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Insulating a garage (converting it to a workshop / studio)

PnhUtHDpji | Posted in Energy Efficiency and Durability on


I’m in zone 6A and am converting a 22′ x 22′ garage into a studio and would like advice about best ways to insulate it. Currently the 2 x 4 walls have R-11 fiberglass in them, as does the ceiling. I plan to put in a plywood floor with rigid foam beneath it (over the slab). But I’m not sure what the best (and most cost effective) approach to the rest would be. Currently there’s no drywall, just the bare insulation.

Could I leave the batts and put rigid foam over them?

Should I remove the batts and spray closed cell in?

Is there a choice for the walls I haven’t mentioned?

Should I plan to spray the rafters, or can I spray in the ceiling joists instead and leave the small attic space vented?

Is there a gradual approach that would be workable across a few seasons?

My budget is tight, but I would be happy to do the project in sensible stages. I just want a really cozy and efficient space.

Any advice would be greatly appreciated.


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  1. jklingel | | #1

    Dana: I would check on the diy forums, too, as this is, IMO, a bit more tailored for there (though no one is going to shout for posting here, as we are all welcome). You'll hear various opinions on what to do, no matter where you ask, so I'd suggest you locate one of the online heat loss analysis calculators, which are free and accurate enough, and play with different scenarios. Don't be intimidated with their spread sheets; they look ugly, but are simple to plug your data into, really. Then you'll have an idea of how much different scenarios will cost you, and how much you'll save on heating bills. Then you can make the call based on your agenda. You can then throw other questions into the mix, too, like: How long in the heating season are you going to heat the place? To what temp? How often? Etc. Consider, for ex, dense packed cellulose, and maybe even adding an inner wall to get a thick insulation blanket. Rock wool, cellulose, and cotton are also other batt insulations you could use instead of the 'glass that you have. Blowing cellulose over, or instead of, your 'glass in the ceiling is commonly done, too, and very effective. Be sure to air seal like a demon; that is rule #1. But again, it's all in the numbers, so a heat loss "analysis" should be done first, IMO. Good luck on the "shop". john

  2. PnhUtHDpji | | #2

    Thanks John, I'll post on the other forum, too. I did 't know about the heat loss spreadsheets, so I'll check them out.

    I'll be wanting to use the space year round. I work from home many days, and this will be the place I'm at most often.

    When you say to air seal, do you mean to use spray foam? Or is there another approach?

    Thanks again,


  3. jklingel | | #3

    There are a few "ways" to air seal, but mostly it is just "plug up every penetration between inside and outside". That means electrical, plumbing, venting, attic doors, cracks around windows, etc. Search for "ADA" (airtight drywall approach) and you'll see about sealing the sheet rock and using it as an air barrier. You'll have to decide about the vapor barrier or vapor retarder debate, but I'd research that for your area; many places do not need a true vapor barrier, and they (poly sheeting, ie visqueen) can be problematic in certain situations. Vapor retarding paint may be your best option. You'll have to read your area's code and see if they mandate either a "vapor barrier", or something like "a vapor retarder of 1 perm or less" . Visqueen has a permeability of about 0.06, which means it is essentially a vapor barrier, while vapor retarding paint is somewhere around 0.5 or 0.8. The "1 perm or less" lets vapor travel in and out of the insulation, and the benefit is if you ever do get some "water" into the insulation then it can sneak back out. That is why the exterior should be 5x more vapor permeable than the inside. A 1 sqr inch hole will let as much vapor into a wall as a whole sheet of sheet rock, so you want to stop air movement. If no air enters the wall, very little vapor will because diffusion is not a good way to move a lot of water. BTW: Your 'glass insulation is OK, but it is impossible to pack those batts in perfectly, and typical 'glass allows a lot of conductive loops, which robs you of heat. A more dense, vapor permeable batt is better. Spray foam can be real good, but research what type you want; open or closed cell. It is expensive, but it has its place in homes. I have no use for it, but you may in your situation. Here and will fill you with many hours of reading!

  4. Foamer | | #4


    Talk to a couple of spray foam contractors in your area and get quotes on spraying your walls before you get lost in the many DIY options. This is a small space and if you get the air sealing right, it should take very little energy to heat and cool. Foam will insulate and air seal in one application so go with it if it fits your budget.

  5. kevin_in_denver | | #5

    Besides the wall assembly and the attic, the biggest heat loss will be through the edges of the slab.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "When you say to air seal, do you mean to use spray foam? Or is there another approach?"

    A. For more information on air sealing, see Questions and Answers About Air Barriers.

  7. PnhUtHDpji | | #7

    Hello everyone,

    Thanks for all the information you've shared. I've been following up on it.

    Further questions:

    1. Would it be OK to put rigid foam over the fiberglass and air seal those panels? I thought I might build out the walls a bit that way.

    2. Would it be sensible to build up the floor with 2 x 6s and rigid foam or to use a somewhat insulating material (like Delta-Fl) under the plywood?

    Thanks again.


  8. GBA Editor
    Martin Holladay | | #8

    Q. "Would it be OK to put rigid foam over the fiberglass and air seal those panels?"

    A. Yes.

    Q. "Would it be sensible to build up the floor with 2 x 6s and rigid foam?"

    A. You can do that if you want. But there are two disadvantages to 2x6s:
    (1) Loss of headroom, and (2) Thermal bridging.

    Q. "Would it be sensible to use a somewhat insulating material (like Delta-FL) under the plywood?"

    A. You can use Delta-FL if you want, but remember -- it is a vapor barrier, not a type of insulation.

  9. kevin_in_denver | | #9

    Another way to insulate the floor would be to lay down a few inches of the high compressive strength XPS under a floating floor. Click together laminate is the easiest type of floating floor. Maybe the greenest floating floor would be click together cork.

    Since XPS is rated up to about 100psi, it would feel as solid as any normal floor.

    It also may be money well spent to insulate the perimeter of the garage slab, going down as far as you can easily get, say, 12"-24". A good energy model might help you determine the optimum thickness and depth. You excavate the dirt with a pickaxe, place the foam, and backfill quickly. No fasteners or glue needed.

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