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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    Tricky to explain so bear with me.

    We're extending the basement of an existing listed building, the image below shows a simplified ground floor (left) and basement plan (right). Blue shows existing footprint and green shows new areas.

    Due to listed status our options are limited so it's IWI to all walls. New build basement walls will be retaining so generally we're maintaining the same approach here with solid masonry walls and IWI, any EWI would likely be crushed. Yellow walls are lined out to achieve the same U-value (0.28 W/m²K) as above ground.

    What I'm wondering is how much we need to insulate the new basement walls shown in red. They are not 'perimeter' walls in the sense that they will not be exposed to outside air, as they are completely beneath the ground floor footprint above. Following FosterTom's theory that the soil directly beneath a building will reach an equilibrium point if continuously heated, I'm wondering what our options are for these walls.

    It would be ideal if we could reduce the IWI depth here as space is very restricted. I concede that the perimeter 'enclosure' for the soil here is just the existing building's foundations, so won't be insulated, but is there still a case for reduction?

    Finding it difficult to find ways to model this scenario, although I'm gradually getting to grips with THERM if anyone can give me some pointers?
    • CommentAuthortony
    • CommentTimeFeb 10th 2015
    what will be behind the new walls? soil or air?
    Soil on the outside, heated air on the inside. Ground levels mean the yellow walls may be exposed to air but the red ones are completely buried below the building.

    The red walls are solid reinforced in-situ concrete. Basement is around 2.5m high internally.
    • CommentAuthortony
    • CommentTimeFeb 10th 2015
    Then you can reduce the insulation to the red walls, I would rather see higher levels of insulation on the outside walls and even accept none on the red ones! likely you will do a thin layer though.
    • CommentAuthorringi
    • CommentTimeFeb 10th 2015
    You should put some insulation on the red walls, if only to stop condensation in summer.

    But how do you stop BC making you install a vapor barrier on the inside of the insulation?
    Thanks for the answers. Sounds like I wasn't completely on the wrong track, but that we'll definitely need something there to avoid surface condensation.

    It's a cavity drained system so any hydrostatic ingress should be dealt with behind the lining. Tricky part is what type of insulation to use in front of this as we have woodfibre above ground but I'm not sure it's suitable below DPC.

    Tony, we can't go above 0.28 on other walls for this reason - WUFI says OK to achieve this but anymore would risk the dew point.

    I guess I need to get a BCO to confirm what U-value they would accept first, I was just hoping to build an argument before getting them to name a figure.
    • CommentAuthortony
    • CommentTimeFeb 10th 2015
    In my view the soil behind the wall has an insulation value.
    The supplier of wood-fibre I use recommends v high density EPS below d.p.c./below ground. Don't know what it's called, but it is yellow and a bit eye-watering!
    Posted By: Nick ParsonsThe supplier of wood-fibre I use recommends v high density EPS below d.p.c./below ground. Don't know what it's called, but it is yellow and a bit eye-watering!

    Over here EPS that is high density and yellow in colour is called XPS:shocked:
    Yes, but it isn't - really! It is, not to be rude (!) a load of balls, but much denser than standard EPS.

    BTW, and off-topic, I used some EPS that my client had saved from a skip. It had lots of pibk bits in, and they were *sharp*! Anyone come across this?
    Posted By: Nick ParsonsYes, but it isn't - really! It is, not to be rude (!) a load of balls, but much denser than standard EPS.

    Maybe, but over here EPS is white with a colour band on the edge to indicate the density. XPS is yellow, pink or blue depending upon the manufacture - yellow is the most common around here
    I know what you mean. I have yellow XPS here, and blue, and pink, but I do also have the yellow HD EPS - honest!
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