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Community and Q&A

Garage Insulation – walls and roof?

e4miller | Posted in General Questions on


I have enjoyed learning from all of the great content on this website – it has guided me through several projects and raised my interest in building science and improving the energy efficiency of my home.

My current question has two parts – 1) should I insulate my attached garage, and 2) if yes, how can I do this correctly and in a cost effective manner?

Some background information: My house was built in 1963 – split level with attached 2.5 car garage (one shared wall). I’m in climate zone 5 (Chicago area). The garage has a flat roof and is un-insulated. The wall that is shared with the house is dry walled (and I assume this wall is insulated, but the drywall was in place when I moved in). The walls are open 2×4 on 16″ centers with a “USG Insulating Sheathing” behind the cedar siding, which appears to be original and in good shape.

I recently bought the house (almost 2 years ago) and use the garage as a workshop for projects around the house and hobby woodworking. On cold days that I need to get work done, I use a small propane heater to take the chill out of the air. Ideally I would like to install an electric heater out there to take the chill off without introducing moisture like the propane heater does. I am also wanting to clean up the look of the walls and hang cabinets for storage. I am thinking I will use drywall.

My first priorities with the garage (completed last summer) were to replace the entry door to the house with a fire rated door with spring loaded hinges and air seal around that opening. Second was to run a sub panel to properly power my woodworking tools. This summer I was thinking that I would replace the side entry door (it is leaky), and perhaps replace the un-insulated garage door with something better (it also has a dent on the front, so my wife wants this done anyway). I have read several threads about the advertised vs. actual r-value of garage doors and considered not replacing, but honestly anything has to be better than the existing door.

As far as insulation goes… for the walls I like the idea of the cut & cobble detail using polyiso or EPS and an expanding foam around the edges (perhaps paired with fiberglass). I don’t have much to do – only two walls, so this seems reasonable. As for the roof… I have 2×10 joists, the garage has 2′ overhangs and no soffit vents. I have been reading about vented vs un-vented assemblies and the materials/techniques used. Honestly, I am stumped as to how to approach this correctly in a cost-effective manner. I am not convinced that insulating this roof is a good idea, but it seems pointless to insulate the walls if I do not also insulate the roof.

I look forward to everyone’s advice.


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  1. charlie_sullivan | | #1

    Cut an cobble is fine as a hobby project, but it's not really a good use of time or (relatively) expensive insulation compared to conventional fluffy cavity fill, which can be fiberglass, mineral wool, or dense-pack cellulose. The thermal bridging of the studs undercuts the advantage of the higher R-value of the foam. But you can put foam over the studs on the interior to get rid of the therm bridging and that will make a bigger difference.

    I'll try to come back if nobody else gets to your roof question first--out of time for now.

  2. user-2310254 | | #2


    Have you read this article?

    On the cut and cobble, it seems to be an imperfect solution if you have other options, as Charlie suggested.

  3. e4miller | | #3

    Charlie - Thanks for your feedback - you make a good point about cost (both time and materials). As far as eliminating the thermal bridging. I am familiar with the Bonfig wall assembly - strips of foam on stud edges with furring strips over top to attach drywall. However, I am not that keen on bringing the walls in that far (1/2" drywall, 3/4" furring strip, and whatever the minimum foam thickness is, I'm guessing 1/2"? - almost 2" in total). Is there anything wrong with attaching the foam and then using long drywall screws to fasten the drywall directly over top?

    Steve - Thanks for the second opinion on the wall insulation. I had read this article. Based on my review, I felt that my only good option was un-vented and the only way to accomplish this without tearing off the existing roof was to use spray foam. I understand that closed cell SPF is expensive and my concern is the cost-benefit at this point since it is not a conditioned space. Re-reading this article just now, the exterior application of spray foam roofing material seems like a possible option - I wonder how common/successful this is. I'd be interested to hear your opinion as to the best option.

  4. charlie_sullivan | | #4

    Here's a good article about interior foam.

    It discusses the issues with just putting drywall directly over foam. It can work fine, especially for a garage.

    Interior foam isn't popular for the reasons discussed in the article, but few of them apply to a garage. The one thing that would give me pause for a garage/shop is the easy of mounting shelves, etc. on the wall. You might want a layer of OSB over the foam under the drywall to make it really easy to mount anything anywhere on the wall. OSB instead of the drywall might be OK too but I'm not sure about fire code for a garage.

    As for the roof, instead of spray foam, I'd look for reclaimed foam insulation boards to put above the roof deck. It's a well established approach. The only downside is the need to re-roof above it so if you weren't due for re-roofing for another decade or two it's expensive. And if you don't find reclaimed foam, new foam boards are somewhat expensive.

    I'd be temped to rig up a vented roof system using high-efficiency low-flow computer fans, one in each rafter bay, but that's a risky approach because there are no established design guidelines and you might have a few fans fail after a few years and not know it, and the next occupant of the house might be confused or oblivious to what that system is about.

  5. e4miller | | #5

    I checked with my inspector and I’m allowed to use OSB. I like your suggestion and plan to cavity fill with fiberglass, apply a thin layer of foam and install osb over top.

    The home inspector thought the roof over the garage was in good shape 2 years ago, so I may be getting into territory where unvented becomes too expensive to justify at this point. I plan to price out a couple of the options presentd in the gba article. The idea about computer fans is interesting...I was discussing with a friend and he brought up the idea of connecting them via Bluetooth or WiFi to my smarthome setup so that a failure could be detected. Plus the possibility ofl control of fan timing/speed seems like an added benefit of that connection.

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