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Insulate flat roof only on Single Skin outbuilding?

Discussion in 'Roofing and Guttering' started by a546345, 6 Dec 2019.

  1. a546345

    a546345

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    I have a single skin block-on-edge outbuilding. I need to put a flat roof on it, probably EPDM from an online place. I've installed one of these before with good results.

    The outbuilding will be used as a workshop, containing pillar drill, saw etc. It will not normally be heated.

    I think the most heat gain/loss would be through the roof,

    A normal shed flat roof could be just joist, OSB then EPDM. I think this would get very hot in Summer and cold in winter.

    If I don't insulate the rest of the building, is it still worth putting insulation in between the joists on the roof as a kind of hybrid roof to reduce the temperature variation?

    Alternatively I could just build a proper warm roof with joist, OSB, vapour barrier, insulation, OSB, EPDM, so that I could then use insulated plasterboard on the walls for a proper room. This is obviously more expensive, so considering whether it is worth doing the hybrid roof instead as I don't really need the full insulation?
     
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  3. ^woody^

    ^woody^

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    Better to insulate to remove the risk of condensation on the deck underside rather than for the purpose of warmth.

    As the shed won't be heated, I suspect this would be a situation where a ventilated cold roof would be better, with insulation at ceiling level and an air gap above.

    To insulate the roof fully or form a warm roof, you would need a PIR board insulation product not cheaper quilt, and this would be much more expensive and unnecessary for a unheated shed.
     
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  4. a546345

    a546345

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    That makes sense. Makeup like this then? I do have 75mm cavity PIR board I can use.

    upload_2019-12-7_12-42-2.png
     
    Last edited: 7 Dec 2019
  5. ^woody^

    ^woody^

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    Yes that is the principle
     
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  6. a546345

    a546345

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    Looked at this again, one end of the joists will now be closed off, as it will be more of a lean to, with a ledger board against the wall on one side.

    Would a cold roof still ventilate enough if one end is sealed?
     
  7. ^woody^

    ^woody^

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    It would really need venting at both ends.

    If abutment ventilation is too much faffing, then go with the hybrid roof. But fill it with insulation. If you don't have enough PIR, then use PIR under the deck and quilt below.
     
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  8. a546345

    a546345

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    Thanks. Going for abutment venting to get proper flow.
     
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