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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthordelprado
    • CommentTimeOct 17th 2017
     
    Do they exist out of the box? If I full fill my suspender timber area with eps I had may as well use a different kind of joist, especially as some have shown signs of damage from the old fireplace area which had no DPC
  1.  
    Conduct what - heat, moisture, noise..........
    •  
      CommentAuthordjh
    • CommentTimeOct 17th 2017
     
    I-joists - the kind with OSB webs - have lower conductivity than conventional timber joists, I believe. That's why they are used in insulated timber frames. I don't know about metal web ones. They both cost more than conventional timber though.
    • CommentAuthordelprado
    • CommentTimeOct 17th 2017
     
    Sorry I should have said I meant something which might not rot if moisture in the area is high (so i beam out) and something which is not particularlly thermally conducitve for obvius reasons, so thats metal out
  2.  
    For something that is a poor (relative to other things?) thermal conductor and does not rot is a big ask. But then if we are talking relative values and steel as the bench mark then concrete beams don't rot and are way much better than steel although wood is better than concrete (but not that much if your starting point is steel) but of course wood rots.
    And I am assuming a typical building budget so exotics like glass fibre or carbon fibre are out.
    Within a typical budget go for concrete beam and mitigate the thermal performance with EWI
    • CommentAuthorgyrogear
    • CommentTimeOct 17th 2017
     
    Posted By: Peter_in_HungaryWithin a typical budget go for concrete beam and mitigate the thermal performance with EWI


    +1

    Concrete beams also good for thermal mass & temperature stabilization...

    (If making concrete floors, please allow for sufficient box-outs including for future occupiers' weird retrofit ideas...). ( they will thank you...)

    gg
    •  
      CommentAuthordjh
    • CommentTimeOct 17th 2017
     
    Posted By: delpradoSorry I should have said I meant something which might not rot if moisture in the area is high (so i beam out) and something which is not particularlly thermally conducitve for obvius reasons, so thats metal out

    You won't get permission from the BCO if the design leads to levels of RH that can rot joists, so I think worrying about non-rotting joists is a waste of time. Use treated timber if you must but even that shouldn't be necessary.
    • CommentAuthordelprado
    • CommentTimeOct 18th 2017
     
    DJH - its a hedge against the risks of EPS full filling, in case i go down that route.

    So I guess I just need to use concrete joists, sitting on some short of thermal bridge on the floor stacks - is that what you are saying peter?
  3.  
    The concrete beams will be in the wall (I presume) so either some sort of thermal break between the wall and the beams or EWI to insulate the wall and the beams together
    •  
      CommentAuthordjh
    • CommentTimeOct 18th 2017
     
    Posted By: delpradoDJH - its a hedge against the risks of EPS full filling, in case i go down that route.

    In the real world, things should perform better than the design calcs, and the design calcs will (a) demonstrate no risk of condensation or mould etc and (b) include a safety factor anyway. So I think you're mad to consider the risks of rot - it would indicate a major design cockup that would have you rethinking your entire plans anyway.

    By all means consider concrete beams as an alternative to timber or I-joists for other reasons.
  4.  
    Posted By: delpradoIf I full fill my suspender timber area with eps I had may as well use a different kind of joist, especially as some have shown signs of damage from the old fireplace area which had no DPC
    Regular timber is fine, the cold bridging at that point is minuscule. The timbers will be at less risk of decay after pumping the void with EPS bead than before because the heat-loss from the house is more concentrated to the un-insulated areas, so the floor joist ends will be warmer. I left a few floor-boards loose in one house we did and went back to measure the moisture level for a few years, the moisture level of the joist in the middle of the floor was similar to the moisture content of the same joist at the wall/floor junction.
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