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Community and Q&A

Kneewall vapor retarder

Mwm1400 | Posted in Energy Efficiency and Durability on

I know similar questions have been asked previously, but I’ve hard a hard time determining which answer was right for my situation/climate. I have knee wall attic spaces on both sides of a dormer window. These walls currently have kraft faced R-19 batt insulation installed between 2×4 studs (with facing towards the interior), and I am planning to air seal these walls with foam board over the batts. A contractor mentioned using foam board with foil facing for a radiant barrier, but this seems that it would create a double vapor barrier with the faced insulation. 

I live in the southeast in Zone 3. What would be the best option for installing an air barrier over the current insulation without causing moisture issues? 

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Mwm

    Your plan to insulate and air seal the back of the kneewalls is okay. In your mild climate you do not have much concern for condensation, and wetting from other sources is unlikely here, unless you have an unfortunate roof leak. Anyway, the faced batts will still allow some inward drying. You may find this article helpful. It points out all of the deatils you need to consider: Insulating Behind Kneewalls.

    1. Mwm1400 | | #2

      Brian - Thanks so much for the reply and the article link. I don’t think it’s feasible for me to insulate the rafter bays and bring the area into the envelope based on space/access issues. I’m planning to add 2” furring strips to the studs to avoid compressing the R19 and will add 1.5” foil faced Johns Manville polyiso boards on top. Any other recommendations? Thanks again

  2. charlie_sullivan | | #3

    That sounds good. Foil faced boards are easy to tape for good air sealing. To get the full benefit of that, you need to think through what your air barrier is at the floor level, and how to connect to that, and at the roof. That's discussed near the end of the "establishign an air barrier" section of the article Brian linked, and illustrated in the picture shown above that section.

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