I posted before about using tigerfoam. I used in in my addition. My garage ceiling was sheetrocked by my builder, and I'm sure he did the least he could get away with. Part of my bedroom is over the garage. I'm thinking that they didn't used to build living space over garages before. Now we all see why. If someone could figure out how to retrofit insulation onto a sheetrocked garage ceiling, that would be great. For stockrex, you don't have to fill the entire space for the Tigerfoam to be effective. Just a few inches will probably stop the air leaks. Then, you can fill the rest with the same batting that you take out, if it's clean. I think that it's best to figure out where the airleaks are coming from first. My powder room shares a wall with my garage. It was always cold before we renovated, but now it's freezing. When I had a wall mount sink installed, I decided to have an access panel behind it in the garage. Cold air was flowing in through the electric plug. I sealed that. It's better, but the walls are still very cold.

We have a similar set up to yours, except that our bonus room stays more comfortable in the winter. I believe that the flooring underneath is not adequately insulated...and there isn't much between the roof and the sheetrock either. That said, we've adjusted the air conditioning vents to make sure that room is getting proper ventilation. An air conditioning pro we used to go to church with suggested it was a ductwork issue. we always forget to work on it in the winter when we can be in the attic. Last year I added an inroom air conditioner. If the window in the room was anywhere but the front of the house, I'd just put in a window unit for the extra ooomphf the system needs. Instead, I found a unit at Home Depot that vents through the window and takes the edge off the temps. I use the room as 'my' space--sewing, writing, etc. so it doesn't get used every day. When I hit the lottery (grin) I'll rip out all the sheetrock and install radiant barriers and better insulation all around, especially from the garage side. (And add a bump-out and windows, too.)

I agree JDS, mainly because of garage-to-house door leaks at weatherstripping and jambs to rough framing but usually it's mudsills needing the most attention. Bonus rooms often share garage walls (mudsills) and the stack effect can be more effective at sucking garage air up, than wind or exhaust fans at sucking the air sideways into the house. Spray foam at garage ceiling/bonus floor is a good suggestion to prevent this but also pay attention to the airtightness of garage wall cavities. Standard construction practices of fiberglass batts and not proving airtightness with a blower door is sure to result in poor separation of garage air and bonus room (house) air.

If the rest of the house has forced air, why wouldn't you extend that to the bonus room? Didn't specify if attached/detached... If detached, depends how you heat the garage. If in-floor heat, obviously extend that upstairs. If modine/infrared, I'd run the gas line upstairs to a gas fireplace. If its a portable shop heater or something, electric fireplaces today throw out a decent amount of heat. You can do mantle units to slide around the floor. Heatilator/Simplifire make wall mounted units you can either recess in or hang like a TV, and they put out decent heat. Otherwise I have a Dyson hot fan that oscillates and heats up decent, but depends how much juice you'll have to this garage