Insulation over metal ceiling with room above

[jaycoflyer]

Can't seem to find a discussion that covers my scenario, but if someone can point me to existing thread, I'll be happy to redirect. My 30x40x12 garage has heated floor, sheetrocked walls with R-21 (vapor barrier in). Ceiling is ribbed metal directly on attic trusses. My plan is to insulate directly above metal ceiling (truss depth ~7") for now. Planing to use unfaced batts thinking that metal serves as vapor barrier. (I also considered that faced batts might actually trap moisture instead of allowing it to rise on up and out). Later, I have plans to finish attic space, but for right now, leave it unfinished. Above attic trusses is OSB underlayment with asphalt shingle roof. Garage used mainly as hobby shop, storage, etc. Nothing that I expect will introduce moisture. Mid-Atlantic location has dramatically varying temps/humidity over course of the year.

Questions:
1) Is my assumption of metal ceiling as sufficient vapor barrier with unfaced batts directly on top OK?
2) Once I finish attic in future, should I remove insulation between ceiling and attic floor above?
3) Any other suggestions or flaws in my plan?

Thanks!!

 

[Mattlt]

Interesting. I've never heard of metal sheeting being considered a vapor barrier before. I will tell you that I put plastic up before I installed my metal ceiling, then blew in insulation from the top.

Assuming your metal is already installed? If so, I would install FACED bats from the attic side, faced side toward the metal (down), this will serve as the vapor barrier.

No need to tear out the insulation when finishing the attic space. You're likely going to want to provide the attic with its own heat / cooling mechanism when it's finished anyway.

Assuming this attic was engineered for the weight of the room above the garage in the first place?

 

[jaycoflyer]

It was my assumption that metal ceiling provides a vapor barrier. Maybe a bad assumption on my part. It IS already up and I should have had them install poly first but too late.

Your suggestion about FACED batts with paper down is something I considered also. That might make the most sense.

Yes, attic trusses were oversized to allow for beefy attic.

Others?