Hi, this is Astro's first post. I am renovating a very small ground floor flat in an old house in Bulgaria and have exposed 2 outer stone walls in the bedroom (approx. 18m2 total). Since winter temperatures can reach -20C I am wondering whether it would be a good idea to insulate the external surface of the walls (eg 5cm AAC), or is 50cm thickness of stone (no cavity) sufficient insulation in itself. Or maybe 10cm ACC or polyurethane squares would be advisable? Maybe exposing walls will make the room too difficult to heat in winter and I am mad to consider it at all, and should insulate the walls on the inside! The lower third of the longer wall is underground, too. The builders have dug an external drainage trench and laid a plastic membrane against the external surface. I am wondering if it might be worth laying insulation (eg polyurethane squares) against the membrane, but am assuming that earth plus waterproof membrane is sufficient?

Any ideas much appreciated...

Ken, thanks for your comments. Fantastically helpful. I will make sure to externally insulate, and it sounds as though a better insulator than Ytong/AAC concrete is needed, too.

Best regards,

Astro

the trouble with giving advice in this sort of way is that we do not get to see the building!
a neighbour behind you will presumably heat his own space so what you are concerned about is only the party wall junction.
you will need to either overlap the insulation externally by about a metre beyond the party wall (if you can get your neighbours permission) or insulate the return face of the internal party wall for about a metre or into a return face such as a chimney breast. above is more of a problem because the insulation needs to be weatherproofed and this will not be easy unless you can take it up to the underside of the roof: probably not very practical. On balance, and without seeing the building, it is likely to be more practical to insulate internally! sorry.look elsewhere on the forum at insulating barn walls where you will find lots of good and sometimes competing advice on that!

Bottom line is do you external insulation and encourage you neighbours to do the same.

Thanks Ken and Tony. I am going to take your advice to insulate externally - I am assuming it will help a lot even though the neighbours above and next-door are not going to insulate?

Would you recommend also internally insulating the lower portion (eg 60cm) of the walls to reduce heat loss through them into the ground (and from there to the outside atmosphere, too) and maybe the lower 1.6m where one of the walls is a metre underground. Or is it true what they say and heat mostly rises! Maybe the relatively high thermal conductivity of stone means that partial insulation of a wall or insulation of one wall but not the adjoining one will provide a negligible insulation effect since heat will vanish straight out behind the insulation (this would effect my decision to externally insulate of course!)? Maybe insulating walls on both sides is inadvisable in itself? Does condensation come into any of these decisions? The upper part of the internal walls is too attractive to cover really and has some insulation above in the form of an enclosed air gap in the eaves. I assume that insulating the wall in the external drainage trench is unnecessary.

The movements of heat remain as mysterious as ever! Thank God the plumbing is almost finished!