Insulating ceiling or roof of attached garage to new construction house

[BakingFool]

I am building a new home in NY (zone 5a). It will have a 28'x28' living area, 22'x26' garage, and a 14' long 11' wide entry between the two. The living area and garage have metal 3/12 shed roofs, each sloping upward towards the center and overlapping the flat entry roof - shown in the diagram (a portion of a draft for revision that has been digitally manipulated).

The entry wall is 9' high, and mosty adjacent to the 9' high exposed garage wall - creating about 120 sq ft of common wall. The living area and garage roofs are planned currently to be closed cell spray foam, unvented.

I am considering the heating and cooling impacts on the house from the garage, if it is not heated or cooled - which I anticipate to be the norm. I may wish to heat the garage on occasion, to work on a car or a project. I don't envision cooling the garage, but if I added a minisplit I could do both.

In winter, the enclosed garage may delay warmup of the air adjacent to the common wall, adding some heating load - but I don't know if it would be significant. In summer, the enclosed space between the garage ceiling and roof may become quite hot. Note that this space is mostly above the top of the conditioned space of the entry.

If I use batt insulation to insulate the garage ceiling, instead of spray foam in the roof, I assume I would have to vent the roof. Speaking of venting, should I be considering venting that space? Lastly, since I am not seeking to conditon this space continuously, might putting in a moderate amount of R value be in order?

Thank you in advance,
Don

 

[steveray]

Always easier to put insulation in ahead of time and probably minimal cost if they are there anyway...

 

[BakingFool]

Always easier to put insulation in ahead of time and probably minimal cost if they are there anyway...

I agree - but I am looking for a clue how I might go about it. One question is, insulate the ceiling of the garage, or the roof. Another is, how much insulation - the normal amount as if the garage is conditioned continuously, or less. I am wondering if insulating with spray foam the common wall might be advised to aleviate most of the concern of how the unconditioned garage might affect comfort and energy impact on the house.

I realize this may not be the right forum - I tried to post this on the site I thought would be a better fit, but I get error messages when I submit the post.