Insulated plasterboard all walls or just external?

[rreanor]

I'm renovating some bedrooms where the plaster has blown - old original cement/plaster over 210 double brick (no cavity).

I was originally just going to fix insulated plasterboard over the bits where i've removed the plaster (i.e. chimney recess walls) but since refitting my entire central heating system i've found that these rooms lose their heat quickly, and i'm just wondering whether i should do all the externally facing walls (so one more basically).

I assume you dont have to insulate the internally facing walls, since these will not lose heat anywhere near as much. Is that correct?

Also, i just want to double check how to accurately calculate the correct thickness needed, as the ones i've bought for the recesses are 52mm celotex pl4040. If these are too thin, i can use them in another room.

 

[Mr Chibs]

Yes, no need to do internals.

Thicker the better, within reason.

I would go 50mm minimum insulation and 12mm plasterboard as a minimum.
If you can use the thinner here and there and get thicker Stuff for the large walls.

Are you insulating the floors?