17' x 24' garage, 16 o.c., 2 x 8 constr. Ridge vent only goes 6 feet into it from the house side, have installed styrofom rafter vents on that part. What should I do about the remaining 11 feet (8 rafter bays)? Already have the R-25 faced rolls. I don't have any heat source in the garage nor will I ever. Am i going to destroy my roof deck if I install the insulation on that area without addressing the venting? Mat

Depends a lot on where the relative garage & outdoor temps stabilize. Maybe.

It would be safer to at least maintain a gap between the fiber & roof deck (even without adding vent chutes or a ridge vent.) Pay attention to air-sealing it from the interior side- don't leave any big gaps/rips to convect air into the insulation and it'll probably be fine.

the entire soffit on both sides is vented, it wouldn't be much of a hassle to run chutes on the rest of the rafter bays. I suppose it would probably be the safest bet to maintain the air gap.
Thanks again
Mat