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Garage insulation

Ben G | Posted in Energy Efficiency and Durability on

I am about to insulate a detached garage in climate zone 7. The homeowner wants it to be insulated on the main level as well as the attic level (roof plane). The ground level has 6″ walls and 8″ ceiling joists. The attic level has 4″ walls and 6″ rafters. I am wondering if I can insulate the walls and ceiling joists with batts and then put a 2″ layer of foam on the underside of the ceiling joists before finishing the interior. In the attic, I will run a continuous 1″ air space soffit to peak and run a ridge vent. I will use 1″ xps foam to make this space, then fill the remaining space with fiberglass. Can I then cover the bottom of the joists with 2″ xps or will I be creating a moisture problem. This garage will only be heated, not air conditioned. Homeowner wants both levels insulated seperately to have a warm garage, and so the attic storage isn’t 120 degrees in the summer.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ben,
    You'll get different opinions on whether the foam / batts / foam sandwich is a good idea. Here's my opinon: if you go that route, use 1-inch EPS instead of 1-inch XPS as your ventilation baffle (because EPS is more vapor permeable than XPS). But as long as your rafter bays are dry on the day you do the work, and as long as your ceiling is installed with attention to airtightness, I think that either EPS or XPS will work -- the risk of moisture problems is very low.

    Just make sure that the total R-value of your roof insulation meets minimum code requirements. For more information, see How to Build an Insulated Cathedral Ceiling.

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