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Flash and Batt Insulation in Climate Zone 6

hesacarpenter | Posted in Energy Efficiency and Durability on

Hi there,

I build and renovate homes in cottage country north of Brockville, Ontario Canada. We’re about 30 miles north east of Ogdensburg NY which puts us in climate zone 6 . I read with interest the article in the latest issue of FHB about flash and batt insulation. We’ve made extensive use of sprayed polyurethane foam insulation both in new homes and in renovations. Just as often, we’ve fought with the local building officials about the use of a class I vapor retarder on the warm side of wall assemblies insulated with up to 4.5 inches of polyurethane (about R25). They want it; we say it’s redundant and can potentially lead to moisture damage.

My question is: if we were to insulate a 2×4 stud wall using the flash and batt method, is the R7.5 called for in IRC Table 1102.5.1 sufficient being that the Table assumes that rigid insulation is being installed on the outer face of the wall leaving the entire 2×4 stud bay available for batt insulation?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,
    If you are using the IRC table as a guide -- the same table that formed the basis of my recommendations in Calculating the Minimum Thickness of Rigid Foam Sheathing -- but you end up installing thinner batt insulation than assumed in the table, you're safe.

    The thicker the foam, and the thinner the batt insulation, the safer you are -- whether the wall has exterior rigid foam sheathing or spray foam on the inside of the sheathing.

    So if the table assumes that you will install 3.5 inches of fiberglass, and you actually install only 1.5 or 2 inches of fiberglass, the wall is safer, not more dangerous.

  2. xFzgt3mPgm | | #2

    not an answer, sorry but further discussion. just posted similar question on FH breaktime. I would like an opinion on the flash and batt method for a new house in the Toronto, Ontario area. 2X6 walls I would propose 2"- 2 1/2" closed cell foam then R13 fiberglassbatts (possibly compressing slightly as required to accomodate irregularities of foam) thoughts please
    ? what about interior vapour barrier as there seems to be a lot of conflicting beliefs about whether or not it is required and if as Michael has suggested is could potentially trap moisture? Thank you for any info you could provide.

  3. GBA Editor
    Martin Holladay | | #3

    Doug,
    The amount of spray foam insulation is fine. Don't install an interior polyethylene vapor barrier.

    For more information, see these two articles:
    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Why Flash and Batt Makes Sense

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