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How best to insulate newly built garage with flat roof

briancl | Posted in Energy Efficiency and Durability on


I live in Chicago (city of), Zone 5, and I have a new detached 2 car garage 20x20x11. The construction is wood frame with metal siding, drywall finished on the inside. I’d like to heat this garage to 50F during the winters in a cost effective manner (both insulation cost and energy cost), but I also want to do this right and not cut corners. I plan to heat this garage with a small ceiling mounted gas heater, like a Reznor unit.

The roof construction is 2×12 TJI @ 12″ O.C with only 1/2″ rigid insulation and single ply modified bitumen membrane. No real insulation on the walls or ceiling. This is an unventilated roof. There is no soffit, only a parapet that extends an additional 3.5 feet to accommodate a wood roof deck on top.

My biggest concern is moisture. I want to insulate the walls and ceiling such that I do not have to worry about moisture getting trapped anywhere. I’ve been recommended a few configurations, and the one that sounds most plausible to me is as follows:

Tear out the ceiling, spray closed cell foam to 3″ (R20), fill in with fiberglass 9″ batt (R 30). Replace drywall. Total (R50) for roof.
Fill wall cavities with open cell foam to 3.5″ depth (R12)
Insulate the garage door with some rigid board R4.5

Does this sound like a good cost effective plan that will reduce my energy costs, prevent moisture problems, and allow me to easily heat my garage to 50F in Chicago winters?

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  1. jinmtvt | | #1

    Can you unscrew the exterior finish easily ?

    You will need to focus on 2 things here.
    Move dew point to exterior ( need exterior insulation )

    And air seal perfectly the building to get best benefit of insulation and best moisture control.

    I am not too sure about your roof, i'd favor adding insulation on top of the current roof and then using an EPDM waterproofingmembrane 1 piece + glued on it.
    The current bitumen membrane would serve as inside vapor control.

    Best moiture resistance for walls is to use a peelstick membrane all around ( water and ice shield let's say ..something like that ) under the exterior insulation.
    that would be used or water protection, air leak sealing and vapor barrier ( warm one that is )
    Then scew on requried polyiso insulation to get the R value u wish to reach.

    But 50F ?? that' not too hot .. you could try and find a quick calculator to help determine
    what will be the paybacks of your insulating stragegies once you get material costs.

    Also, what is your local gaz cost? the Reznor unit efficiency ?
    I'd consider a mini split heat pump also for heat/cooling
    ( depending on ur gaz installation price and need to compare efficiency vs cost )
    Some newer units are at SEER26 now, and works down to -25c for heating.

    But hey! what the heck do i know!!!
    let someone else propose simpler ideas, i'm always on the extremist reserve!
    ( sorry i don't settle for nothing less than best ) :)

    seriously, let's see what other more experienced here say about ur situation !

  2. GBA Editor
    Martin Holladay | | #2

    Your plan will work. If you follow it, your garage will be better insulated than most garages in the U.S.

  3. briancl | | #3


    Thanks for the verification.


    I believe your suggestion to insulate from the outside in is optimal, but given how the building is constructed and it's current state, I don't think this will work for me. I have a walkable wooden roof deck on top of the roof membrane which would need to be pulled up to have the roof insulated in the manner you suggest. The metal siding is a non-standard style siding, which would be a pain to completely remove and reinstall. Many days labor. From a cost effectiveness perspective, I believe I am limited to what I described above.

    However, you've suggested I look into a heat pump. This is a new idea, which after a little research may be perfect for me. I do not have a gas hookup at my garage, so I'd have to run a line... adding to my install cost. I know HP's are not best suited for very low temperature climates, but Chicago is on the edge of that threshold, plus I only need this to operate up to about 50F, and if during the coldest days, it struggles to hit that, I'm okay.

    From a total cost of ownership perspective, would there be a >10% difference between an efficient gas furnace and an efficient HP for my 50F heated garage? My cost of electricity is about $0.14/kWh and gas is about $1.10/therm? Traditionally, I figured electric was way too costly, but HP's seem to flip that idea on its head and could be a net cost savings for me.

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