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Community and Q&A

Insulation and vapor barrier with tongue and groove

cobragrover | Posted in Energy Efficiency and Durability on

I am in Southern Indiana which is zone 4a. My wall is this from outside in…Hardie board lap siding, house wrap, 7/16 OSB. That’s where I am now. I have 2×4 walls.
If money were no object I would do 3 1/2 inches of closed cell foam. 2nd choice would be and inch or so of closed cell on the OSB and then net the walls and use cellulose. Even an inch of foam is very expensive so the cheap option is netting the walls and blowing in 3 1/2 inches of cellulose.
If this is the way I go do I need some sort of vapor barrier on top of the cellulose and under the tongue and groove? If I need a vapor barrier what kind do I need? I have read there are different classes of vapor barrier. Anywhere from plastic sheeting, to kraft paper such as on fiberglass insulation. I don’t really want to mess with drywall if at all possible.

Can you help sort this out?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you didn't tell us whether this is a new house or an old house.

    Second, you didn't tell us the location of the tongue-and-groove boards. I'm guessing that you want to use the tongue-and-groove boards as the interior finish material for your walls.

    If you insulate your 2x4 walls with cellulose insulation, you'll end up with about R-13 in the center of each stud bay, and a whole-wall R-value (after de-rating the wall's performance due to thermal bridging through the studs) of about R-10. That's not much. It's too bad that your house doesn't have 2x6 studs or a layer of exterior rigid foam.

    If you want to insulate your stud bays with cellulose, you have to decide whether to follow the damp-spray method or the dense-pack method (using InsulWeb). Either method can work fine. To learn more, see How to Install Cellulose Insulation.

    Once your walls have been insulated, you need to install an interior air barrier. (Tongue-and-groove boards are not a good air barrier). The best air barrier under tongue-and-groove boards is a layer of gypsum wallboard. Don't forget to use airtight electrical boxes, and to seal all of the air leaks at all penetrations.

    If you pay attention to airtightness when you install your air barrier, you don't have to worry very much about vapor diffusion. However, it can't hurt to install a layer of vapor-retarder paint on your drywall before you install your tongue-and-groove boards. The paint will be an adequate vapor retarder.

  2. cobragrover | | #2

    It is new construction. A small cabin of about 700 sq ft. The hardie boards are not on yet so I guess I could do foam board on the exterior but the house wrap is on and the windows are in. Not sure if the wrap would go over or under the house wrap. Also doesn't the foam create problems with the windows and how to finish around them? The T & G boards will be my wall covering and ceiling if I can afford that much wood. I'll use the dense pack method on the cellulose.
    Another idea I had read about was using foam board between the studs in the cavity next to the OSB. Something like 1 1/2 inch thick foam board cut a little small and then foam with the great stuff brand of canned foam around the edges to seal air movement. Then dense pack cellulose.
    Another question...can I add the layer of foam to the inside after the cellulose is blown in and under the T & G?

  3. GBA Editor
    Martin Holladay | | #3

    There are lots of ways to build a wall. If you are considering installing rigid foam, then the best location for the foam is on the exterior side of the wall sheathing. But if you want to put the foam somewhere else, you can.

  4. Expert Member
    Dana Dorsett | | #4

    High R/inch foam between the studs is largely wasted, since it does nothing to block the heat transfer through the roughly 25% (typical for 16" o.c. stud spacing of wall area that is R1.2/inch framing timber, in the form of headers, window & door framing top/bottom plates, etc. If you somehow managed to jam a perfect 3.5" of closed cell foam into that wall, allowing R1 for the gypsum + sheathing + siding you come up with a "whole wall" R value of about R11.5-R12 due to this thermal bridging.

    And it's impossible to do 3.5" of closed cell in 2x3 stud bays since it's damned near impossible to trim flush with the stud edges. Best you're going to do is about 3", which brings the whole-wall R down to about R10- R10.5.

    If you did R13 cellulose or open cell foam in the cavities, the same wall comes in at R9.5-R10 whole-wall. Which means you could hit the complete-fill closed-cell foam performance by adding a half inch of EPS between the sheathing and siding. (Even just a 1/4" fan-fold XPS siding underlayment matches the performance of the more realistic 3" fill.) That amont of EPS costs about 20 cents per square foot, but you'd want to mount the siding on furring through screwed to the studs 24" o.c.rather than nailing through the EPS, to minimize the thermal bridging of a gazillion nails, so call it 30- 40 cents/foot, all-in cost of a half-inch foam-over.

    If you put in two inches of EPS you'd have a whole-wall R of about R17-R18, beating the performance of the all closed cell solution by quite a bit at the same wall thickness of a 2x6 wall, at much lower price point than the all closed cell cavity fill. If you used 2" of 1.5lb foil faced polyiso you would be at 2x the performance.

    Bottom line, save the high-R foam budget for thermally breaking the framing, where it does the most good.

    It's fine to put the foam over the already-installed house wrap, as long as the window flashing is correctly lapped with the housewrap.

    In zone 4a you don't really need exterior foam to be able to use latex-painted gypsum as the interior air & vapor retarder, but if you're using t & g you'll either have to install some sort of air-barrier and class-III vapor retarder between the interior and the cavities, or put a decent amount of R between the sheathing and siding. Using 1.5-2" of foil faced iso works, since that brings the average winter temp at your OSB to well above the typical wintertime dew-points the interior air of houses that have any reasonable ventilation rate.

  5. cobragrover | | #5

    How much R value are you assuming the closed cell foam is? I think closed cell is about R6 per inch so a perfect 3.5 inch wall would be R21 and then if I use your 25% figure that would be be about R15.75 for a perfect foamed wall plus R1 for the gypsum, sheathing, and siding which would put the wall at R16.75.

    I guess in a perfect world we could live in Yeti coolers and our 2x4's would be made of closed cell foam.

  6. cobragrover | | #6

    In my figures I didn't allow the 4.2 R value for the framing, plates, and headers so you can add another R1 which should bump it up to about R17.75.

  7. cobragrover | | #7

    I don’t think I believe ¼ inch of fan-fold siding underlayment (R1) is going to be close to the same effective R value of 3” of cellulose which, with the 25% loss factor you claim, would be about R8.

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