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Dormer roof insulation & ventilation

Discussion in 'Roofing and Guttering' started by xdrive, 24 Oct 2019.

  1. xdrive

    xdrive

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    Hi.

    Can somebody help me with this.

    "LimeStone chips bedded in Bitumen 3 layers of felt on Grade 3 19mm Chipboard
    On 200mm by 50mm Joists at 400mm centre.

    Insulate with 100mm kingspan Between joists And 50mm below joists. Cross battens to roof joists and side fascia boards to be packed of 12mm or Ventilated soffit strip. "

    Do I need cross battens (50x50?) on top of dormer joists for airgap?

    Joists are 8x2 and there will be overhang. Is that where the soffit vent will be? The joists are quite big so would assume sufficient for airgap with insulation?

    Does the 50mm kingspan go on the ceiling and then plasterboard? Would bring dormer height down.

    Cheers
     
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  3. noseall

    noseall

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    Antiquated cold roof. Why not opt for a simpler warm deck method?
     
  4. xdrive

    xdrive

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    Can you explain a bit about the warm deck method and what is need?

    Cheers
     
  5. noseall

    noseall

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    A warm deck describes a flat roof whereby the insulation (current minimum 120mm) sits wholly above the roof joists meaning that everything below the insulation is at room temperature i.e. 'warm'. This method removes the troublesome temperature difference that occurs with a cold deck, thus alleviating the need for ventilation.

    The downside is that fascias have to be deeper to accommodate the joist thickness + furrings + deck + insulation + cocking fillet etc although the latter can be covered by the felt flashing lap or with a trim.

     
  6. xdrive

    xdrive

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    You're a star mate. Cheers (y)
     
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  8. Are you having plans drawn for building regs approval for the dormer and any other alterations to the roof and house :?: They would take into account the proposed construction and insulation values required to comply with bldg. regs .
     
  9. xdrive

    xdrive

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    The inspector been around today. As I don't want to add any insulation below roof joists which will lower internal height further he said to use 125mm insulation. Then he mention another 50mm insulation (unless I'm getting confused)

    On drawings it says "125mm kingspan between rafters , 25mm kingspan below rafters with air gap to sloping ceiling"

    Does anyone know what is minimum size to use? Kingspan? What's cheapest thing to use but would provide adequate insulation? The insulation is quite confusing as lots to choose from.

    There's 18mm ply on top. Below that there are 50x50mm battens. The roof joists which are 8*2.

    Do I also need to insulate floor?

    He also advised to use all pink plasterboard?
     
    Last edited: 31 Oct 2019
  10. noseall

    noseall

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    I thought we were talking about a flat roof?
     
  11. Ian H

    Ian H

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    On mine which is similar I have the rubber on ply/osb, that’s on top of the joists.

    Between the joists top to bottom I have 50mm air gap, 125mm Kingspan, then under the joists a full layer of 50mm Kingspan then plasterboard. The air gap has a mesh to stop wasps.

    Temperature wise it’s great, sound wise it’s ****e.

    40ACBE91-C242-4E61-A82F-118A8021A393.jpeg

    DB12BACF-898F-495C-BA6B-BDC52AF2700D.jpeg

    B8FC4AED-E3E0-4712-AA43-D64723317C39.jpeg
     
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  12. xdrive

    xdrive

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    Sorry my mistake.that part is for rear extension
     
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