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Community and Q&A

Massachusetts Code for Vapor Retarders

stolzberg | Posted in General Questions on

I’m retrofitting a 1900 balloon frame farmhouse in northeast Massachusetts (Zone 5): leaving exterior alone (cement shingle over wood shingle over board sheathing), filling real 2×4 stud bays with recycled polyiso insulation  (cut and cobble), running continuous sheets of 3/4 inch polyiso across studs (taped and acoustical caulk as air barrier), then building inside that 2×3 walls with batt insulation as service cavity. Inspector wants to know about vapor barrier and I’m worried he’s going to require polyethylene sheets.  My understanding of Mass. Code is Class I or II vapor retarders must be installed on the interior side of frame walls in Zones 5-8.   However, it also seems like Class III vapor retarder is allowable if using spray foam.  Is it reasonable to argue that the cut and cobble 4 inch of polyiso (spray foamed around edges) in the bays plus continuous layer of polyiso is equivalent to spray foam and therefore qualifies for using Class III, i.e., sheetrock and latex paint?  If not, does kraft faced batt insulation in the new walls qualify as Class II?  There’s also some references to paint with a perm rating between 0.1 and 1.0 qualifying for Class II.

p.s. I’ve already lost the fireblocking battle; despite having stud bays completely filled with solid rigid foam through which I don’t believe any flame or gas could travel, the inspector is making me fireblock both top and bottom of walls.

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  1. user-2310254 | | #1


    Over the last several years, GBA has been a lot less supportive of the cut and cobble method. The issue seems to be with maintaining the air seal over time as the wood framing expands and contracts. In situations like yours, the experts generally recommend cellulose since it is greener and safer (from a moisture buffering standpoint).

    If you are carefully installing and detailing polyiso on the original studs, I believe that will function as your air and moisture barrier layers. But one of the experts may chime in a more informed opinion.

  2. brian_wiley | | #2


    I believe that Kraft paper insulation qualifies. It’s listed as such in ICC Table R702.7(1)

    It seems like it’d satisfy the code inspector. Maybe one of the more experienced members will chime in about the efficacy of cut and cobbled wall assembly and ways that it might be made safer.

  3. Jon_R | | #3

    Class II vapor retarder paint or kraft facing meet the requirement.

  4. Expert Member
    Akos | | #4

    If your 3/4 polyiso over the studs is foil faced, it is a class I vapor retarder and will satisfy code. Our code allows the vapor retarder to be located in place other than directly behind drywall as long as there is sufficient amount of insulation outboard of it to avoid condensation (in zone 5 this should around 30% of assembly R value).

  5. GBA Editor
    Kiley Jacques | | #5

    Re: “Does kraft-faced batt insulation in the new walls qualify as Class II?”

    From Martin Holladay’s article, Vapor Retarders and Vapor Barriers:

    Since 2007, the IECC has required (in section 402.5) that walls in climate zones 5 (e.g., Nevada, Ohio, Massachusetts), 6 (e.g., Vermont, Montana), 7 (e.g., northern Minnesota), 8 (e.g., northern Alaska), and marine zone 4 (Western Washington and Oregon) have a Class I or Class II vapor retarder — in other words, kraft facing or polyethylene.

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