secondary garage roof insulation

[gaz100uk]

Sorry if this post is in the wrong place or seems a daft question...

Our garage is integral to the house and above it is our youngest bedroom. As this was built in the 70's there is no insulation between the garage roof and the bedroom floor. Being north facing it can get rather cold at times. I’m considering putting some sort of insulation inside the garage, on the ceiling, in the hope it will help keep the bedroom from getting so cold, did think about putting it between the floor joist's but I cant get all the flooring up. So the question is would this make sense and what would be the best to use? I was considering using some of this aluminium loft insulation (fancy bubbly wrap). Or what about polystyrene slabs or even some cavity wall insulation slabs (rockwell type stuff).
Hope this makes sense and hope someone can help.

 

[Deluks]

Being a garage under a bedroom you will need to maintain protection against fire and smoke. I'd go with rockwool full depth and 2 layers of 12.5mm PB, using fireline plasterboards would be better.

You could go with another type of insulation as the pb by itself can satisfy regs for the fire thing but a little extra fire protection can't hurt.

As for using the bubblewrap stuff, you'd be better of ****ing down your leg to keep warm