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Community and Q&A

Detached Garage Insulation

BlackFeet | Posted in Energy Efficiency and Durability on

Hello GBA Community!

I spent about an hour trying to find the answer to insulating a detached garage but mostly found articles about an attached garage.

I will have a detached garage in zone 6 (central Idaho).  I want to be able to turn on some heat when someone is doing a project in the garage. 

I thought I wanted to run pex in the slab.  But thanks to GBA, that seems like a waste.  It seems the best option is ceiling mounted radiant heat (electric) over the “shop area” might be best because it is fairly quick instead of heating the whole slab for those occasional projects.

But now the burning question is …. how to insulate?  I will add ceiling and wall insulation (FG batts) myself.  But should I do anything before they pour the slab?  I noted one person pointed out that an uninsulated slab provides ground source heat.  Our ground source wells show a temp of about 47 degrees, but not sure how that works out closer to the surface (like a slab). 

I would really appreciate your thoughts!  You all have helped me so much so far.  We are breaking ground next week!  Joining GBA was a great choice.

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  1. tallpinescabin | | #1

    I have 2 slabs right now. One is a cabin in process. Right now, it's a wood box over an insulated slab. (R10 under it and up the sides.) The other is my detached garage, no insulation on the slab or in the garage walls/ceiling.

    At the cabin site, (Zone 6, nearly Zone 7,) I've been monitoring soil temps for the last couple years at 6" deep. I also, this winter, have a thermocouple logging slab temps.
    I'm also logging temperatures inside my detached garage at my main house.

    Based on what I've seen so far this season, I think for an intermittently heated structure, you'll be better off insulating only the sides of the slab you're about to pour.

    It seems my insulated slab (as you'd expect) tracks closer to ambient air temps vs the ground. My uninsulated garage, OTOH, seems to get some benefit from the ground coupled, uninsulated slab. In the recent cold, the garage got to 13F at the lowest, while -9F outside. (meanwhile, a shed that I have raised off the ground tracked outside ambient temps almost exactly, in winter. Not much solar gain then.)

    If I were you, I'd frame w/ 2x6s and overhang the slab edge by 1 1/2", and then use slab edge insulation to not lose heat to the air, but I'd leave it uninsulated underneath, to utilize ground heat when unheated. While that may increase the hourly heat loss when you are heating it, you'll be starting from a warmer temp.

  2. BlackFeet | | #2

    Thank you for that information! I am framing out of 2x6. What did you mean by "overhang the slab edge by 1 1/2"" ..... you mean if it is just a slab? The garage foundation is actually footings, then a stem wall which comes out of ground and will be about 9" above grade.

    1. tallpinescabin | | #6

      If it's a slab, yes. If you have a stem wall, I guess the same thing still applies.....But you may want to watch your slab size vs truss span on that.
      The idea being to have the framing overhang the slab edge insulation.
      See attached image from BSC.

  3. charlie_sullivan | | #3

    From the point of view of keeping the space at a moderate temperature so you can get it comfortable easily and quickly, an uninsulated slab is ideal. But you risk the ground underneath freezing and cracking the slab from frost heaving. There are specific guidelines in the guide to "frost protected shallow foundations", but basically if you aren't going to heat the building, they recommend insulation under the whole slab for that reason.

    Another issue is summer humidity. The ground coupling the slab keeps the interior cooler in the summer, but without the dehumidification of real A/C, that cooling will increase relative humidity. I run a dehumidifier in my garage in the summer, at a much larger energy cost than I would like. An insulated slab would reduce the need for that. But maybe that's not an issue in central Idaho? Maybe your soil conditions are also such that frost heaving isn't much of a concern, at which point maybe an uninsulated slab is practical and desirable.

  4. twoodson | | #4

    I have a similar project planned. I had not considered the point that winter temps benefit from an uninsulated slab, only the frost damage issue. I’d like to heat it from time to time as well.

    I had also consider finding a cheap mini split (maybe even a used one) that I could use to heat it moderately during the winter to make commutes a little warmer :). Something like 45 or 50 degrees. Anyone tried that?

    1. tallpinescabin | | #5

      Tom, I've seen some brands that have pretty high minimum heat settings, like say, not allowing you to set the thermostat lower than 61.... Just make sure you know that limit on whatever mini-split you settle on.

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