How Much And What Kind Of Insulation

[bazwillrun]

Have a small brick built extension on back of a property south facing that I intend using as an office.

Around 4m x 3m with a flat, felt roof covering, ceiling cavity around 15-18 inches . Long wall has a double glazed window about 2m x 1m

It has a radiator opposite the window and it makes no real noticeable difference to the temperature, as the vast majority of the heat goes up and out via the ceiling cavity and as a result its always freezing in there,

Would I be better off putting some sort of fiberous insulation in the cavity or something like celotex which has the foil outer layers ?

what thickness to keep it super duper insulated for the respective materials ?

Lastly, what amount of gap between the top of the insulation and the roof
Thanks

[The Builder]

Since it's a flat roof Baz, you'll have to break into the ceiling. Put 300mm of ordinary glasswool insulation. Then patch the ceiling up again.
I'm guess the walls are one brick thick. You really need to insulate them as well. Screw 50mm battens to the walls, put 50mm thick Celotex between the battens and nail plasterboard or MDF (no need for plastering then.)

Or, fix TLX multifoil insulation to the ceil;ing and board it over again.

[bazwillrun]

Ok, thanks for that.
This morning I managed to get the P/board ceiling down with hardly any damage, so that can be patched and reused.
Walls are 2 brick and have a cavity so will be getting them filled in.
Off to B&Q for glass wool now and plumber friend is going to install another large rad, so should be nice and warm when finished

[The Builder]

Cavity? That will help when it's been filled :o)