Is the ceiling presently sheetrock? If so, look at some blown in insulation if there is none in there now. If you use foam panels on the outside, it should be covered with sheetrock to meet code. As for R value, as much as you can.
I'm not trying to scare you, just pointing out something I saw on Holmes on Homes...
The family had a room over a garage that was always cold. Insulating the underside of the garage ceiling wouldn't have helped because of the way the builder framed and insulated the house. There were open spaces where the garage met the house that allowed air to travel up under the floor of the room and along the spaces where the ductwork ran. This let lots of cold air into the room. They had to open up the garage walls and ceilings and insulate all of the open cavities to stop the air flow.
Also keep > My bedroom is over my garage. The bedroom stays cold in the winter and hot
Does the garage have a ceiling? if so what is it. If sheet rock I would determine which way the joists are running and drill 1 7/8" hole in ajoist cavity. I would use a probe to find out if the the cavity is in fact empty. If so countinue to drill
7/8" holes every 5' in every cavity. Before blowing cellulose into the cavities check to make sure the cavities are not open to living space, if so block w/ fiberglass. Also make sure there are no heat producing sources, such as a chimminy or "can" lighting. You will want to fill these cavities on the highest presure setting on the machine to achive a dense pack in the cavities. 2. Are the walls and the attick space of your bed room insulated? If you are not sure drill a hole in a claset wall to explore. Take a look up in the attic to see if you have insulation up there As far as recomended R values go. The DOE recomends an R-38 in open attics and in walls and restricted spaces they recomend 3.5 pounds, blown density per cubic foot
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