Our bedroom is over the garage, and it has always been much colder than the rest of the house. Our garage has drywall on the walls and ceiling and (we assume) insulation in it, but just this year we realized the garage door was not insulated at all. My husband insulated the garage door, and so far it has made a big difference. I know it hasn't gotten super cold yet, but it has been almost the same temp as the rest of the house even on the coldest nights we've had this fall. Also, keep the garage door closed as much as possible. The warmer the garage is, the less it will affect the bedrooms. I don't have any advice about space heaters as we haven't used them- just extra blankets. Hope this helps.