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Community and Q&A

Air-Sealing, Insulating, and Vapor Control for Rim Joist

kieran973 | Posted in General Questions on

Sorry to ask about this a few times, I’m just confused about what’s necessary in order to avoid wood rot. I’m trying to insulate the rim joist in my 100 year old house. I don’t want to use spray foam, and I can’t use rigid foam board since the joist bays are irregularly shaped with lots of wires, pipes, other joists, etc running through many of them. I would like to insulate and air seal the rim joist with low VOC products. Currently, I just have some mineral wool batts stuffed in the rim joist bays by a previous contractor. But I would like to add proper air-sealing, as well as a vapor barrier if that’s necessary. I spoke to 475 on the phone yesterday, and they suggest: 1.) air-sealing: apply one to two layers of Visconn to the interior of the rim joist — they say the 1/8 inch thickness of dried Visconn will create a sufficient air barrier, 2.) insulating: place my current mineral wool batts right up against the Visconn once it dries, and 3.) vapor barrier: cover the interior-facing side of the mineral wool batts with Intello Plus. Is this the correct way to air seal and insulate a rim joist from the inside of the house? Do I really need BOTH the Visconn AND the Intello? Could I get away with just one of these mated with the mineral wool batts? Any and all advice would be much appreciated. Thank you.

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  1. Expert Member
    Michael Maines | | #1

    The Visconn is the air control layer; the Intello can be an additional air control layer but it's primarily the vapor control layer. (NOT a vapor barrier; it simply slows the flow of water vapor so the rest of the assembly can deal with it.) With convoluted joist bays it can be hard to get the Intello fully sealed to the framing to the point that it is an air barrier.

    You need at least one air control layer and one vapor control layer. I avoid foam whenever possible and the 475 approach will work, but it's a lot more work than simply spray-foaming the rim joist, which does everything you need in one shot.

  2. Expert Member


    475 have given you advice that takes into account both air-sealing and the need for an interior vapour-retarder. You could try and combine the two functions inside the batts, but to do so effectively may be more work than doing what 475 suggested.

    When it is just acting as a vapour-retarder the Intello-plus doesn't need to be installed particularly well, but if it is to be an effective air-barrier it will need some backing at the perimeter of each joist bay so you can run a bead of sealant to bed it in and then staple.

    Those to me seem like your two options. Maybe someone else will come along with an easier one.

  3. kieran973 | | #3

    OK, thanks a lot. So it sounds like if I do a thorough job air-sealing with the Visconn, then just doing an acceptable job securing the Intello would be OK. The Visconn can be painted on or sprayed on. The sprayer is $500 - any significant advantage to spraying it as opposed to painting it?

    1. Expert Member
      MALCOLM TAYLOR | | #4


      The only advantage I can see to spraying would be speed.

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