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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    I'm trying to come up with a very rough estimate of the proportions of heat loss from various elements of an existing building. In other words, how much heat is being lost through walls vs roof vs floor.

    To do this I'm using this calculator:

    https://www.changeplan.co.uk/u_value_calculator.php

    Which says it calculates according to BS EN ISO 6946:2017.

    Walls and roofs are easy enough to understand but with the ground floor it's more complicated, because the overall U-value given takes into account the perimeter/area ratio.

    I understand in principle that this means the calculation method assumes a higher U-value at the perimeter than it does at the interior of the floor slab. What I am not sure about is what assumptions it makes about what's happening at the perimeter - does it assume that some degree of wall insulation extends around the perimeter of the floor slab?

    In the instance I am looking at, it's a concrete slab in direct contact with the ground, and I know that there is essentially no insulation extending around its perimeter (the external walls themselves sit on the slab and are barely insulated anyway). In addition, the floor level is a fair bit higher than the external ground level on one side, so one edge of the slab is significantly exposed to the outside air.

    Below are the results I get from the calculator. I am unsure what U-value I should best be using for my heat loss estimations.

    I know it's not the "uncorrected" U-value of 1.626 because that would ignore the reality of what is happening in the inner parts of the floor, in contact with the ground but not really affected by perimeter effects.

    However, I am not sure whether the "corrected" value of 0.356 is realistic, because I'm not sure whether the calculation method assumes a standard modern construction where the edges of the slab are insulated somewhat.

    Any thoughts appreciated.
      Screenshot 2021-09-13 at 12.28.29.jpg
  2.  
    That method is from BRE Information Paper 3/90, which like Nessie, many people talk about, but I have never managed to see myself...!

    The assumptions are equally mysterious.

    Legend has it that BRE did lots of finite element models of buildings with concrete slabs, with lots of different variations like shape, soil, water table, edges, etc. They plotted them all on a graph and drew a best-fit line through them, U=0.05+1.65(p/A)-0.6(P/A)² to give a very wet finger estimate.

    I also assume that BRE would have included the resistance of the concrete, and the surface resistances, in their model, but your pic seems to show them being added on separately (double counting?).

    IIUC their method refers to the P/A of the whole slab, however many people seem to look at individual rooms, giving very high or low P/A (like your example?)
    • CommentAuthorlineweight
    • CommentTime5 days ago edited
     
    Posted By: WillInAberdeenThat method is from BRE Information Paper 3/90, which like Nessie, many people talk about, but I have never managed to see myself...!

    The assumptions are equally mysterious.

    Legend has it that BRE did lots of finite element models of buildings with concrete slabs, with lots of different variations like shape, soil, water table, edges, etc. They plotted them all on a graph and drew a best-fit line through them, U=0.05+1.65(p/A)-0.6(P/A)² to give a very wet finger estimate.

    I also assume that BRE would have included the resistance of the concrete, and the surface resistances, in their model, but your pic seems to show them being added on separately (double counting?).

    IIUC their method refers to the P/A of the whole slab, however many people seem to look at individual rooms, giving very high or low P/A (like your example?)



    For my particular example, it's a 'slice' through a building that I'm looking at - this means that it's roughly a 6x7m rectangle where two sides are external walls and the other two sides are internal partitions and contiguous with the slab that extends to other parts of the building. So, hence my 12m "perimeter", or at least that's what seemed to make most sense to me.

    It's frustrating that these calculation methods are often rather opaque.

    This seems publicly available at least -

    https://www.bre.co.uk/filelibrary/rpts/uvalue/BR_443_(2006_Edition).pdf
    • CommentAuthorLF
    • CommentTime4 days ago
     
    Not read fully as should be working but I think there is a lot of benefit in insulating around the slab/footings. The heat loss through building to 14 C soil in the middle of the slab is low so difference is low and driving force for heat loss. Ignore if not valid input.
    • CommentAuthorlineweight
    • CommentTime4 days ago edited
     
    Posted By: LFNot read fully as should be working but I think there is a lot of benefit in insulating around the slab/footings. The heat loss through building to 14 C soil in the middle of the slab is low so difference is low and driving force for heat loss. Ignore if not valid input.


    Thanks, I agree that insulating around footings/slab is likely to be of benefit but at this point I'm just trying to quantify what's being lost where.
  3.  
    Most frustrating!

    It's common to just consider the exposed perimeter/area of a part of the building (eg one mid-terrace unit, or one room), but I don't think it's very accurate as that's not the same geometry as the original BRE FE models, from which the formula is extrapolating. If you slice up the building into pieces, calculate their heat losses, then add them all together, I don't think you get the same answer as if you calculate the whole building in one go. This might not be the worst of the inaccuracies!

    If we imagine a square building 14.7m each side, it would have the same P/A as your slice, but would it have the same U? (Who knows!)

    Our house has very thick stone walls with minimal foundations, I've been wondering how to account for the longer heat loss path in this case.

    BRE443 advises not to include the carpet or the screed in the resistances, but is a bit vague!
    • CommentAuthorlineweight
    • CommentTime4 days ago edited
     
    I see there's also an updated version of that BRE doc - "2019 for consultation" vs the 2006 one.

    https://www.bregroup.com/wp-content/uploads/2019/10/BR443-October-2019_consult.pdf

    This has quite a bit added to the ground floors section... which I now need to read and attempt to understand.
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