Polystyrene can be a very serious fire hazard, especially if you hold a lighter next to it until it sets alight.
You're talking about a bedroom over a garage, where you park a car, the car can have 11 gallons of petrol in it, now that is a fire hazard.
Polystyrene, like petrol is quite safe if you use your nouse, i.e. know what you're dealing with, and don't do welding in your garage unless it's covered with boarding.
Put 1st layer between joists, level with the bottom of the same, hopefully leaving an air gap. Next layer at 90deg to 1st and finally board over following 1st layer and joists.