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Decrease attic heat in climate zone 2B?

breeseinfo | Posted in Energy Efficiency and Durability on

I am remodeling a 50 x 50 wood structure that did not have Tech Shield or insulation when built. I have build an apartment in the building (1200 sq. ft). with blown insulation to R-38 standards in the attic. However the portion of the attic not over a HVAC space is not insulated. Would 1″ foam board with foil nailed to the bottom of the 2×6 rafters with openings at the top and bottom, (5.5″ of open space between the decking and foam board), be effectivei in reducing the heat in the attic. If an area at the bottom and top would be left open to allow proper ventilation. Possiblitiies (foil, foam board, fan ).

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  1. Expert Member
    Dana Dorsett | | #1

    Would it do something? Sure.

    Would it be effective? Depends on what you mean by that.

    An inch of polyiso with foil on both sides and at least an inch of air-gap on both sides will do more than foil alone or an inch of unfaced foam. A mop-on high solar reflectance "cool roof" material might be worthwhile too, if there is simply no way to put in R30+.

  2. GBA Editor
    Martin Holladay | | #2

    I can't quite visualize what you are talking about. It sounds like there are two different parts of your attic:

    1. The portion of the attic over an HVAC space, and

    2. The portion of the attic not over an HVAC space.

    I'm not sure what you mean by "an HVAC space."

    You inform us that "the portion of the attic not over a HVAC space is not insulated." Why can't you insulate it?

  3. breeseinfo | | #3

    Thanks for the response Martin. To clarify roughly half the building is a garage where there is no need for air conditioned or heated space. The other half is now an apartments that has insulated walls and ceiling. I have floored a portion of the attice above the garage for storage. However it is really hot (+140 F). The original roof did not include tech shield and I am looking for a method to reduce the heat. My thought was to install foam board below the roof rafters with foil rather than foil alone. I would leave air space at the bottom and top so air could flow between the roof decking and the foam board on the bottom of the 2x6 rafter.

  4. GBA Editor
    Martin Holladay | | #4

    As Dana mentioned, any insulation you add to your garage roof will help a little. However, one inch of rigid foam has an R-value of R-4 to R-6, depending on what kind of foam you choose. That's not much.

    For information on insulating a sloping ceiling, see How to Build an Insulated Cathedral Ceiling.

    If your hot garage attic is connected to the attic above your house, you might consider building a wall to separate the garage attic from the attic above your house. If you do that, you should insulate the new wall.

  5. breeseinfo | | #5

    Thanks Dana and Martin
    Since I am in Zone 2A (HOT 9 months our of the year) with little if no chance of freezing weather. My biggest concern is the heat. I am wanting to use a portion of the attic for storage which will NOT be airconditioned and is above the garage which is also not airconditioned. The Hot water heater and central air are located in this portion of the attic. I see a product called Thermax that indicated a R13 value. This looks to be the best alternative to reduce the heat in the attic. If I screw the (foil /foam board) to the bottom of the 2x8 roof rafters this will allow an 7.5" airspace for the super hot air to vent from the bottom to the top of the roof line. My thought this is more effecient than the foil radiant barrier alone and safer from a moisture prospective than full insulation between the rafters. I will leave 4' open at the bottom before I start the sheathing and 4' plus at the top to allow the hot air to escape the roof vents. Do you see any problems. I read the catherdral ceiling information and was not able to relate to the HOT climate issue that I am faced with here. HELP (foam board, thermax, rafter insulation)

  6. GBA Editor
    Martin Holladay | | #6

    Insulation works in hot climates as well as cold climates. In a cold climate, the insulation keeps the heat inside the building. In a hot climate, the insulation prevents the heat from flowing into the building. The principle is the same in both climates. The thicker the insulation, the higher the R-value -- and the better the insulation performs.

    You are correct that R-13 insulation is better than a radiant barrier.

    If this attic includes your air handler and ductwork, it makes sense to create an unvented, conditioned attic -- and if possible, your insulation should have a higher R-value than R-13. You don't want to leave a gap at the bottom of the insulation; nor do you want to leave a gap at the top of the insulation. You want to insulate the entire roof plane. More information here: Creating a Conditioned Attic.

  7. breeseinfo | | #7

    Will it be ok to leave the insulation (blown R 38) above the apartment ceiling since roughly half of the attic space is over the garage which is not air conditioned or heated.

  8. GBA Editor
    Martin Holladay | | #8

    You have not described your attics clearly, so it's hard to answer you question.

    It sounds like you have an attached garage, and that your garage attic is connected to the attic above your house. Is that correct? If it is, you need to install an insulated wall that separates the two attics.

    If you have R-38 insulation above the ceiling of your house, I certainly advise you to leave it there.

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