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Vapor Barrier/Retarder for Garage in Climate Zone 4

Peter 778 | Posted in Energy Efficiency and Durability on

Hello,

I have a single family detached home (built 2004) in coastal southeast British Columbia (climate zone 4) and I’d like to insulate my garage.  It gets quite cold in there in the winter and hot in summer, so I was hoping that with added insulation it would be a little more comfortable.  I thought I would start by seeing how it does with just passive insulation, and potentially add a ductless minisplit later if I find I want it a bit warmer or cooler.  

Not looking to make it a regularly occupied space – just want to have it more comfortable while doing intermittent work in there during the coldest or hottest parts of the year.  We do park one car in the garage which does drip rain or snowmelt for much of the year.

The garage is attached, with a bonus room above.  There is insulation in the ceiling between the garage and bonus room (R28 + 6 mil poly VB, per drawings) and on the wall between the house and the garage (R20 + 6 mil poly VB), but it looks like the exterior garage walls are uninsulated.

I was planning to add R22 rockwool batts to the two uninsulated walls, and then a fiberglass insulation kit for the garage door.  My question is around what type of vapor barrier/retarder to use between the rockwool and the drywall.

My first thought was to use 6mil poly, as is used elsewhere in my house, which seems to make sense for winter when it should be warmer inside the garage than outside.

But from reading this forum and elsewhere I see that CertainTeed MemBrain is potentially a better choice here, thinking about the summer months when it will potentially be warmer outside the garage than inside (either just due to insulation, or in future some very occasional cooling from the mini split).  If anybody is able to confirm whether my understanding is correct, that would be greatly appreciated!  

Also, does anybody happen to know whether the BC building code permits the use of this type of vapor retarder for this use case?

Thank you in advance!

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