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Exterior wall construction layout — Am I on the right track?

MrEd72 | Posted in Green Building Techniques on

I am in Zone 5a (one county north of 4a) in central Indiana. We are currently building a home and detached garage. We will be building the garage first and living in it while we construct the house.

I want to apply some green building techniques to the garage as a “test run” for the house. Please comment on my design:

2″ XPS under slab with radiant heat, 2″ XPS on exterior and basement walls.

Exterior walls (from interior to exterior) will be 1/2″ drywall, 2×6-16″oc with 3 1/2″ batt insulation over 2″ open cell foam, 7/16″ osb sheathing, 3/4″ T&G XPS, “HardiWrap”, Hardi-Plank siding.

Roof: (from interior to exterior) will be fiberglass batt to fill rafter cavity over 2″ open cell foam, 7/16 OSB sheathing, 2″ XPS, 7/16 OSB sheathing, 30# felt, with “Ice&Water shield” eave, asphalt shingle or standing-seam metal roofing.

I realize that the garage will not be as tight as the house but we want to use similar techniques.

Thank you for your input.

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  1. user-928793 | | #1

    skip the foam and batt, opt for dense-pack cellulose. The foam will likely leave a non-flat surface which can cause non-insulated air pockets in between the foam and batts, these airspaces can seriously impair the R-value of the system. Additionally cellulose is likely to be cheaper. Replace the OSB with CDX ply- OSB is very prone to moisture damage. Why 16" O.C. framing and not 24"?

  2. Jim Bannon | | #2

    You're on the right track, but you might consider going with at least 1" of foil-faced polyiso board on the exterior wall instead of the 3/4" XPS. That would up your R-value, better thermal break for the framing/sheathing. Some have suggested that a taped foil face actually eliminates the necessity of the housewrap but I don't necessarily agree. 2" of polyiso would be even better actually, but then you would probably have to add vertical strapping to hang the siding.

    4" of polyiso on the roof would be my ideal. What are you planning for rafter depth? Will the roof be hot or cold?

  3. MrEd72 | | #3

    I have been back and forth on both issues CDX vs OSB and 16" OC vs 24" OC. cost mostly driving the OSB choice and it seems what i am reading tells me OSB has a greater shear strenght. 16" is needed for support of the second floor I-Joists with 28' span 16"OC, I thought of going 24" on the gable ends but thought I should keep everything the same. Also, my framer is reluctant to go 24" OC with the Hardi-Plank siding

  4. wjrobinson | | #4

    Use foam in one place or no place. Not on two sides of sheathing ever.

  5. MrEd72 | | #5

    Hot roof with 2x10 rafters,
    The XPS would be taped and the house wrap is primarily for a drainage plane behind the siding

  6. MrEd72 | | #6

    My understanding (ignorance) was that open cell would breathe. Should I put xps over studs then sheathing? but that would decrease my shear strength. I want the thermal break so I may need to eliminate the spray foam?

  7. wjrobinson | | #7

    Ed, my friends that use foam sheathing use it. Those that use spray foam use it and no foam sheathing. Then there are people on this site that prefer cellulose and double stud walls.

    You are making up details in your head that are more than one way combined. Too much of a good thing is not always better.

    This site and Building Science Corp have plenty of details you can purchase or find online for a system to use. Just don't make up your own out of the blue from this and that.

  8. user-928793 | | #8

    Also check out Martin's blog about minimum exterior foam thickness for climate zones.
    The 3/4" T&G foam wouldn't be enough according to the IRC for your climate zone. You would need and 1 1/2"
    In a 2x6 wall with dense-pack cellulose and 2" of XPS you would have a clear wall value of R-29, and you would have addressed the thermal break issues.

  9. user-928793 | | #9

    The Hardi-Plank siding warranty is probably the limiting factor for stud spacing. One reason not so fond of Hardi-Plank (siding choices shouldn't limit my structure choices). The IRC (2006 - your current code in Indiana) allows for 2x6 studs supporting one floor, roof and ceiling to be spaced 24" O.C. (Table R602.3(5)). Unless you are in a restrictive seismic zone then 15/32 CDX ply will offer plenty of shear strength.

  10. user-928793 | | #10

    From what I understand about ice dams it seems to me that an unvented hot roof is begging for them to occur. You could keep your details of foam and batt in the rafters but don't also include that exterior foam. Consider using 2x furring strips on the sheathing then your second layer of sheathing and then roofing. This would allow you to keep your roof cool by having ventilation and eliminate the ice dam problem.

  11. GBA Editor
    Martin Holladay | | #11

    1. You are describing the use of open-cell spray polyurethane foam with a flash-and-batt system. That won't work. Flash-and-batt always requires closed-cell foam, not open-cell foam, for two reasons: (a) a thin layer of open-cell foam doesn't create an air barrier, and (b) a thin layer of open-cell foam doesn't have enough R-value to keep your sheathing above the dew point.

    2. As others have pointed out, you don't want to sandwich OSB between two layers of foam. Put foam one one side of your sheathing, not both. My vote: the foam goes outside of the sheathing, not between your framing members.

    3. Pay attention to minimum foam thickness, as others have pointed out. (See Calculating the Minimum Thickness of Rigid Foam Sheathing.)

    4. Slabs with in-floor hydronic heat need better insulation than slabs without hydronic heat. You will never be able to add more insulation under your slab at a later date. If I were you, I wouldn't use less than 4" of XPS under a heated slab. Don't forget to include vertical insulation at the slab perimeter.

    5. You won't have any ice damming problems with a hot roof, as long as you address thermal bridging through the rafters, pay attention to air sealing, and include above-code levels of insulation.

  12. 2tePuaao2B | | #12

    I believe that you said that you wanted to apply some "Green" techniques to your building.
    Please be sure to research current and potential problems, health and enviroment, that are associated with the use of foam products. I hardly recommend the use of petro chemicals as green building material. Greenwash at best.
    Robert Riversong could probably offer some of the best advise available today but he has been barred from this site.

  13. jbmoyer | | #13


    In new construction there is nothing that beats a double wall with dense pack cellulose. I have done a cost analysis of different wall assemblies-- double walls, SIPs, ICF, exterior insulation/2x6, spray foam/2x6, and double walls are the most cost effective.

    Realize 1" of foam on the exterior is not enough insulation for you climate. 2+ inches of foam creates all sorts of cladding fastening and drainage plane/WRB/flashing issues.

    I've given up on exterior foam in new construction for these reasons, not to mention the environmental impact of plastic.

    Also, any wall should be air tight. Make sure your builder and home designer/architect understand this. If they don't have air sealing in their details, job scope, and sub contractor agreements, find different building and design professionals.

  14. wjrobinson | | #14

    Brett, good informative post to do with the choice of not using foam. I use foam sheathing and Icynene spray foam, but am very interested in becoming as truly green a builder as possible over the rest of my building career. The RPI natural foam is coming along fast too as an optional product. I believe our future will benefit from both high and low tech and mostly from the combination of both which the mushroom foam is.

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