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Exterior XPS rigid foam in Climate Zone 3A

Stephan H | Posted in General Questions on

I am building a new house in far northeast Texas, which I believe is climate Zone 3A.

I am planning on traditional fiberglass and/or cellulose insulation between studs, and a vented attic with vents in the soffit and a roof ridge vent, and blown-in insulation on the attic floor.

My plan for the exterior walls is first OSB sheathing, followed by 1″ of rigid XPS foam, Tyvek house wrap, then Hardie cement-board siding.

Does anyone see a potential problem with the design of the exterior walls as far as the rigid foam causing moisture to be trapped between itself and the OSB?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    1. Green builders avoid the use of XPS. Instead, use polyiso or EPS. More info here: "Choosing Rigid Foam."

    2. In your climate zone, your exterior rigid foam can be any thickness you want, without worries about wintertime condensation or moisture accumulation.

    3. Of course it's always possible to trap moisture between exterior rigid foam and OSB -- but only if you do a terrible job installing your water-resistive barrier (WRB) or your flashing. Get those details right, and everything should be OK. Remember, including a rainscreen gap is always a good idea.

    More information is available in the following two articles:

    "How to Install Rigid Foam Sheathing"

    "All About Rainscreens"

    1. Stephan H | | #3

      Martin - If I use polyiso, the only option I have in my local building supply is a foil-faced version. Is it acceptable to put my house wrap exterior to the foil face? In other words, to have studs>OSB>foil-faced polyiso>housewrap>concrete-board siding in that sequence from interior to exterior? Do I need to tape the polyiso since it will have housewrap covering the seams?

      1. User avatar
        Dana Dorsett | | #4

        Polyiso is still preferred, and yes, tape the seams with a temperature rated foil tape, even though there is housewrap going over it in your stackup.

        1. Stephan H | | #5

          Thanks, Dana. So it is acceptable to put things in the sequence that I outlined in my question to Martin? Will use your suggestion and tape the seams.

          1. User avatar GBA Editor
            Martin Holladay | | #6

            Yes, you can do that. Just remember that your window flashing must be integrated with your WRB. More information here: "Where Does the Housewrap Go?"

          2. User avatar
            Dana Dorsett | | #8

            Yes, the stackup sequence as-outlined works, provided the housewrap laps properly with the window flashing. But putting the housewrap on the exterior side of the foam gives up a potential ~R1 of performance, since there isn't an air gap between the foil facer and the next layer, which in your stackup is the housewrap.

            With only 1" of foam the aesthetic differences between innie vs. outie mount on the windows is negligible. With the housewrap between the foam and OSB the exterior side foil can be utilized for thermal benefit, and if a crinkled type housewrap the OSB can dry to the exterior as well as the interior, whereas with the OSB flat to the foam it's only drying toward the interior, which is fine. In the good/better/best of worlds there's a hint more resilience to having crinkle-wrap between the OSB and foam.

      2. Stephan H | | #7

        Thanks, Martin and Dana. I will have 'outie' windows, so will integrate the window flashing with the WRB. Just wondering - what is the purpose of taping the seams on the polyiso since I will be covering the polyiso with the WRB house wrap? Will I really lose that much more heat if I leave unwrapped? I thought the main purpose of the polyiso was to stop thermal bridging.

        1. User avatar
          Dana Dorsett | | #9

          Foam can never "...stop thermal bridging...", it only reduces the effects of that thermal bridging. A 2x6 is about R6.6, adding R6.5 polyiso cuts the heat flow through tbe framing fraction by about half, but it's still heat flow per unit area than the part between the studs. With R20 cellulose the you're looking at R20 + R6.5 = R26.5 at center bay, but R6.6 + R6.5= R13.1 at the framing, still twice the heat flow.

          While, thermal bridging it the primary reason for the foam, since it's already going up, get the most you can out of it. The redundancy in air barrier layers is cheap and effective, and taping the seams also makes the foil facer a pretty good backup drain plane , also "worth it." Foam board changes dimensions over time, and even an 1/8" air gap between sheets becomes an air convection channel and bulk moisture channel. Taping blocks that, making the assembly more reliable & resilient over the long term.

          1. Stephan H | | #10

            In my area, although in far northeast Texas, building with 2 x 6 framing is unusual unless it is a plumbing wall. My house, and almost all I've ever seen here, uses 2 x 4 framing with r-13 insulation between studs. I'm afraid that using an 'innie' window design, with house wrap between the osb and the exterior rigid foam, will blow my builder's mind. Hence, my plan for the 'outie' window, with the house wrap exterior to the polyiso.

            Easier to plan on the layer sequence that I outlined above, which will give me r-13 in the walls and r-6 with with polyiso. Our concern is more loss of cool air inside during the summer than it is heat loss in the winter.

  2. User avatar
    Dana Dorsett | | #2

    As long as there isn't an interior side vapor barrier such as polyethylene sheeting behind the wallboard or foil/vinyl wall paper is should work fine. The window flashing has to extend to the exterior side of the foam if that's where the housewrap is going to be.

    If it's not too late, swap the XPS for foil faced polyisocyanurate. The labeled R5 for the 1" XPS is only warranteed to R4.5, and even that might be optimistic in 50 years. The performance of XPS drops as it's high global warming potential HFC blowing agents diffuse out over time. At full depletion it's performance will be R4.2, the same as EPS of similar density & thickness. Polyiso is blown with pentane at about 1/200th the lifecycle warming potential, has a higher R/inch, and is less dependent upon the blowing agent for performance. At 1" foil faced polyiso is labeled R6 - R6.5, (at 3/4" it's labeled R5) and even with age & temperature it will still beat R5 in your climate, more if using the low-E characteristics of the foil by designing in an air gap/drain space between the foam and siding.

    With even a small air space between the foil facer and the next layer it adds another ~R1 of average performance. Using a crinkle type housewrap (eg Tyvek DrainWrap) between the foam and OSB offers some amount of capillary break and a drainage path, and a drying path to the exterior for the OSB, and doesn't interfere with the thermal performance of the exterior foil facer.

  3. User avatar
    Dana Dorsett | | #11

    >"Easier to plan on the layer sequence that I outlined above, which will give me r-13 in the walls and r-6 with with polyiso. Our concern is more loss of cool air inside during the summer than it is heat loss in the winter."

    With a very careful design it's now possible to hit Net Zero Energy with a PV array that still fits on the roof in the Red River Valley Texas with a 2x4/R13 + R6 continuous insulation wall, but it's easier with 2x6/R20 + R6 c.i.. With the 2x4 + 1" foam stackup the "whole wall R" after thermal bridging comes in at around R16- R17, with a 2x6 + 1" polyiso it's about R22-R23. Backing off to half-inch polyiso with a 2x6 wall would be about R20 after thermal bridging.

    You are in US climate zone 3- take a look at the whole-assembly R values in the zone 3 row of Table 2, page 3 of this document:

    They're suggesting that R20 whole-wall is in the the middle of the obviously financially rational range, but a bit more would still be rational in locations with lower than US average construction costs.

    Though it wasn't intended for that purpose, in zones 5 and lower that turns out to be about what it takes to hit Net Zero fairly easily.

    Heat rejection at peak siding temperature is another plus for having a foil facer and an air gap on the exterior side in your climate, but if it's easier to get contractors to build an outie-mount than an innie correctly, it's a fairly academic difference.

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