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Community and Q&A

Insulating detached garage in Climate Zone 6

newsole | Posted in Energy Efficiency and Durability on

I would like to insulate my existing detached garage in Minneapolis.  It’s 20×22 with a shallow, 2×4 framing (walls and roof), an unvented hip roof, and an uninsulated slab. 

-Hope to use ceiling mounted radiant heat (run primarily at night) to heat garage to a baseline of, say, 30* during the winter.  For comfort reasons.
-Be able to heat to maybe 50-60 degrees for shop use a couple times a week.   I’m not sure what temp I’ll actually want since I’ve never worked in a space like this before but I know I won’t want a huge lead time to get temps up to a tolerable level.
-Keep the cathedral ceiling for clearance reasons  (its not really a cathedral ceiling since there are ceiling joists but I do not want to create an attic).

I had almost settled on R-20 closed cell spray foam directly on underside of roof sheathing, then r-13 fiberglass batts in walls covered by osb.  However I’ve been reading a bit more, and it seems like open-cell foam on the roof sheathing is fine in this circumstance since interior moisture will remain low.  Then I was thinking why not just do open cell foam everywhere (leaving it exposed) and skip the osb.  Price is a concern, but not the most important consideration.  This will not be a diy project regardless.

Any input on the best choice for materials and what r-values make sense given what I’m hoping for would be greatly appreciated. 

Thank you!


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Tyler,

    There are some pretty savvy GBA users when it comes to calculating the effectiveness of varying levels of insulation. I am not one of them. I typically differ to code minimums as a starting point and a few rules of thumb to increase performance from there. I'm sure you'll get some more feedback soon.

    That said, I think that you are going down a sensible path with the open cell spray foam, which will give you some thermal performance (albeit, not too much in a 2x4 roof) and go along way towards air sealing the garage. As you know, open cell spray foam it is not typically advised against a roof deck, but you will be heating so minimally that you won't likely have any moisture issues. I know that you are on a budget and are considering no interior finish in the garage, but you could include an interior vapor retarder as extra protection against vapor drive and moisture problems.

    I look forward to reading some other's responses to your question.

    Also, have you read this? How to Build an Insulated Cathedral Ceiling

  2. newsole | | #2

    Thank you Brain!
    Yes, I have read that article. I've tried to read any/all relevant articles and postings on the site but I'm not sure just what portion of the standard advice is applicable to me. Martin's article about open cell foam and dampness leads me to believe that that particular concern is not applicable.

    One reason I like the idea of open cell foam on the roof sheathing is the idea that an r-20 layer will cover over the 2x4's completely which I'm assuming will give me significantly better performance than the closed cell only between the trusses.

  3. Jon_R | | #3

    Review code on "exposed foam".

  4. newsole | | #4

    Thanks Jon. I have tried to review the code. In doing so I’ve only seen reference to “occupied spaces.” Since the garage is detached I did not think it applied. I could easily be wrong. However I understand why the precautions exist, so perhaps it would make sense to to cover it regardless and to definitely not leave the walls exposed. I’ll look into coatings and whatnot for the ceiling.

  5. Coreyjnorris | | #5

    I am in a similar boat.
    I live in southern mn
    I have a 25x20 garage
    2x4 walls and trusses.
    Vented soffit along the side's with 3 vents in the roof and I plan on heating in the winter to work on cars.
    I would poly and sheeth tape the entire cieling other than an access panel and then sheetrock over that and then blow in cellulose followed by a piece of kraft faced stapled the the final back side of the access panel?

    For the walls would spray foam work well?
    Or would you recommend something else
    If I go spray foam would poly be required under the sheet rock but over the foam?
    4# I like it warm in the "shop" new home brand new garage just looking for the best way to capitalize on my investmemt. Would also be running a ceiling hung gas heater on a thermostat to keep around 50-60° give or take depending on how cold it is. Occasionally thawing out a car our sled and then running a squeegee to pull out excess water afterwards.

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