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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: Basement heat loss

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    Any thoughts on floor insulation depth for my basement ? It’s three meters deep all round on a 250mm slab with thick layer of tanking beneath. Ground is solid clay but the perimeter is filled with granite stones 20-70mm

    Ground temps at that depths are pretty constant so it seems I don’t need as much as a normal ground floor?
    The architects just put 150mm but that seems excessive.
    Please and thank you!
  2.  
    IMO put the same amount of insulation as you are putting on the walls. (I can't see much difference between the walls and the floor in a basement)
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      CommentAuthorfostertom
    • CommentTimeApr 27th 2023 edited
     
    I'd maximise insulation to the perimeter walls but leave it out completely on the floor. If the basement is part of the internal environment of the house i.e. free to exchange heat with the rooms above, then over a year or two the subsoil beneath the floor (and in a 'bulb' extending out sideways and a bit upward) will also 'fill up' with heat from the house to become part of the internal environment at a massively stable temperature year-round, diminishing outward, and after that year or two, having 'filled up', drawing no more heat from the house. Its intermediate temp (at floor surface) will reduce hest demand in winter and reduce overheating in summer. The good (3m) depth of the 'coffer dam' insulated perimeter walls will put the warm 'internal environment' (both basement room and 'bulb' of subsoil) a good distance (thickness) of soil away from the cold ground surface around the perimeter.
  4.  
    Soil has an insulation value around 1 mK/W, compared to insulation materials which are between 0.02 and 0.04 mK/W, and depending very much on the type of soil and how wet or dry it is.

    So every 1m thickness of soil is equivalent to 20-40 mm of insulation.

    If you think of the basement floor as equivalent to a normal ground floor, but with 3m of extra soil thickness piled up all round the outside, that's like having (3x20 to 3x40) = 60-120 mm of extra insulation value from the extra depth of soil, in addition to the usual soil insulation value beneath a ground floor.

    You'd probably still want a bit more than that in a normal ground floor, maybe add another 100mm underneath the basement floor? Won't cost much in the scheme of things, compared to digging the hole and pouring the concrete.

    Need more than that in the basement walls because there is less than 3m path of soil from them to the outside ground level.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 27th 2023 edited
     
    That's true - the equiv of 60-120 of woodfibre (0.04) underfloor (or a bit more, as the curved mean path length will be more than 3m) against loss from the very perimeter of the basement floor; but
    a) that path length increases, and thus equiv insulation thickness, against losses from points closer to the middle of the floor;
    b) the 'given' temp gradient in the subsoil, from 3m depth up to surface, will be only 10K or less, instead of 21K thro a wall, thus that 60-120 will be worth 120-240.

    So raw winter floor heat loss may be a little greater than with say 150-300 of wall woodfibre; that is to be balanced against the advantage of a massive block of year-round temp stability within the thermal envelope.

    This does depend upon really good insulation to the basement walls, as a 'coffer dam' of insulation with very little loss/leakage thro it, to force that path length that heat loss has to take to the surface.
    • CommentAuthortony
    • CommentTimeApr 27th 2023
     
    I had no insulation under my basement floor. No tanking in the walls 🙂

    I did put 25mm of EPS under a floating floor. I have an interseasonal thermal store under the house which changes the approach
    •  
      CommentAuthorfostertom
    • CommentTimeApr 27th 2023 edited
     
    25mm at the surface is a worthwhile compromise, to kill any cold risk to bare feet!

    Your interseasonal cold store is just an augmented/mechanised/not so 'passive' version of what I've proposed.

    No tanking because what? french drain guaranteeing unsaturated ground hence zero hydrostatic pressure?
  8.  
    Thanks all. So do I risk none or put a bit in? It’s never straightforward haha. Walls are Nudura with 68mm foam either side of 8” concrete core. We had to put tons of rebar in the slab and walls as well. It was like a faraday cage.
    I’ll ask my building control officer what I can get away with. Maybe…..

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