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Rafter Bays, Foam Board, Fiberglass Batts, and Venting

goldenfab | Posted in Green Building Techniques on

I live outside Prescott, AZ. Zone 4. New construction, vaulted ceilings everywhere, 16″ deep TJIs, 8/12 roof pitch, standing seam metal roof attached directly to 5/8 OSB roof deck. I was thinking of using 1/2″ EPS foam board with the foil face up against the TJI top web (bottom of) which would leave a ~2″ air gap between foam and OSB roof deck. Soffit to ridge vent would channel through this air gap only. Directly below and touching the foam would be R-49 fiberglass batts. Drywall would be at attached to the bottom of the TJIs and contact the fiberglass batts and would also create an air barrier. I’m guessing I would should leave a small gap between the foam board panels so I’m not creating a second air barrier? Is it going to be an issue with fiberglass batts sandwiched between the drywall and the foam board and not having an air gap? Seems to me the R value would be improved from the foil face on the foam (radiant barrier benefit) and and less wind washing on the fiberglass but I don’t know if I should be concerned about moisture buildup. Thanks, -Adam

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Adam,

    The worry about sandwiching materials in a wall or roof is about vapour, not air movement. The more air-barriers a roof has the better, so I wouldn't worry about leaving gaps in the baffles. However, foil-faced EPS is vapour-closed, meaning there is no good path for moisture to move through and get to the vent channels so it can dry, but gaps won't help with that either. In most climates you need a more vapour-open material for the baffles, which could be as simple as using un-faced EPS, or plywood, or house-wrap. However in dry Arizona you may be just fine with the foil faced.

  2. goldenfab | | #2

    Thanks for the reply. I think I understand what you are saying about vapor vs air movement. Yes very dry here so my guess too is it would be fine. As far as I can tell there is nothing in the code that prohibits it. I'm thinking of going for it. If anyone has experience as to it causing problems please let me know. Finishing up plumbing and electric now and then will start on insulation.

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