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Am I making a mistake building my house with a sealed attic?

4x4crew | Posted in Energy Efficiency and Durability on

I am building my own home and I want it to last for my son, and be energy efficient. My plan is to build it with a sealed attic with spray foam on the underside of the roof decking. On the top side I want to use a synthetic roof paper covered with a preferred radiant barrier. Then put down 1×4 vertical battens, and then a layer of 2×4 horizontal battens, and finally my metal roof.

My thinking was, since the roof can draw air at the bottom and vent it out of the top I should drastically reduced the amount of heat entering the attic. Any moisture under the metal would be dried by the air flow. And if I used open cell foam moisture could actually dry out of the attic and through decking.

But the more I read on it all the more I get confussed.

Can someone tell me the best practice for central Arkansas which is zone 3a?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Without an air gap between the radiant barrier and adjacent layers, the benefits are exactly ZERO, but with the cross-ventilated battens as-described it'll deliver about R2 on a sloped roof, a bit more during the cooling season on a very low-angle roof.

    In zone 3A you need at least R5 out of R38 (13%) of the total R to be above the roof deck to keep the roof deck dry enough over a winter. See TABLE R806.5:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

    You can get there with a continuous layer 1" of rigid foil-faced polyiso or 1.5" of foil-faced EPS strapped to the roof deck with your furring/battens through-screwed to the rafters with pancake head timber screws, and 8-9" of half-pound density polyurethane on the underside of the roof deck.

    It's probably easier to use 2x4 furring and only one direction rather than cross ventilating it. Depending on how the fastening system for the standing seam works, you may be able to cross-ventilate it by segmenting the furring by leaving 1' gaps on some regular spacing.

  2. GBA Editor
    Martin Holladay | | #2

    Eric,
    If it's facing upward toward an air space, the radiant barrier will provide a small thermal benefit -- at least until it gets dusty. (Once it gets dusty, it doesn't do much anymore.) In general, investing in insulation usually gives you more bang for your buck than investing in a radiant barrier.

    Most manufacturers of synthetic roofing underlayment (which is usually vapor impermeable) do not allow their products to be used unless there is a ventilation channel under the roof sheathing. In your case, I'm not sure whether this stipulation is important from a performance perspective, but it's worth mentioning that ordinary asphalt felt would probably be a better choice here.

    Your roof assembly will work, but other assemblies may be cheaper and better performing. Your approach doesn't affect thermal bridging through the rafters. To address thermal bridging, it's a good idea to have some rigid foam above the roof sheathing (my preferred approach).

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