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Community and Q&A

Insulating an Attached Garage

mvetting | Posted in General Questions on

Looking for some advice on a project in in Climate Zone 6 that I’ve been researching for a while.  I’ll try and lay out all the details up front.  If I miss something please chime in.

-Garage is about 1000 square feet
-Planning to install 60k BTU (83% efficient) gas ceiling heater
-Exterior of studs is 1/2 OSB with Tyvek housewrap and vinyl siding, there are some small sections (~ 3 ft high) of stone on lower areas next to garage doors
-Typical 3 car garage with deep end stall wrapping around corner of house
-16’x8′ and 9’x8′ R13 insulated garage doors
-Small service door and small window
-House wall construction is 2×6 while garage is 2×4
-Planning to use Bonfiglioli approach to fir out 2×4 walls to 2×6 for additional insulation
-Furring was going to be 1-1/2″X1-1/2″ XPS, High Density EPS or Polyiso glued to  1/2″ X1-1/2″ strips for drywall attachment
-Foam/plywood strips will get glued and screwed to studs with foamboard adhesive and 3-1/2″ construction screws.
-About 90 ft of wall will need to be insulated/finished
-Walls are 9’3″ tall on interior to flat ceiling
-Plan A: R23 Roxul Mineral Wool Insulation in walls
-Plan B: Fiberglass R19 in walls
-Was planning CertainTeed MemBrain on walls but I can’t find it anywhere at the moment
-R49 blown in fiberglass in ceiling
-Ceiling and along house already drywalled with 5/8″ type-x panels
-Garage attic is just open, no living space above
-Current uninsulated walls will be drywalled, then whole garage taped, mudded, and painted.
-Don’t believe there is insulation or vapor barrier under slab

I’m feeling like I’m overkilling this for a garage but it’s in my nature as an engineer by trade.  I figure about $500 more to do the furring plus my time to break the stud thermal bridge and get another 2″ of insulation on the walls.  I’ve already sealed the stud bay from inside using acoustical sealant.  I bought the house after built so I don’t know the air sealing that happened on the exterior during build.  If I had to guess it was minimal which is typical near me.

Where I’m torn now is the unavailability of the MemBrain Smart Vapor Retarder.  I’ve seen where some say you don’t need a vapor and I’m especially wondering if I really need one considering this is in a garage.  The house walls will already have a vapor barrier for that conditioned space.  I was not planning a vapor barrier/retarder in the ceiling other than the drywall/paint since I don’t want to take down drywall to install something.  I know there are options like  a thin layer of closed cell foam prior to blow in but I think this will get very pricey.  Another thing I’m considering is just using fiberglass kraft faced insulation and here once again I’m wondering how picky I would need to be with sealing the seams with tape.

The most I plan to do is heat garage to 65-70°F in winter when working out there.  Also tossing around maintaining 45-50°F in winter with the heater rather than completely shutting off hence the extra effort for some better insulation.  I know in summer with the door closed it can stay pretty cool in a well insulated garage from the concrete slab .  I’ve done lots of reading on the site and haven’t really come across a similar situation to mine.  I don’t mind expending a little extra effort if it will pay off in the long run with comfort/utility bills/longevity.  I’d appreciate some advice from the community.  Thanks in advance!

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  1. qofmiwok | | #1

    The similar sized garage for my house currently under construction in CZ 6B has 2x6 with 2" external rockwool, R49 ceiling, impeccable air sealing, and sub-slab insulation, and based on that I'd say your 60k BTU heater is waayyy overkill. My modeling came out to needing a small fraction of that. Around 10k peak load.

    1. Jon_R | | #4

      Peak load = everything in the garage is very cold and you want it all heated up quickly.

  2. mvetting | | #2

    Thanks for the reply. I will admit I didn't complete any heat loss calcs prior to this attached spreadsheet. I had gone off the recommended sizing by the manufacturer. Looking at the calcs I think I'd be in the same boat as you if I had slab insulation. Unfortunately, we didn't build the house and I suspect there is no insulation under the slab. I also think the slab is going to get colder than ground temperature with no insulation so the 55°F estimate is likely low. I'm also calculating a pretty severe case of 0°F outside and 70°F inside the garage which I'll likely never go this high. So based on this it seems the heater may be properly sized.

    An update on the insulation and air barrier debacle. I've decided to leave the walls alone and just use Rockwool R-15. Then I'm planning to use the air sealed drywall approach with vapor retarder paint and no vapor retarder. Decided that furring out the walls didn't seem to be worth the extra effort.

  3. Jon_R | | #3

    > sealed the stud bay from inside using acoustical sealant

    As far as I know, manufacturers do not recommend this. So I'm curious - where is the evidence that this is long term better than a good sealant that hardens somewhat and bonds?

  4. walta100 | | #5

    I like the idea of the low efficiency heater as it has no way to trap any water that could freeze and destroy the unit unlike the high efficiency gas heater that condenses water that could freeze.

    I also like the hugely oversized heater so when you open the doors filling the space with cold air and 2 tons of frozen steel covered in ice you have lots of BTUs the warm everything up in minutes not days.

    I say a manual J is great for building that are generally heated everyday to more or less the same temp but that does not sound like how this garage will be used. To my ear it sounds like the heat will be used to keep the garage from falling below freezing most of the time and on rare occasions it will be heated to 70° to work on a project.

    My guess is if you built a computer model of the garage with BEopt anything more than R13 wall would not recover its costs at 49° in the next 20 years.

    If you want more than a wild guess you need to build the BEopt model.


  5. BirchwoodBill | | #6

    You forgot to include sensors to monitor temperature and humidity.

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