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Suspended timber floor over insulated concrete floor ~ insulation needed?

Discussion in 'Building Regulations and Planning Permission' started by bestendeavours, 4 Jan 2016.

  1. bestendeavours

    bestendeavours

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    We have built an extension and had the base built for us ~ the usual layers of hardcore, sand, DPC, rigid insulation, more DPC and poured concrete. Because we want a 'traditional' timber floor, we allowed for joists and floorboards.
    At last inspection BCO said we needed air-bricks to ventilate under the floor. I pointed out that the floor was insulated and waterproofed etc (which he had forgotten) so he then said we had to insulate between the joists so there was no gap under the floor. This means a hefty extra expense and more insulation than the floor really needs. Why not a space under the floorboards?
    If it is to do with internal moisture 'collecting' and condensing in the space, would a VCL under the floorboards suffice?
    I don't usually question a BCO, but his response was a bit vague as if he wasn't quite sure. I do appreciate it's not the standard way to do a floor ~ being a mix of two types of construction!
    Any guidance gratefully received!
     
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  3. ^woody^

    ^woody^

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    The BCO is correct and a vcl won't stop air condensing in voids. Fill it with polystyrene if things are tight.

    The floor should have been designed with battens set into the concrete, to which you could fix your floorboards flush with the surface.
     
  4. bestendeavours

    bestendeavours

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    Thanks so much for speedy response.
    The words 'should have' are much used in our house!!
     
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  5. freddiemercurystwin

    freddiemercurystwin

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    Yes I agree the BCO is right and polystyrene is pretty cheap though I’m not sure I concur with woody’s batten solution.
     
  6. bestendeavours

    bestendeavours

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    Thanks for your response!
     
  7. wessex101

    wessex101

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    If the floor insulation is below the concrete slab the floor slab will be "warm" and therefore surely you wont get a dew point in the timber floor void above. If you pack out the void with insulation this could bring the dew point higher in the construction and you could end up with a bit of minor sweating on top of the slab but a polythene membrane under the floorboards would help reduce that.
    I would be inclined to leave the insulation out if the existing insulation under the slab is adequate but probably put the polythene under the floorboards just to make sure.
     
  8. Millwright

    Millwright

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    You've had the concrete 'floor' made as it should be. Your choice of floor covering is timber boards, so I don't understand why building control is asking you to insulate your floor covering? The actual floor has already been insulated.

    The airbricks he originally asked for would only have been because traditionally with suspended floor, the DPC is only on the brick piers supporting the floor joists and not the entire concrete sub floor, or even more traditionally, only earth below. You've already got a complete insulated and damp proofed concrete base.
     
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