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Query about insulating a suspended floor

Discussion in 'Building' started by ClareG1964, 12 May 2021.

  1. ClareG1964

    ClareG1964

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    Hi, I'm insulating my old workshop (timber frame, walls sit on brick & concrete block walls), and am currently about to start insulating the suspended floor. Have taken out the old chipboard flooring to expose the joists. The plan is to cut 90mm Xtratherm insulation boards to fit between the joists. Then cover the lot with a vapour barrier. Then put down ply flooring.

    The ends of the joists sit on a "shelf" on short wall of bricks (and they sit on loose laid strips of a damp proof type material). There is a DPC on the top of the bricks and under the sole plate (ie. level with the top pf the joists). Diagram attached.

    If I put insulation boards between the joists, the tops of the boards will be level with the DPC. The insulation boards will therefore be effectively sitting below the DPC. My question is - does this matter?

    If the insulation boards must sit above the DPC, then my floor will end up being raised by 90mm (plus flooring). Which will be a right pain.

    (And yes there are air bricks but have already established ways round that. It's the DPC that I'm now worrying about!)

    Thanks.
     

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  3. Djangobanjo

    Djangobanjo

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    I have this issue, I just cut out the insulation around the air brick so there is plenty of room for air to flow through - I little bit of floor without insulation is not going to make a huge difference.
     
  4. ClareG1964

    ClareG1964

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    Yes, I'm OK with the airbrick issue. It was the fact that the insulation might effectively be bridging the DPC that worries me. ie. If the insulation is butted up to the wall below the DPC, possibly against the edge of the DPC in places, then it would technically be bridging the DPC, but could moisture actually travel from the (possibly) damp area below the DPC, up through the insulation? Does stuff like Xtratherm conduct moisture? Probably not I guess.... Just need some clarification (hopefully a "Yes it'll all be FINE, stop overthinking") before I commit!
     
  5. JohnD

    JohnD

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    heat loss through a floor is mostly by draughts. There is no convection, imperceptible radiation, and little conduction.

    So IMO there is no need for very deep insulation down the entire depth of the joists.

    You will need to take extra precautions near the airbrick to prevent draughts, especially undr the skirting board where there is usually a gap.

    I find mineral wool is a more certain draught-blocker, because it is easy to stuff into irregular gaps with no precision cutting and fitting required. You could possibly use your membrane or dpc as a support. The advantage of a membrane is that any spills or leaks through the floor will eventually dry up.
     
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  7. ClareG1964

    ClareG1964

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    Thanks. Have already purchased the 90mm Xtratherm... so am kind of committed to using it! It will be supported by treated battens fixed to the joists.

    I want to avoid any draughts coming up from the cavity below the floor, which is currently well ventilated by airbricks.

    But is butting the Xtratherm up to just under the DPC going to cause a problem?
     
  8. Mr Chibs

    Mr Chibs

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    It won’t cause a problem.

    I insulated my floor with rockwool ( I know different to you) but after that, I covered the whole floor with building paper before putting the floorboards down.

    Result - draft free.
     
  9. ClareG1964

    ClareG1964

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    Thanks Mr Chibs! Did you have the same issue as me, with the DPC being below the top of the joists?
     
  10. Mr Chibs

    Mr Chibs

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    Yes, the risk of bridging is small. It will be a big improvement over an uninsulated floor.
     
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