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Install foam board and housewrap?

adam.johnson25 | Posted in General Questions on

I am going to replace the old masonite siding on my house with LP Smartside and have a few questions. Currently my walls go (from the inside out) gypsum board, poly, 2×6 framing with unfaced fiberglass batts, black fiber board for sheathing, tar or felt paper, and then the siding.

I was planning on ripping the existing tar paper off and putting tyvek on the the entire house. Then I was told to put 1″ foam board over the Tyvek and then the new LP Smartside lap siding over the foam board. I live in Montana so we have cold winters and hot summers with little humidity. Is this a good plan?

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  1. GBA Editor
    Martin Holladay | | #1

    In your climate zone (Zone 6), the minimum R-value for rigid foam insulation installed on the exterior of wall sheathing on 2x6 walls is R-11.25. So your plan to install 1 inch of rigid foam won't work. To learn why, read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    If you install exterior rigid foam, you'll need 3 inches of EPS or polyiso. (Green builders try to avoid the use of XPS, since XPS is manufactured with a blowing agent that has a high global warming potential.)

    For more information on how to install rigid foam on the exterior side of your wall sheathing, see How to Install Rigid Foam Sheathing.

    You should also consider installing a rainscreen gap between the siding and the rigid foam. For more information on rainscreen gaps, see All About Rainscreens.

    Either Tyvek (a brand of plastic housewrap) or asphalt felt (tar paper) works fine as a water-resistive barrier (WRB). For more information on WRBs, see All About Water-Resistive Barriers.

  2. Dana1 | | #2

    At 3/4" thickness asphalted fiberboard is good for about R2.8, and it is fairly vapor permeable- it was a good choice for that stackup & climate. To preserve outward drying capacity you could use 2" rigid rock wool instead of foam board that would bring the fiberboard + rock wool up to R10.8, which is sufficient dew point control at the fiberboard/fiberglass boundary for all but the coldest parts of the state. With rainscreened siding you wouldn't even need that much rock wool to be moisture safe even in zone 7.

    If foam board, 2" of unfacedType-II (1.5lbs per cubic foot nominal density) EPS would add another R8.4, bringing the fiberboard + foam up to R11.2 (performing at R12-ish or higher when it's below +25F outside) and it would still be above 1-perm, still a Class-III vapor retarder, which is adequate drying capacity. Going with only 1" (more than 2 perms) would still be pretty safe, but not as safe at 2", but at 3" it's a class-II vapor retarder, which can be an issue.

    Putting 3" of foam board on the exterior (any type) brings the vapor permeance down to where it creates an inherent moisture trap: interior polyethylene (0.05 perms) and a sub-1 perm (sub 0.1 perm, if foil faced polyiso). That becomes a potential problem, since any bulk water incursions could then take a year or more to dry. If taking it to 3", unfaced EPS would be the best choice, since it would still be over 0.5 perms.

  3. adam.johnson25 | | #3

    I really don't want to use anything over 1 inch because of windows and the furring strips I would need. I think if the 1 inch is a bad idea, I will just leave the foam board off and just replace the tar paper with Tyvek and install the siding. A local contractor had just told me to use the 1in foam as it made a bid difference on tightening up the house. Thanks for the advice!

  4. Expert Member
    Dana Dorsett | | #4

    If it's 1" unfaced EPS, it'll be fine- it's over 2 perms.

    Even 1" unfaced XPS would be fine- it's still above 1 perm (though not as green, due to the blowing agents used) .

    What you currently have has a performance of about R16 whole-wall ( all-in, including the thermal bridging and the R-value of the fiberboard and interior wallboard). Adding an inch of EPS would bring it above R20, cutting the heat loss of the wall areas by about 20%.

  5. adam.johnson25 | | #5

    Thank you Dana, this makes me feel much better haha! I will go with the 1" unfaced EPS then. Do you recommend a different housewrap over Tyvek?

  6. Dana1 | | #6

    Tyvek is fine.

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