How to insulate detached garage?

[BillWIII]

In NJ, I have a 20 x 22 (440 sq) detached garage with gable roof. I’m installing a 18k MrCool mini split heat pump. I plan to use the garage for a hang-out, multi-purpose space. The walls are bare cinder block, the ceiling is open 2x6 joists, with planks on top for the attic floor. The roof is 6/12 pitch. I never use the attic for storage and don’t plan to. I’m having a 100 amp service put in.

My question is: how to insulate? My plan was 4"closed cell spray foam directly onto the planked attic floor. To prevent the foam from being seen from below, through the plank gaps, I planned to have the insulation company apply black tape to the openings between the planks. Since I’m only going 4” thick, I didn’t plan on baffles for attic venting. I don’t have any ridge or other vents in the roof.

I also planned to insulate the garage door with rigid foam.

How does this plan sound? I was quoted $2,950 for spray foam. I thought the foam will prevent moisture vapor from getting into attic, so I don’t need to worry about venting the attic (there’s no vents there now) What do you think? Is there anything else I should be considering?

Also, they also proposed an optional DC-315 Ignition barrier. Do I need this? I don’t plan to sleep in there.

Thank you! - Bill

 

[u2slow]

Attic will need to breathe somehow. Mine draws air through the eaves and up through the ridge vent.

For the ceiling, I would do batt insulation and poly vapour barrier. Probably R40 on top (and vent channels), or R22 from below. Stuff I can do myself.

 

[BillWIII]

after some research, it seems the way to go is to put an air barrier on the attic floor. Make air-tight everything between the living space below and the ceiling. Put R39 insulation on top of the air barrier. Vent the soffits and the ridge.

 

[BillWIII]

And sorry u2slow, I just looked and I see you posted just what I planned to do. It took me some looking on my own to see what you suggested is the way to go. Thank you. -Bill