I live in a zone 5 area in Ohio in a house build in the 60s. The garage is 20x20, attached to the house on two sides and shares a roof with the house. Current garage isn't insulated - the attic has blown in insulation over the conditioned part of the house but none over the garage; the garage door is uninsulated; the two exterior walls are not insulated either (the garage door wall is drywall, the other one is painted pegboard). Floor is a concrete slab on grade, most likely without insulation or vapor barrier. We don't have noticeable moisture on the slab today, though.
I'm having the garage door replaced with an insulated one (for noise and durability, not just insulation) and am thinking about adding some more insulation.
I use the garage for cars, and the goal with insulation is to have the cars warmer than outside during winter. I might use a 110v construction heater on a timer in the morning if that would help and won't be too cost-prohibitive to run. The garage has no appliances/utilities in it and has no issues with being uninsulated today.
Easiest path for the walls would be to pull off the pegboard, put fiberglass in the cavities (2x4 studs with chipboard sheathing) and put the pegboard back on. I really don't want to replace the pegboard with drywall, but might consider putting drywall in the stud cavities to cover insulation for fire safety, then hang the pegboard over that (just as cosmetic covering, since it's already painted to the same color as the rest of the garage).
For the attic just fill with blown-in or batts (potentially as part of adding insulation for the rest of the attic over the conditioned space).
My main concern is how to manage moisture in the garage. I've read pretty conflicting opinions about the use of vapor/air barrier. We have rain on the cars and in the winter snow melting off the cars for sure, but no other moisture sources.
Would the fact that parts of the walls would be left uninsulated provide "an easy way out" for the moisture, leaving the insulation alone? Would it benefit having Kraft faced fiberglass or using a 6 mil poly vapor barrier before putting back the pegboard (potentially covering with drywall in the cavities under the pegboard, which would compress the insulation a little)?
Same question for the attic insulation. It doesn't look like the existing insulated parts of the attic have a vapor barrier, and there are no moisture/mold issues.