Freezing extension what can I do?

[EasilyConfusedIndith]

We live in a terrace with a long, thin kitchen and bathroom extension which is freezing in winter. In fact it is already starting to get chilly in the bathroom.

There is already something inthe cavity walls and although it is thin and rubbish it means we can't get the walls insulated properly in case it reacts with what is already there. The lofs space of the extension is insulated although we could put a bit more in and most likely will buy a roll and do it ourselves.

Anything else?

The kitchen isn't too bad as it is the first room and next to the room with the stove in (solid fuel heating) but the bathroom gets so cold you just dread needing a wee and showers are a leap in, leap out and run affair. It also turns into a swamp in the winter even though we always open the window after showers.