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Which attic configuration is best?

Trevor Bowden | Posted in Energy Efficiency and Durability on

I am going to build a home in Melbourne, Florida, climate zone 9B or 10A depending on the source. The home will have wrap-around porch varying in depth from 6 – 10 feet around the majority of the exterior. I will be installing a white metal roof and cold formed steel trusses. I have read a fair amount of literature on the topic of attic/roof configuration, including papers, reports and forums. There are two configurations which I believe would perform the best energy-wise. The first is a vented attic with ductwork contained in the conditioned space below and insulation above the ceiling. The second is an unvented, conditioned attic with open cell spray foam insulation under the roof deck and which contains the ductwork. I have not found any detailed information comparing my two desired configurations (both with a white metal roof) and I’m not sure which simulation tool would be the best for comparison. Does anyone have insight on which configuration might perform the best thermally?

If I chose an unvented attic, I’d try to reduce the surface area to insulate and reduce the volume to condition. With that in mind, I would not apply insulation in the attic space above the porch and garage, but instead add wall sheathing to extend the home exterior wall plane to the roof deck and apply the insulation to that vertical surface. I am wondering though if this would cause any thermal performance problems due to stagnant air being baked in those unconditioned, unventilated volumes during hot summer days. Would this approach cause any thermal issues?

Although there are a number of other considerations, including moisture control and cost of installation, I’d like to get some advice on which attic configuration would be the best thermal performer. Any feedback would be appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Trevor,
    Either approach would work, as long as you pay attention to airtightness, and as long as you make sure that your house has an uninterrupted thermal boundary.

    A hot attic above a garage will not cause any moisture problems.

    The biggest potential problem is your choice of steel roof trusses. If you want to use spray foam to create an unvented conditioned attic, make sure that you come up with some way to address thermal bridging through the steel framing. You would need to be sure that all steel members are fully covered with a thick layer of spray foam. It would probably be better to install all of your insulation above the roof sheathing if you insist on using steel framing (for example, by installing a SIP roof or thick nailbase).

  2. Trevor Bowden | | #2

    Mr. Holladay,

    I thought I had responded already, but I guess I had a computer operator error. I apologize. Thank you for the feedback. I will certainly research thermal bridging and avoid it.

    I still have a couple of questions. With the second configuration, the unvented attic, I would not add ventilation to the unconditioned attic areas over the porches and garage. Thermally, do you believe this would cause adverse effects in that the unconditioned attic space would conduct too much thermally into the conditioned attic space? My unproven theory is based on a pool of standing water being heated by the sun. If the air in the unconditioned space was stagnant, do you think it would heat up and cause problems thermally?

    My last question. Overall, comparing the two configurations, which might perform best thermally?

    Thanks,
    Trevor

  3. GBA Editor
    Martin Holladay | | #3

    Trevor,
    Q. "Do you believe this would cause adverse effects in that the unconditioned attic space would conduct too much thermally into the conditioned attic space?"

    A. If you have two types of adjacent attic spaces -- one conditioned, and one unconditioned -- you obviously need a wall (or sloping roof) to separate the two spaces. Since one side is conditioned and the other side is unconditioned, this wall (or sloping roof) needs to be insulated and air sealed. Once the barrier between the two spaces is insulated and air sealed, you don't have to worry about thermal transfer.

    Q. "Comparing the two configurations [a vented unconditioned attic without any ductwork, and an unvented conditioned attic with ductwork], which might perform best thermally?"

    A. Either configuration will work fine, as long as you do a good job of air sealing and as long as your insulation is thick enough. Careful workmanship and good insulation specifications are much more important than which configuration you choose.

    Choose either one, and then concentrate on doing a good job so that there aren't any air leaks, and so that your insulation is thick enough.

  4. Curt Kinder | | #4

    Whatever way you keep the ductwork out of a vented hot attic, you will be ahead of 95% of our fellow Floridians.

    I strongly recommend you have blower door test done after air sealing but before drywall goes up. Results are often very enlightening and can guide a much tighter home.

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