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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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    I recently got a set of proper stairs fitted to access the roofspace, which I had floored with 18mm T&G chipboard, laid over glass fibre insulation, some 20 year ago.
    The joists are 7" deep.
    How do I best, or indeed even reasonably "super-insulate" within the 7" depth constraint of the existing joists.
    Since I failed to anticipate and got the top tread of the stairs to tie-in with my existing roofspace flooring
    The chipboard will not be hard to lift btw.
    Lift the chipboard and construct a raised floor with timbers across the joists to get the same height as the stair treads and create an extra step just inside the loft, then fill with soft insulation. IMO board insulation is too much of a faff getting it to seal at the timbers. You will loose head room - is this an issue? but lots cheaper than using exotic insulation. If you tie the first and last timber to the rafters and screw the chipboard to the timbers then there should be no issues with racking.

    Or could you insulate between/under the rafters so the space is warmish as well as accessible?
    Thanks,I was aware of the possibility of "counter battening" to increase the depth of the floor, but my stair rising is a full 8" so kinda reluctant to match the floor rise to this height, and I only really need to "super-insulate" the middle third of the floorplan, this being the middle bay, about a 1/3 (of a traditional 3 bay house design) which lies over the landing, the bedrooms are currently empty and unheated, since we live downstairs(due to the arthritis crippled wife )
    and I want a flat floor throughout.
    Longer term I intend to insulate between the rafters, so the roofspace is a kinda a half/heated buffer space, and since it is used for third generation storage of stuff too-good-to-throw-out, it needs a bit of heat, and indeed perhaps a little ventilation imho.
    I had envisaged fitting (say 50mm thick) PIR sheets between the joists, cut tight and squashed down on top of the glass fibre insulation, because I need a bit of headroom for the multiple ceiling inset 12V light fittings, and with the chipboard on top I did not figgure air leakage should be a particular issue.
    thanks again
    Marcus, I did something similar, by spending time and money insulating the joists, but subsequently insulating the rafters which made the joist insulation redundant. In retrospect, if there is any chance you will insulate the rafters someday, then get straight on to it and don't bother doing any more to the joists.

    Also, the joists will be thermal bridges through the scheme you mentioned, reducing the benefit of adding PIR onto your existing insulation. The scheme Peter mentioned avoids thermal bridges.

    Your existing wool (7" deep?) between joists is about U=0.31.
    Squashing 2" of PIR between joists will improve it to about U =0.26
    Peter's scheme would be about U=0.15
    Fixing say 70mmPIR between rafters and the same beneath rafters would give about U=0.18, although over a bigger area (worse) than the ceiling would be
    I might just double up on the fibre insulation since I imagine it was only 100mm thick, back in 1996/1997.
    Might have been 150mm, but I doubt it, and anyway it does not fill the depth of the 7" deep void.
    So a cheap and cheerful start.
    With about 225m2 of rafters to insulate, I baulk at the thought of starting, though a good task for the winter.
    Thanks again,
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