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Conditioned attic without exterior foam?

green654 | Posted in General Questions on

No zone 5. We decided to go with a conditioned attic after roof was put on. Roof does not have exterior foam insulation. Can we still create a conditioned attic with just rockwool in attic rafters? We also have a ridge vent so it’s not completely sealed.

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  1. Expert Member
    Malcolm Taylor | | #1
  2. Expert Member
    Deleted | | #2


  3. W Ramsay | | #3

    FWIW, our builder did closed cell foam on the underside of our roof, a 'hot roof'. All of the roof trusses are clearly visible when we have frost or snow due to thermal bridging. Most of the top cords of the trusses are, IIRC, 2x10 and so are not fully enveloped in foam. We cut in to the knee wall of one section and sprayed the underside of the top cords (1-2") and that largely eliminated the thermal bridging in that section. Unfortunately it'd be extremely difficult to go back and do that for the bulk of the roof. SO, if you do closed cell then make sure the top cords and a bit of the webbing is sprayed as well.

    All of these melt lines cause me some concern with longevity of the roof (cedar shake).

    The insulator also skimped on the foam. Even though much of the roof has valleys that make vents to the ridge impossible, he installed 1.25" vents and then sprayed to a TOTAL thickness of 8" from the sheathing so we actually only have about 6.75" of foam which is below the 8" needed for R-49. Irritatingly MN's top code inspector approved it, even after this fault was pointed out. Had he foamed to have proper R-49 he also would by default have encased the top cords w/ some foam and eliminated most or all of the thermal bridging.

    There's a thread on here somewhere that I started about this that you might find interesting but I was unable to locate it.

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