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Community and Q&A

Unvented attached garage attic

Cody Sibell | Posted in General Questions on


I’m in the process of an exterior renovation turning a poorly-built 1970s subdivison ranch into a pretty good house.  The sheathing is being stripped, cavities re-insulated with Roxul, new plywood, Henry Blueskin peel-and-stick WRB, .75″ Dow Thermax, Slicker rainscreen and LP Smartside. The attic is being converted to unvented by spraying 3 inches of closed-cell foam against the roof deck and filling the remainder of the cavity with open-cell.

The home has an attached garage that shares the same gable roof with the conditioned spaces. The side of the garage is separated from the main attic by sheathing. The rear of the garage roof is open to attic space over a laundry roof and bathroom, which will be sealed off.

The exterior garage wall faces west, and the roof has a west facing gable, so the solar heat gain in the summer is extreme.  There is no ridge vent. To combat the summer heat gain, I plan on insulating the garage roof and exterior wall to the same degree as the rest of the house. I’m wondering if I need to provide for mechanical ventilation of the garage. Despite all the insulation, I presume there will still be some solar gain in the summer that will raise the garage temperature above ambient during the day, and it would take forever to cool down at night.  Am I on the right track, or will the garage be fine sealed up like a thermos (excepting the 16′ overhead door)?

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  1. Cody Sibell | | #1


  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Cody,

    People often vent garages because of air quality issues, specifically to remove exhaust fumes. For this you can install an exhaust fan. However, ventilating a garage to condition it, particularly in the summer may not be a great idea as summer air tends to be humid and could potentially cause moisture problems. Though the opposite is true in the winter, you could dry a garage by bringing in outdoor air. I'm not convinced that you'll have overheating problems once the space is well insulated and air sealed. I'm also not sure why it would matter. Are you planning on using your garage as living space?

  3. Cody Sibell | | #3

    Thanks for the reply! I’m not planning on using it for living space, just storage and cars. I was just concerned that a completely sealed west-facing garage, even if insulated, would gain heat during the day and never lose it at night. I suppose that doesn’t really matter - it’s just a once in a lifetime project and I’m trying to get everything right. We’re I to decide to ventilate it for air quality or conditioning reasons, would balanced interlocked 200cfm intake and exhaust fans seem reasonable?

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