Specifically I’ve seen Pro Clima Contega HF and GreenGlue Sealant recommended for a couple similar type of locations from previous questions asked here. I think I should stay away from spray foam here right, as I’m not filling a cavity?  Any ‘consensus’ on what’s best here?

I live in southern NH.

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  1. Expert Member
    Michael Maines | | #1

    I don't think you'll find a consensus, partly because there are so many different approaches. My two favorites: with a slab-on-grade (preferably a slabless slab), simply extend the vapor retarder over the perimeter wall. With more conventional construction, I like Siga Fentrim tape applied to the exterior.

    1. conor_mc | | #2

      Thanks for the reply Michael! I understand I may not get a consensus but I was hoping for a few pros/cons or if someone saw my configuration and responded back they used XX product for the exact same or similar configuration and it worked well.

      This is not new construction and I posted an imgur link showing the location where I need to do this. I believe I asked the question the wrong way. This isn't a 'sill sealant', but more of air sealing the internal sill plate to concrete foundation wall interface. I pointed an arrow to the interface in the attached picture.

      1. Expert Member
        Michael Maines | | #4

        I see now. My first choice would be to remove the lower courses of siding and use Siga Fentrim tape on the exterior, but assuming you'd rather not do that, based on what I can see, my next choice would be Pro Clima Contega HF on the interior. Like Malcolm says, if you are planning to add foundation insulation, which is probably a good idea, that might change the recommendation. I would not use a silicone or typical polyurethane caulk because it won't stay attached to the concrete over time.

        1. conor_mc | | #5


          I'm definitely interested in the tape option. I've been working on mitigating a rodent infestation and I've had to deal with carpenter ants in the past so it seems like taping up that seem would be helpful for those issues as well as air sealing. What's the downside? Just the potentially ugly aesthetic of having tape peaking out underneath the siding? I have vinyl siding so taking off the bottom layer seems reasonably doable to me. If I go this route, can you tell me if I should attach the tape to the bottom of the WRB or to the house sheathing directly? I attached a sketch to illustrate my question.

          1. Expert Member
            BILL WICHERS | | #7

            It wouldn't hurt to put the tape behind the WRB, then put another layer of tape on top of the WRB. It's always a good idea to do flashing type things in a way where gravity helps you, which would be with the WRB over the primary tape layer. If you put the tape only over the WRB, gravity now works against you, trying to get water under the tape layer if it runs down the surface of the WRB.


          2. conor_mc | | #9

            Thanks for the Reply Bill. The website isn't letting me reply directly to you for some reason. Option C of both under and over seems good, and I hear what you're saying about Option B is better than A because of any water potentially getting behind the WRB would have trouble getting out.

            Is the only negative going this route an aesthetic thing of having tape sticking on my foundation wall top? Is there any way to hide that or get around it?

          3. Expert Member
            Michael Maines | | #11

            Conor, Fentrim is available in a gray color that matches cured concrete pretty well. You only need a 1/4" to 1/2" lap onto the concrete; siding often hangs that far below the bottom of the sheathing.

            I should have mentioned that I like using tape on the exterior because the tape is part of the continuous air control layer, which I typically place at the outside face of sheathing. If you are using the interior wall plane for your air control layer, that changes the calculation to some degree. In any case, if you don't want to see any tape on the exterior, you could carefully clean the concrete, possibly use a compatible primer, and use Contega HF or another flexible sealant at the inside of the mudsill and the outside where it meets the sheathing.

            If you use the exterior tape option, I agree with the others--best would be to shingle-lap everything so you aren't relying on adhesives, but Fentrim (and other high-quality tapes) are expensive so you may think twice about it. It's also extremely sticky, and we rely on chemical adhesion for all kinds of important things, so I would prioritize having a continuous air control layer over mechanically lapping the seams. In other words, I would tape onto the face of the WRB.

          4. conor_mc | | #14


            Thanks for the reply. That certainly makes sense if you're putting the air control layer at the exterior surface. I know I have a WRB but I'm not sure if it's taped at the seams or not. I agree the Fentrim tape is more expensive that I thought it was going to be! So you would recommend Option A above, where the tape goes over the WRB and on to the concrete foundation?

      2. Expert Member
        Michael Maines | | #15

        Conor (response to #14)--yes, as long as your WRB is in good shape I would vote for option A. Just be sure to roll or paddle the Fentrim to make sure the adhesive is fully activated.

        1. conor_mc | | #18

          Will do thanks! And I saw they sell a primer for it as well so I'll look into that.

  2. Expert Member


    How are you planning on insulating the concrete stem-walls? That may influence what advice you get.

    1. conor_mc | | #6


      To be honest I haven't finalized that detail yet as I have bigger fish to fry (attic air sealing and re-insulating). I jumped the gun on this project to help seal up all of the mouse holes in my basement.

      I know it's an energy penalty but I'm a little weary of covering the concrete stem-walls with anything at the moment because I have a couple small cracks that leak water occasionally. I'm getting gutters put on the house to help mitigate that issue moving forward.

      How would the advice change based on what I could do to the concrete wall?

      Thanks for the help I really appreciate it!

      1. Expert Member
        MALCOLM TAYLOR | | #8


        That makes sense. Prioritize what needs doing most.

        If you were adding foam to the interior and top of the stem-walls now, you might have used that as your air-seal, and not taped the exterior.

        1. conor_mc | | #10

          If I had foam coming up the stem wall and then over the exposed concrete ledge, how would I air seal that detail to the sill plate?

          1. Expert Member
            MALCOLM TAYLOR | | #12


            Where is your primary air-barrier? Do you taped sheathing, or are y0u planning an interior one?

          2. conor_mc | | #13


            The house is 23 years old and I've only owned it for a year. I doubt the OSB sheathing boards were taped at the seams but there is a Tyvek (or similar) WRB. I don't know if that's taped at the seams or not. I am doing my best to air seal from the inside e.g. spray foaming around windows, air sealing in the basement, air sealing the attic at the drywall (floor of attic), etc.

          3. Expert Member
            Deleted | | #16


          4. Expert Member
            MALCOLM TAYLOR | | #17


            Then it makes sense to use an interior sheet air-barrier (either poly or a variable perm membrane depending on where you are) on the basement framed walls. That will descend to the top of the stem-walls, and one easy way to terminate it is to run a bead of either acoustical sealant or Contega HF on the intersection of the concrete and sill-plate, bed the bottom of the membrane in it, and secure with staples. Then no separate air-sealing of the sill-plate is necessary on either the exterior or inside of the framing.

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