Our 1930's semi is rendered on the back and sides, with stone cladding on the front. The downstairs seems to get cold quite quickly in the evenings when the heating goes off (and is almost suffocating when its been on for a few hours!). There's an old extension/conservatory off the back which has a single skinned dwarf wall with old metal double glazing (which gets a lot of condensation on). The roof is a flat, felted roof. This extension goes directly into the kitchen, whilst the living room entrance to it has metal patio doors which does a good job at keeping the warmth into the living room.
I can't easily think of a neat/clean/easy way of keeping the kitchen separated from the extension except by having a large heavy curtain between the two - and we like having it open as it currently is. Putting in a door would be difficult and we don't like that idea (due to us many reasons such as liking it being open, and the layout of the extension's side door being close).
Also, the old metal double glazing in the living room is badly fitting so does have a small draught - though this isn't noticeable with the curtains pulled.
Anyway - I was wondering whether cavity insulation would be wise, though I'm also thinking that the coldness of the extension is possibly the largest heat loss for the kitchen (which has an exterior wall), and in the living room, there's actually little wall to insulate (as the window takes up a significant amount of the exterior wall). Also, my concern with cavity insulation is that the render is in good condition and we don't want lots of unsightly holes drilled in it. Same with the cladding on the front (though I assume you could drill between the stones, as long as the mortar gap was larger than the hole needed).
Any suggestions? Secondary double glazing is a possibility for the living room (can't afford to replace the double glazing at present). Any idea of approximate costs of this (£100 a window, £500, £1k)?
Basically, cost is very important to us - but I'd like to try and improve things a little.
BTW - we have insulation up to the height of the joists in the loft - and at some point we'll board the loft out, and possibly raise the floor and add extra insulation whilst we're at it - but the upstairs isn't too bad on temp I think.
Thanks
D