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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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    • CommentAuthorSolar bore
    • CommentTimeJul 16th 2007
    was talking to builder working on extension. near to me.
    So far built its a single skin using 4" blocks I asked are you not building an inner skin and how can it meet current regulations - his reply we are Dry-lining it with insulated plaster. will that satisfy regulations.
    No, I do not know of a plaster which comes anywhere near as effective as conventional insulations

    It could be that he plans to use a very thick insulation laminated plasterboard or suchlike. Even so, there are structural implications when only having a single skin masonry wall. I expect the Building Inspector will have something to say
    To answer the thread question, the standard u-value requirements for extensions are more onerous than new housing.

    Walls 0.3W/m2K
    Floor 0.22W/m2K
    Roof 0.16W/m2K [insulation at ceiling level]
    Roof 0.2W/n2K [insulation at rafter level]

    These can be 'traded off' by proportional area provided that no element has a u-value which exceeds 0.7W/m2K. Doing this is not worth the effort in my opinion.

    You can do a weighted average U-value calculation for the whole house plus an extension built according to the U-values in Mike's post and compare it to the extension insulated how you want to do it plus the house as it will be after the job is complete (e.g. you could up rate the loft insulation in the whole house as a trade off for higher levels of glazing in the extension). I've just done this where I had an uninsulated conservatory slab that I wanted to use for an extension.

    The extension I'm building has single skin block walls with external insulation. There are some guidelines in part A of the building regs that set out what thickness of wall is required in certain circumstances e.g. the side walls in my extension are 100mm block but the front wall has to be 215mm because it contains large openings. It is a bit complicated...
    Hi Chris, seems like you made the 'trade off' work for you. Did you need to do a SAP assessment?
    No Mike, although there was mention of this at one point. The building inspector just accepted a summary spreadsheet showing a weighted average U-value calculation (along with a copy of my workings). Basically, by increasing the depth of the insulation in the loft in the existing house, I compensated for the lack of insulation in the existing concrete slab. Quite a cost saving so was worth the effort in this case.
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