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Will Bin (shellac-based primer) over 1.5″ of 2-pound foam over 6″ of polyiso in 2×10 rafter bays give me a vapor barrier?

John Gorman | Posted in Energy Efficiency and Durability on

In a Massachusetts coastal home I have 6″ of polyiso in the rafter bays. If I spray 1.5″ of 2 pound closed cell foam I get close to a vapor barrier. Will painting/sealing the cured foam with a shellac based primer (like Bin or Killz) give the code required warm side vapor barrier?

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Replies

  1. Torsten Hansen | | #1

    John,
    The closed cell foam will give you a perm rating of less than 1, which should be all you need.

  2. GBA Editor
    Martin Holladay | | #2

    John,
    The polyiso is a vapor retarder. The spray foam is also a vapor retarder. You've already got your belt and suspenders, so save your money and skip the shellac.

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