ceiling insulation in basements below a garage

[Cinci-hal]

This is a question not only of what the residential code says but also a question of what is the best practice for the insulation in the ceiling of a basement that is below a garage. I'm not sure if that part of the insulation envelope is considered ceiling, floor, slab, or something else. The garage floor will be concrete supported by steel beams with metal pan decking. There will be 2x furring at the underside of steel beams with 1/2" drywall. I'm guessing closed cell spray foam on the underside of the tray and beams, but I'm unsure of the minimum thickness or additional insulation in the remainder of the cavities between the steel beams. The garage and the basement under the garage are an addition to an existing residence in climate zone 5A (Mason Ohio). The garage will not be insulated or heated. The basement under the garage will be conditioned and will have an opening to the rest of the existing basement.

 

[Mr. Inspector]

Needs R49 per 2018 IRC

 

[steveray]

R60 or 49 maybe here.... for cavity...if you are foaming the whole thing you might get a gimmie down to 15 or so

 

[Inspector Gadget]

FYI, our office (Canadians, so no authority, different codes) would treat this as a cathedral ceiling. We'd also be quite diligent on a vapour/air barrier to prevent penetration of carbon monoxide.

 

[classicT]

That would be a roof/ceiling assembly as it relates to the building's thermal envelope.