Go to Forum Home Building Simulation Ground floor U-value

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    • #31008
      Alan Clarke
      Participant

        I notice using PHPP that the effective U-value of a suspended floor is significantly higher than the same floor as ground bearing. In the case I am looking at the floor is concrete plank with insulation over, and the void beneath is unventilated, with no walls to outside (as it is 500mm below ground level).

        Therefore I presume the only difference in heat transfer is the evening out of the soil temperature by convection in the void, so the soil below the floor near the perimeter is a bit warmer than it would be with a ground bearing floor.

        Is this a realistic assessment of the physics, or am I taking the equations beyond their normal application?

        Would it make thermal sense to try and fill the void below the concrete planks to revert to a ground bearing type of floor?

        Alan

      • #35567
        Eco Design
        Participant

          Regardless of U values, it would be sensible to fill the void in case it filled with ground water, which presumably is a possibility if it is below ground!

        • #35568
          Mark Siddall
          Participant

            Alan,
            In terms of building physics perhaps this may be of interest:
            http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1

            Cheers,
            Mark

          • #35569
            Tom Foster
            Participant

              link doesn't work, for me

            • #35570
              Anonymous

                On a perhaps related topic, I finally got a rough model to meet the 15 kWh criterion in PHPP. I had to fiddle with wall heights and window psi-values and combine small windows into larger groups. None of which surprised me.

                What did surprise me was that decreasing the Floor Slab U-Value increased the heat lost through the floor! I've used a slab on grade with perimeter insulation.

                Does anybody else recognize this? I presume it's deliberate in which case it might be described somewhere? I haven't had time to check how the spreadsheet works this out.

              • #35571
                Anonymous

                  What did surprise me was that decreasing the Floor Slab U-Value increased the heat lost through the floor! I've used a slab on grade with perimeter insulation.

                  Does anybody else recognize this? I presume it's deliberate in which case it might be described somewhere? I haven't had time to check how the spreadsheet works this out.

                  This is very interesting Dave. Can anyone clarify if this is the case?

                • #35572
                  Anonymous

                    You're quite right, Alan. Mea culpa.

                    I'm not quite sure why PHPP doesn't link the two cells directly, or at least have a more informative note in the worksheet cell. There's presumably some circumstances in which the cells should contain different values, since the worksheet does link the cells for the slab area. But I haven't yet found any explanation.

                    Still, at least I know what I need to do to keep the model honest!

                    Thanks, Dave

                  • #35573
                    Alan Clarke
                    Participant

                      The cells were linked in the blank PHPP spreasheet, but it is editable, so not surprising you thought it OK to enter data there. Going by the comment on the cell, I think it is editable just for unheated basements, where there will effectively be a floor U-value above the basement and another one for the basement floor, but haven't investigated further. (And I agree that if that's so it would be better if PHPP could just ask for the different U-value only when necessary.)

                      Alan

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