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Air Gap and Thermal Control Continuity

matt2021 | Posted in General Questions on

For an extension room I am creating, from an existing porch, I would like to redo the roof, so as to have some rigid insulation on the exterior.  I will be using Hunter Cool-Vent panels (the ones with 1″ ISO board as part of the panel).  Under the panels, against the existing roof’s deck (which is plywood), I will apply additional rigid insulation, most likely 2″ XPS, which I happen to have.  (See the attached picture for the type of assembly I am looking to achieve.) Between the ceiling’s rafters, there will be R23 Rockwool. I have a question that has to do with the existing structure: Under the existing plywood, there is a 1.5″ air gap, then there are 1″ tongue-and-groove boards nailed to the rafters.  So, the Rockwool insulation will be against the T&G boards.  Does the existing air gap represent a problem for my intended assembly?  Doesn’t it interrupt the insulation layers, and cool down the back of the existing plywood sheathing, on top of which I intend to apply the rigid foam insulation, the Hunter Cool-Vent panels, then the underlayment and shingles?  Will such a gap create a loss of heat from the room?  (Should I “shield” the Rockwool insulation that will be between the rafters with some type of moisture or some type of air barrier?) Of course, I hope the feedback will be that I have nothing to worry, and that I can leave things underneath the existing plywood sheathing as they, and build the new roof assembly on top of that sheathing.  I would greatly prefer not to remove the plywood; the 1.5″ gap has some wires that run through it (for the ceiling lights); the T&G gives structure to the roof/ceiling; and there is no easy way to fill that 1.5″ air gap.  But I am worried that I might be missing something, and my intended assembly not work because of that air gap. (My question is made urgent by the fact that I need to buy the Hunter panels AND need to know whether to submit to my township’s building office the current plan, with the air gap, or modify it.) Thanks for any input!

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Replies

  1. Expert Member
    Michael Maines | | #1

    What climate zone are you in, and do you know the ratio of exterior to interior insulation required to minimize the risk of condensation?

    More information here:
    https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
    https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    The IRC (International Residential Code) for an assembly like yours requires the insulation layers to be as close together as possible, because when there are gaps--aka interstitial spaces--bad things can happen, such as condensation or convection currents that render the outer layer of insulation ineffective. It's best to fill the vent space, but at bare minimum you need to air-seal and insulate the vent openings at the top and bottom of the vent channels.

    What is your reason for going with vented nailbase panels rather than conventional nailbase?

    1. matt2021 | | #2

      Thank, Michael! In answer to your questions:

      - Zone 5 (hence, unless the 1.5" air gap represents a problem, I have the correct ratio between exterior and interior insulation; it will be a min. of R15 on the exterior and R23 on the interior).

      - There are no air vents, on any of the sides, for that 1.5" air gap, though, if need be, I can have any possible openings taped or the like.

      - Shall I try to block air/vapor flow on the interior, between the rafters and, if so, using what in your view?

      - I would have been happy to go for a regular nail base, and have an additional 1" of insulation rather than an air gap. However, I cannot find a roofer who understands the idea of a completely unvented roof, AND I have found the Cool-Vent panels at a fraction of the cost (and that, too, matters). So, all things considered, if that 1.5" air gap does not create problems, I'd like to go for that assembly. It would be a great roof, I think.

      What do you think?

      1. Expert Member
        Michael Maines | | #3

        Your total roof R-value and the R-value of the exterior portion are lower than what the IRC requires for new construction, but your ratio just squeaks by at 40% exterior/impermeable.

        It would be good to fill the existing vent space if possible, even if it's not perfectly done, it will cut down on heat loss. You might try dense-packed cellulose or fiberglass from the exterior.

        I don't see any problems with the vented Hunter panels, just no performance benefit either, other than possibly a very slightly longer shingle life. But cost-savings are always desirable.

        1. Deleted | | #4

          Deleted

          1. matt2021 | | #5

            Michael,

            As you might recall from my other post, things have evolved. I will have to abandon the plan described in this post, and propose something else to the township. That's sad, but in my circumstances there's no viable way around the township's objections, unfortunately.

            Your advice will still be helpful when pursuing the alternative I am now thinking of. So, again, thanks!

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