Bonus room above garage

Venting with min 1.5" baffles from the eave or soffits and installing gable venting fan above that window. Then fill the rafter bays with fiberglass insulation. If there's enough head room, first cover with 1.5 or 2" xps boards then drywall. You will need ac anyway but probably a small one. Then heat in winter with propane heater. Propane heaters must have a fresh air intake. If mine, I'd have a smoke and co2 alarms too. Floor will need insualtion. Stuffing more insulation doesn't work. Example, stuffing r19 into 2x4 wall reduces the insulation into r13. It was discussed in the forum. You can put more into the wall but must seal all air passages and exposed insulation to air. Example, you can put thick fiberglass batts on that wall but if air is swishing around in that area, r value will decrease. You can leave that area windy but then you must encase the insulation.