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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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    • CommentTimeSep 7th 2013 edited
    • CommentTimeSep 7th 2013
    Neither Tom, was about the affects of insolation on thermal mass, both passive and active. All within an insulated envelope.
    1) we won't have any more valves etc than if it were a "bog-standard" central heating system.
    2) running pipes under the roof has proven only minimally more expensive than not bothering ... (granted it's a new roof)
    3) we're on a slope, and deep digging not an option, so this is also testing the use of insulation to force a long path from ground to slab
    4) once it's commissioned we'll have, at the very least, a series of fuel bills to compare before-and-after ... not a game changer I know, but informative nonetheless assuming other variables like occupancy, unit price and weather have been taken into account.
    Can anyone please shed light on appropriate boundary conditions, specifically film coefficients, as they pertain to materials which are not immersed in air e.g. EPS to soil, sand blinding or Hardcore etc? I gather that surface resistance does not manifest the same way in this application? At the moment I've created a new boundary condition and taken a tabulated soil resistance of 1.5m2K/W (from Dansk Standard 418 page 37) to give a coefficient of 0.66W/m2K but I'm really just stabbing in the near darkness!!!
    • CommentAuthorEd Davies
    • CommentTimeJan 9th 2022
    AFAIK “film coefficients” only apply when at least one material is a fluid. Are you sure that number given in the Dansk Standard is not an assumed resistance for the bulk soil?
    Hi Ed

    I'm not sure no, but I suspect your hunch is probably correct as the tabulated value is given for:

    "Ground supported floor, from 0.5 m above to 0.5 m below the terrain"

    I attempted to extract this coefficient from the 1.5m2 K/W resistance given in the table (1/1.5=0.666W/m2K?)

    The first thing I tried was simply to delete the film coefficient for the elements in contact with the ground however the isotherms don't look even vaguely correct when I did this so I reasoned that I needed to find some value to attribute to that boundary condition order to generate a vaguely plausible result.

    <blockquote><cite>Posted By: Ed Davies</cite>AFAIK “film coefficients” only apply when at least one material is a fluid. Are you sure that number given in the Dansk Standard is not an assumed resistance for the bulk soil?</blockquote>
      Screenshot 2022-01-09 at 12.06.09.png
    Zero coefficient
      Zero coefficient.png
    0.66 coefficient
      0.66 coefficient.png
    • CommentAuthorEd Davies
    • CommentTimeJan 9th 2022 edited
    Certainly looks like that 1.5 m²K/W (and the other values in their table 6.9.1) is supposed to be used as the net resistance of the whole soil-to-air path so makes sense as your boundary condition, so not really a “film resistance” like that Rsi [¹] figure.

    It's odd how they have the resistance increase from 0.2 to 0.8 m²K/W as the depth down the outside wall increases from 0 to 2 metres then suddenly jumps to 2 m²K/W. I wouldn't expect an exact match as this is all presumably a simple approximation to extending the finite-element model out into the soil but it does seem quite a big jump anyway.

    [Âą] Irritating how Rsi for the internal surface resistance clashes with Rsi for resistance specified in SI units as opposed to in US customary units.
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