Insulation advice for cold bedroom above garage

[mezzaw]

House built in 1991 (Bovis)
Regular block and brick, had cavity wall insulation 5 years ago.

The bedroom above the garage gets a cold floor in winter. The garage ceiling is double skin insulated plasterboard (about 50mm total thickness). But the ceiling / floor joists are empty and end with a vented soffit above the garage door. Cold air enters and travels down the joists cooling the bedroom floor.

I'm prepared to take the bedroom floor up (chipboard). Looking for advice on what steps I can take to add/improve the insulation to this room.

I wonder what kind of inslutation I can put under the bedroom floor? Can I block off the ends of the joists behind the soffit?

What about in the void, and the small wall that runs under the dormer (with radiator on)? I'm prepared to take the rad off to allow a large-ish access hole behind the rad to access this space if need be.

 

[^woody^]

 

[mezzaw]

Thanks woody.

Lifting the chipboard floor and filling with quilt - yep can do that. And sealing each joist cavity off under the dwarf wall with celotex yes can do that from the bedroom too reaching down between the joists.

The challenge is going to be getting enough access to behind that dwarf wall. It's a wide room - +5.5m with two dormers. Walls have just been painted and new skirtings down so I'm not going to get approval for ripping that off.

Maybe I could take off a 60cm wide slice off the garage ceiling all the way along to gain access to the void from below. Any other ideas?
I don't want to/can't take a large amount of garage ceiling away - too much junk in there at present!

How could I refit that garage ceiling portion to retain the fire-resistance?

 

[^woody^]

Unfortunately, all the options are crap, you just need to pick one that's less crap than the others!

Access from the bedroom
Access from the garage
Line the face of the wall in the bedroom.

 

[mezzaw]

Very true!

If I take a section down from the garage ceiling, is there a way to put it back up to retain the fire proofing?

Making plasterboard mess in the already messy garage is my best approach I reckon