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Community and Q&A

Detached garage attic is 30-40 degrees hotter

bogey9651 | Posted in Energy Efficiency and Durability on

I have an insulated 28×30 detached garage main level with an uninsulated attic. on an 80+ degree day my garage stays comfortable in the mid 70’s later in the day. The attic on the other hand is 115 degrees or more and is unbearable to do anything other than to retrieve something and get back down the ladder. I have read your opinions on residential home and attic spaces but was wondering if the was an effective way to lower the attic temperature. Obviously the ridge vents aren’t enough. Am I just resigned to living with this and limit my work time to cooler months? This garage has shingles for a roof covering. Thanks in advance for your reply.

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  1. Steve Knapp CZ 3A Georgia | | #1

    Where is the insulation located in the garage attic? And where is your location or climate zone?

  2. GBA Editor
    Martin Holladay | | #2

    Insulation is installed to separate the hot exterior from the cooler interior. If you want to keep your garage attic cooler -- either because you store things up there or because you work up there -- you need to install insulation that follows your sloping roof line.

    Most people don't care if their garage attic is conditioned. If you care, you need to install insulation. Here is a link to an article that explains how to insulate a sloped roof assembly: How to Build and Insulated Cathedral Ceiling.

  3. Jon R | | #3

    If you want to occasionally drop the garage's attic temperature, you can use fans.

  4. Walter Ahlgrim | | #4

    If you have not read this blog you should.

    How many square feet of attic floor do you have?
    What is the net free area of your ridge vent?
    What is the net free area of your soffit vent?

    I would recommend 1 sqf of ridge vent for every 300 of attic floor and 2x that area in the soffit.

    This ventilation is mostly to control moisture inside the attic and keep the roof below freezing to prevent ice problem.

    I would not install any powered vent fan as they generally pull air from the living space.

    Please remember attics are not designed for human habitation and are not always going to be comfortable places to spend your time. If you want to spend time up there bring it up to code with a proper stair case, floor joists, floor, walls and insulation.


  5. Anon3 | | #5

    Solar attic fan

  6. bogey9651 | | #6

    I was interested in letting the extreme heat escape the attic so It is tolerable. I don't need to work up there but don't want to feel as if I need to evacuate the attic in short order. I only insulated the ceiling of main floor not the attic ceiling(roof).

  7. bogey9651 | | #7

    Thanks Walter. I was concerned about making the main space too warm by making changes. Our local code does not allow interior stairs in garages. It would be nice to do so and insulate the entire structure.

  8. GBA Editor
    Martin Holladay | | #8

    I don't think that you'll see much benefit from adding soffit vents, gable vents, or ridge vents. In a garage, solar-powered roof vent probably won't lead to problems, as long as you don't have a gas-powered water heater in your garage (the fan might cause backdrafting problems if you have a water heater), and as long as you aren't running an air conditioner in the garage (if you are operating an air conditioner, the attic fan would increase your energy bill and suck hot air into your garage).

    Attic fans have more disadvantages than advantages. If you don't want to spend the money for roof insulation, you could install a radiant barrier on the underside of your attic rafters. That would lower the attic temperatures somewhat -- at least until the radiant barrier gets dusty.

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