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Community and Q&A

Insulating an unheated basement garage

Dean Jacobs | Posted in Energy Efficiency and Durability on

Hi guys.

I have been framing out and insulating my basement garage. One side wall and the rear wall are below grade and I’ve built/framed one wall so far. I’ve used R-10 2″ of rigid foam on that wall and framed it out. I was going to use unfaced batts and then cover with drywall. I am also insulating the ceiling with rigid foam and using sprayfoam to air seal the edges and then use batts for the rest of the ceiling. Our master bedroom is above the garage.

So the thing is a friend of mine comes around today and said it was pointless insulating the garage interior walls as there will be no difference in temperature between the inside the outside. I can’t imagine and uninsulated basement garage with cold concrete walls being warmer then an insulated garage with 2″ of rigid foam and fiberglass batts.

Is this true?

Thanks
DJ

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Replies

  1. Dean Jacobs | | #1

    I should add I'm in the Seattle area.

  2. Donald Endsley | | #2

    Since your garage is in a basement it is not pointless. The basement (and thus the garage) will be warmer than the surrounding soil, and the outside air temp in winter. Since Seattle has more heating degree days than cooling degree days it makes sense to insulate and minimize that heat transfer.

    Since you have an attached garage, and one that is in the basement, you need to ensure that there is more air pressure in the house measured at the floor than there is in the garage measured at the ceiling. With attached garages you risk sharing air between the spaces, which in most cases results in increased VOC levels in the house. A basement garage may even be riskier as the stack effect can cause increased air movement from the garage to the house. The best solution is not having any form of attached garage, the second best is ensuring you have a higher air pressure at the pressure boundary on the house side. The easiest way to do this is to use a small exhaust fan in the garage, I'm not aware of any testing to recommend a cfm level but my back of the envelope calculations say 10-25 cfm. The optimum CFM rate would be something that your blower door contractor should be able to test for. All you need is enough to ensure you have air movement from the house to the garage under operating conditions (worse case is with the range hood and a bath fan running most likely).

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