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Community and Q&A

Attached garage addition: insulation?

Garret Olson | Posted in Energy Efficiency and Durability on

Hello all,
I am in Minneapolis, MN region 6, 1930’s home.

I currently have a 19’x19′ attached garage that is CMU, uninsulated and a portion of that is tucked under an existing sunroom. Next month we are going to extend the garage, by redoing the second stall of the original 19’x19′ and enlarging it to take our current cars and bring height up for drainage.

So, I will have a single 8′ stall with a portion open and under a sunroom and a larger 20×22 area of CMU with a floating deck above.

Current specs on the new garage portion include a frost footing 48″.

I would like to insulate this area. Currently, I plan on 2″ XPS under 4″ slab for new area and am thinking of 2″ XPS on top of “old” slab with an over-pour (the old slab is several inches below new pour).

Walls are currently bare CMU and open joists with deck above. Plan on pex in slab, for future hookup.

My plan was to build a 2×4 wall under portion of garage that is tucked under the sunroom which then could be insulated to bring that tucked area into semi-conditioned space. This would help the temps in the sunroom. Could do mooney wall or double stud? Any suggestions welcome.

As far as the garage proper, not sure what the best option on garage CMU walls and between joists. Is this a situation where a 2″XPS install is sufficient on walls and either packed cellulose, 4″ xps or CCspf ~2″ is sufficient? I suspect my shared wall needs to be at least R20 and sealed. Just not sure about the garage wall and ceiling stackup.


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  1. GBA Editor
    Martin Holladay | | #1

    This sentence has me confused: "My plan was to build a 2x4 wall under portion of garage..." This wall is under the garage? I don't get it.

    Whatever you finally do, you had better trace your thermal envelope carefully. Any discontinuities in insulation may indicate a thermal bridge. If you are making an envelope, connect all the parts. For example, is the XPS on top of the old slab continuous with the XPS under the old slab? If not, is this discontinuity a thermal bridge?

    The same question should be asked at the top of every wall: does the wall insulation join the ceiling insulation, or is there a discontinuity?

  2. Garret Olson | | #2

    Thanks Martin for your response. I am attempting to attach a graphic that will assist. My house has an odd "level" issue where things are stepped in large areas down to basement level, driveway is 6" above garage slab, which then is 6" above area under sunporch which is 6" above basement slab.

    So, on the edge of the slab, where it meets the CMU, I should have a 2" XPS "border, thereby disconnecting the slab from the CMU thermally, correct? Similar on shared wall, etc. as you mentioned.

  3. J Chesnut | | #3

    Don't know exactly your current circumstances or goals but I recommend separating your garage and house proper both thermally (to reduce heating costs) and with proper air sealing (to keep fumes from entering your house).
    I assume you want to insulate your garage to keep it warmer in the winter. Insulation slows heat loss but you still need a heat source to keep the garage at a desired temperature. I think heating garages is a waste of energy. If starting your car in extreme cold is a concern this can be accomplished with much less energy with a plug in for the motor.
    Having said this if you had a nice south facing window in the garage with a high SHGC you might be able to retain some heat in the garage however keep in mind garage doors are no where near air tight and therefore heat is quickly lost out them.

  4. Garret Olson | | #4

    Thanks for your response J Chesnut. My goal for the garage is to have a space that is partially insulated and prepped for radiant in case future owners want the heated garage or want to convert the space. I do have a hybrid vehicle, and unfortunately it's battery is useless in the cold months, so I have been considering a loop for one stall, however seems like overkill. Unfortunately, I don't have a vehicle with built-in battery heater coils as in the 2012 leaf.

    That being said, I agree that the wall between garage and living space needs to be sealed and insulated. However, what level do I take the shared wall? Most of the garage is below grade, over 4 feet. However, it also has a large leaky garage door, so does that shared wall get an r20 buildup with cellulose or flash and batt? Or should I go higher? My guess is r20 is ok given that I'm going to insulate the garage floor and walls but I'm not sure as a large portion will be exposed and there will be more heat loss than in a basement where all walls are below grade.

  5. Dick Russell | | #5

    Regarding sub-slab insulation in the garage, if there might later be heat in that slab, then more than 2" of XPS would be appropriate. On the other hand, if the garage is not heated, then the pickup of ground heat through an uninsulated slab and through the sub-grade walls, while low, still will moderate the temperature inside the garage quite noticeably. I tend to agree with the opinion that heating a garage is an energy waste in most cases.

    Regardless of whether there will be heat in the garage, replacement of that leaky door with one that is insulated and has better edge seals should be in your list of upgrades.

  6. J Chesnut | | #6

    I agree with Dick. Go with more than 2" of subslab insulation. I would say at least 4" and consider using type IX EPS because EPS production has less of a global warming impact than XPS production.
    Subslab insulation will help reduce condensation on the garage slab in the summer. Remember to install a proper vapor barrier under the slab before pouring.
    I've been involved with several energy retrofit designs for homes in Minneapolis and one recurring problem we run into is uninsulated slabs so if your planning ahead and insulated the slab for future retrofits you are an enlightened homeowner.
    How much is enough insulation at the shared wall? More is better but there is little advantage to insulating your shared wall beyond the R-value levels of your home's exterior walls.

  7. Garret Olson | | #7

    Ok, thanks again. I will increase the insulation to 4" and go with EPS. I was considering EPS but I wasn't sure of local code regarding EPS. I understand EPS also maintains R-value over time and is slightly cheaper per inch than XPS. Drawback is a reduced compressive strength right? However under a slab it should be fine.

    Is a vapor barrier necessary? If I went with 4" of EPS would that be enough of a vapor barrier and/or would it alter the temperature enough to limit significant condensation? I would let the slab "breathe" ?

  8. GBA Editor
    Martin Holladay | | #8

    Be sure to include a layer of poly (or a similar vapor barrier material) between the rigid foam and the concrete. You certainly don't want your concrete slab to "breathe."

  9. Morgan Audetat | | #9

    A heated garage is a luxury to be sure, but calling it a waste is a matter of opinion and local. Not running your car for 20 minutes to warm it up is a waste. Then of course there is the relative cost. My two-car garage cost $20.00 to heat last January here in Minneapolis.

    With a new door and suitable perimeter insulation, you may have above freezing temperatures year round. Only a proper heat load analysis will tell for sure.

    Just Google Audetat for local help.

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