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Garden office summer house - Vapour barrier needed

Discussion in 'In the Garden' started by wood2e1, 12 Aug 2019.

  1. wood2e1

    wood2e1

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    Hi,

    I am sure you have all been asked this a thousand times, but I cannot find an answer.

    We are building a wooden framed garden office/summer house, using plywood for the walls, and OSB for the roof.

    We are then going to externally 'clad' the walls. The roof will have
    'Galvanised Steel Roofing Sheets', which I believe I put a plastic sheet/breathable membrane on the OSB then batons then Galvanised Steel Roofing Sheets

    The problem I have is that I cannot work out if there is a need for an external vapour barrier i.e underneath cladding, or internal in between the joist, and before I insert the insulation board between the stud work?

    My understanding is I need one, but I do not know which one.

    Obviously, I want to give as much protection from water as possible, but I don't want to end up with condensation everywhere!

    Can anyone help with this?
     
  2. flameport

    flameport

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    Two different things.
    Plastic sheets do not allow anything through, membranes allow water vapour through but not water.

    Generally - plastic sheet on the inside of the building against the insulation, to prevent water vapour migrating into the insulation where it will cool, form condensation and eventually mould and rot.

    Breathable membrane on the outside of the building, to prevent rain entering the insulation, but allowing any moisture that may get in to evaporate away.
     
  3. wood2e1

    wood2e1

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    oh right, so in short both... :)

    from the outside

    a) Cladding
    b) batons
    c) Breathable membrane
    d) Plywood
    e) plastic sheer
    f) Kingspan
    g) Plasterboard/internal boarding

    Cheers :)
     
  4. flameport

    flameport

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    a) Cladding
    b) batons
    c) Breathable membrane
    d) Plywood
    e) Kingspan
    f) plastic sheet
    g) Plasterboard/internal boarding

    Plastic on the warm side of the insulation.
    although if it's the solid PIR insulation, taping over all the joins with the appropriate tape is sufficient, as it already has a foil facing to block moisture.
    Foil backed plasterboard is another option.
     
  5. wood2e1

    wood2e1

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    Hi

    Sorry for the delay in responding, that is great many thanks for that.

    And so I use 'foil backed insulation' the foil side faces outside? And with this there is no need for plastic?

    One final question but about the roof, we are going to use, Galvanised Box Profile Roofing Sheet

    So for that from the outside would the roof structure be the same?

    a) Galvanised Box Profile Roofing Sheet
    b) batons
    c) Breathable membrane
    d) OSB
    e) Kingspan - between the joist and tape level with the joist.
    e) Plastic sheet
    f) plasterboard/internal boarding

    The joists will be 6inches deep.

    For some reason I do not know why I remember someone telling me between then joist I should leave a small gap, so for a sake of argument, the joist is 6inches but the kingspan should only be 5 inches and flush to the joist, tapped into place. This 1 inch air gap is exactly that just to allow air to flow?!

    maybe to stop condensation?!!?
     
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