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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorAstro
    • CommentTimeMay 25th 2007
    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...
    • CommentAuthorken davis
    • CommentTimeMay 28th 2007
    assuming you walls are limestone which has a thermal resistivity of 0.65 ( for 500mm a U-value of about 2)then you will need 42.6 times the thickness of expanded polystyrene equivalent, in other words the wall on its own is completely useless!
    the wall and 100mm of expanded polystyrene will give you a U-value of 0.3, or 0.16 if you have 200mm!
    (if you use a polyisocyanurate board such as celotex then half that thickness will do the same job).
    For some of the pros and cons of internal versus external insulation get hold of the relevant best practice guidance from :
    www.est.org.uk/housingbuildings (its free).
    personally i would always go for insulation on the outside as you then have the thermal storage capacity (and look) of the stone wall but of course you need to protect the insulation externally.
    • CommentAuthorAstro
    • CommentTimeMay 30th 2007
    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,

    • CommentAuthorAstro
    • CommentTimeMay 30th 2007
    One thing just occured to me: I have adjoining neighbours both above and behind my flat whose walls are not externally insulated, so heat in the stone wall has a route out. Does this mean that externally insulating my part of the walls will give little improvement in insulation and I should consider internal insulation (even if it hides the lovely stones/beams). Will stone walls without insulation (in a small room) lead to astronomical heating bills?

    • CommentAuthorken davis
    • CommentTimeMay 30th 2007
    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!
    • CommentAuthorAstro
    • CommentTimeMay 31st 2007
    Thanks for all your advice Ken. Astro is very grateful.
    • CommentAuthortony
    • CommentTimeMay 31st 2007
    Bottom line is do you external insulation and encourage you neighbours to do the same.
    • CommentAuthorken davis
    • CommentTimeJun 2nd 2007
    yes, external is best if you can do it but getting a number of people to agree and put in the necessary funds can be difficult.
    • CommentAuthorAstro
    • CommentTimeJun 4th 2007
    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!
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