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Do you seal the sill interior if you’ve sealed the sill on the exterior?

Ryan_SLC | Posted in General Questions on

Don’t laugh. Don’t make we mental gymnastic the title again 🙂

I applied Prosoco Joint stuff on the exterior on the seal and then put Siga Fentrim overlapping this onto the sheathing and foundation.

Is there any harm to sealing the sill with tape/caulk now from the interior?


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  1. Ryan_SLC | | #1

    I'm sorry to bump this thread. However, my nightly work while the kids are sleep is moving to my addition's conditioned crawl space :)

    I'm increasing the now contract placed 2" R10 Foamular insulation to another EPS layer, my Stego 15mil vapor barrier jut arrived today to replace the new but really abused 6mil, and then I'm thermal barrier covering the rigid and my finished closed spray foamed rim joists with Rockwool Comfortboards and Safe 'n' Sound.

    My inspector accepts the qualities of the smoke and fire as printed on the Foamular NGX, though this is funny because it's not "good" enough to leave exposed as even Owens clearly states. However, he absolutely requires thermal barrier on the spray foam on the rim joists I've done. I'm doing the full mineral wool Rockwool for my own piece of mind.

    What I'm still unsure about is that exposed plate staring at me (seal is underneath).
    -Acoustical caulk it? Tape it?
    -For insulation, build a rigid foam 2" house around it? Spray foam it?
    -Leave it alone and cover over with rockwool?

    Outside should be considered sealed by Fentrim and Prosoco Joint Filler.

    Thank you!

  2. Expert Member
    PETER G ENGLE PE | | #2

    Our local Siga rep advises that their Fentrim tape is moisture-permeable enough that sealing the bottom flange on windows (or the bottom of the window if no-flange style) is acceptable. In the past and with other systems, the guidance has been to not seal the bottom of the window to allow for drainage of incidental moisture. The caveat here is that the Fentrim should be the only product used to make that connection between the window and the exterior WRB. I'm not sure what you did with the Prosoco, but if that's another layer of sealant in this joint area, it would most likely make a seal that will stop any incidental moisture from weeping out of the bottom of the window opening. Probably not a fatal error, but you would now be relying on the window itself to be 100% waterproof for life. Sometimes that's a good bet, sometimes not, and you won't find out unless/until there is a problem. Regardless, you probably still want to air seal the inside of the sill to the rough opening or interior air barrier.

    I'm not sure about your other questions. A sketch or photo of the area might help.

    1. Ryan_SLC | | #3

      Sorry, I do appreciate the response. This isn't a question window detail. I mean interior sealing the mud sill on the crawl space concrete wall.

      1. dfvellone | | #4

        I’ve got a similar concern with the bottom plate/sill on my slab-on-grade. Gasketed and caulked to the slab, and sealed and insulated on the exterior, so I’m looking at it from the inside and wondering why not seal it/insulate the interior facing portion as well.

        1. Expert Member
          MALCOLM TAYLOR | | #5


          Secondary air-barriers provide a belt and suspenders approach. it's an individual judgment call as to whether they are necessary or redundant.

          1. Ryan_SLC | | #6

            Thanks Malcolm.

            But what would that look like? 2 inches totally encapsulating the sill plate or just let is rest with caulk on the seams?

            Thank you!

        2. Ryan_SLC | | #7

          Thanks defvellone.

          I am glad I am not the only one wondering.

          Everything I see is cover the entire area in spray foam or just caulk and leave it alone. But they are justpicture and I can't find any articles directly addressing what to "do."


  3. Ryan_SLC | | #8

    Does anyone have an idea on how or if you should insulate the exposed sill plate?

  4. andy_ | | #9

    I think this is clearly a case of you wanting to do this and looking for a justification. Go ahead. You have my permission, ne, my blessing. Go forth and air seal!

    Will it make any difference? Probably not, or maybe a little in this case, but you really want to do it so you don't have to think about what might have been had you not done it so that's worth the cost of a tube of caulk.

  5. Expert Member
    Michael Maines | | #10

    Like the rest of the house, it's important for there to be one really good air control layer; additional layers are good as long as vapor control is considered but they aren't necessary.

    It's a judgement call whether the extra effort and expense is worth it to you. I will say that on most houses, the sill plate is a major source of air infiltration, so it's not a bad place to put in some extra effort.

  6. Ryan_SLC | | #11

    So in one night I found out what you DON'T do.

    Context; insulated my rim joists with 3+ spray foam, R15 on crawl space wall, plate has the cheap pink capillary break, and I acoustically caulked the PT plate at the bottom between the open space of the concrete/pink/PT. Wall and plate have felt my hand "dry."

    So what you don't do is put a European vapor perm tape on the plate, over the exposed horizontal concrete, and onto the rigid foam where the small gap between the wall and foam were covered. My thought was tape it and then come back and put 2"+ EPS onto it.

    Went down to work last night and there was not only moisture but actual pools of water on to of the tape. I won't pretend to know what caused so much water that it went through the perm rated tape to leave puddles on top of a space that has been otherwise dry...but there you go.

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