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Using EPS for roof insulation?

Matt Bowman | Posted in GBA Pro Help on

I am building a home in zone 6A. It is a reclaimed barn frame that has gone through engineering, engineer wants double 9.5″ LVL’s 24″ on center. I would like to use 8″ 1.5lb EPS between the rafters giving the R value required. My thought was to spray foam the corner where the rafter meets the interior sheathing and while still curing slip the 12′ lengths of EPS in. Then sealing any other seems w/ spray foam. This would also provide for 1.5″ vented cold roof. The roof system would consist of (from interior out) pole rafters, 3/4″ reclaimed wood sheathing, Grace triflex, then the EPS block. Would this be a correct use and do you see any problems with it? Thanks!

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

    The best way to install rigid foam insulation is as a continuous layer on the exterior side of the sheathing.

    In general, it's a bad idea to cut the foam into strips and install it between the rafters. If you cut the foam into strips, you introduce opportunities for air leaks and voids, and you guarantee the fact that you will have thermal bridging through the rafters.

  2. User avatar Expert Member
    Dana Dorsett | | #2

    In zone 6A if you put 5" of EPS above the exterior roof deck (~R24) you'd then be just fine putting 9.5" of cheap cellulose (~R32) in those cavities between the exterior roof and the reclaimed planking, and have an R50-ish roof (after factoring in the thermal bridging). If 5" of EPS is too thick, you can get there with 4" of polyiso.

    That would be slightly over the IRC 2012 prescriptive R49 center-cavity value for attic insulation for zone 6, but roughly double the performance of the cut'n'cobbled ~R32 foam approach.

    It's a simpler installation, and would likely be cheaper in the end too.

    Keep the Triflex membrane in the stackup, but on the exterior of the outer roof deck, under the foam.

    The cellulose can be damp-sprayed from the exterior prior to putting up the Triflex, and would not need to be "dense packed" in that stackup- anything in the neighborhood of 2.5lbs/cubic foot won't settle when temperture & humidity stabilized by the exterior foam. (Dry or damp-spraying "stabilized" forumla would be further insurance against settling.) The exterior foam is sufficient exterior R to protect the roof deck from wintertime moisture drives- no interior vapor retarder necessary.

    Materials-wise EPS and iso run about 10 cents per R per square foot, cellulose about 3 cents. The reduction in foam thickness from R32 to R24 saves about 60 cents per foot on the foam, but that 60 cents pays for about R20 (or about 2/3) of the cellulose. With full sheets of foam you don't have the labor and scrap rates of a cut'n'cobble, and you save the material & labor cost of sealing all of those seams with expanding foam too.

    The type of roofing matters. If composite shingles you'll be buying a nailer deck of half-inch OSB over the foam, through-screwed to the LVLs with pancake heat timber screws 24" o.c., which is a cost adder which may put you somewhat higher in cost, but its twice the performance, easier to build reliably, and more resilient in the end.

  3. Matt Bowman | | #3

    in your scenario is there an OSB layer attached to the LVL's then the layer of EPS then the roof deck?

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