thanks everybody for advice and for discussion-
the bit behind the rayburn is the hardest to insulate so i'm
concentrating on that,
the rest of the walls and windows could do with some too
(and when i repair the ceiling the major heatloss through the
floorboards will be cut).

However its just occured to me that maybe
I could put kingspan on the outside of the building
because that wall is ugly outside, and is badly rendered
(thereby using the brickwall as a heat store)
[g]

fred wrote:
In article , george (dicegeorge)
writes

But as heatloss is proportional to temperature difference
and the back of the rayburn may get quite hot
there could be quite a bit of heat loss through the walls,
worth doing something I think
(as well as increasing the air gap)
[g]

Out of curiosity I did a few calcs:

Assuming zero degrees outside and a wall surface temp behind the rayburn
of 60degC then you lose 240W for a 2m area on the single brick (9") wall
you described.

Just an inch of celotex on that would drop the loss to 75W (2" gives 45W).

By comparison, if your kitchen wall is 3m high by 4m wide then the 10m
not behind the rayburn loses 400W (zero outside, 20deg inside).

Total current loss 640W vs total loss for 1" behind rayburn 475W so it's
still going to be a pretty lossy wall.

If you stuck 1" of celotex to the whole wall then stuck on some
plasterboard then the loss for the whole would be 200W which sounds a
bit better.

If you do just decide to go for insulating behind the rayburn, I would
hack the plaster off the wall immediately behind (which is probably 1"
thick) then infill with 1" celotex with a sheet of masterboard over the
top with a good overlap for protection.

How does that sound?