Plastic vapor barrier for detached garage in PNW?

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blehmsect

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My detached 2 car garage is used as a workshop and now that winter is here I need to insulate. I bought some faced bats to insulate the walls and I also plan to use the same to insulate the roof (not the ceiling, I want to slide items up there for storage and plan to keep it open). For the roof I will use the 'wavy boards' under the insulation to keep air flowing to the vents at the top of my walls.

I have read so many "DO THIS!" "DON'T DO THAT!" posts about vapor barriers. Considering my PNW climate, and my use of my detached garage, do I need to put plastic wrapping over my faced insulation? Or is faced insulation enough of a vapor retarder for my scenario? Long term I plan to put up sheet rock on the walls, but I will leave the roof unfinished with faced insulation (so I can use my ceiling as a surface for quick storage).

So, Is my faced insulation enough or should I use un-faced insulation and a plastic vapor barrier?
 

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Bob Reynolds

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My detached 2 car garage is used as a workshop and now that winter is here I need to insulate. I bought some faced bats to insulate the walls and I also plan to use the same to insulate the roof (not the ceiling, I want to slide items up there for storage and plan to keep it open). For the roof I will use the 'wavy boards' under the insulation to keep air flowing to the vents at the top of my walls.

I have read so many "DO THIS!" "DON'T DO THAT!" posts about vapor barriers. Considering my PNW climate, and my use of my detached garage, do I need to put plastic wrapping over my faced insulation? Or is faced insulation enough of a vapor retarder for my scenario? Long term I plan to put up sheet rock on the walls, but I will leave the roof unfinished with faced insulation (so I can use my ceiling as a surface for quick storage).

So, Is my faced insulation enough or should I use un-faced insulation and a plastic vapor barrier?
The manufacturers do not like you to leave the paper vapor barrier exposed because the paper is a fire hazard and fiberglass can be a skin irritant or cause lung damage if you breath this product. You can put drywall over it or you can put paneling or even pegboard over it. You could go with an encapsulated insulation and that would eliminate the need for a vapor barrier.
 

Sparky617

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With your truss roof system storage is going to be very limited in your attic. The trusses are designed to handle the roof load, not a floor load. If you want to insulate the roof deck, I'd look at poly-iso spray foam. It doesn't need covered with drywall and has an R8 value per inch.
 

Bob Reynolds

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In looking at your pictures and seeing that you are in Seattle, I would go with 24 inch wide R38 Batt insulation with the paper vapor barrier facing into the garage at the ceiling level between the trusses. The paper vapor barrier will allow the insulation to be stapled to the bottom of the trusses. Most of your heat loss in going to be though the ceiling so this is a great place to start if funds are limited. You can then insulate the walls as you get some extra funds. The walls will take an R-15 batt.

I would put a fold down attic stairs in the ceiling if you want to use the attic area for limited light storage.

Since this garage is detached you could use OSB on the ceiling in place of the drywall. It's going to depend on what your future plans are for the detached garage. Drywall does not do well (long term) in unheated spaces.
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