Warm Roof and Insulation Between Joists

Warm Roof and Insulation Between Joists

Constructing a warm roof on a new extension with parapet walls, I will have surplus amounts of insulation left over and am considering adding this insulation to the underside between the joists to double up the insulation, would this install have any negatives?

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Comments (3)

  • Steve @ London Flat Roofing
    Steve @ London Flat Roofing Reply

    Good question, theoretically when building a warm roof the Vaper control layer goes on the warm side of the insulation. If you put insulation under the decking and between the joists you lower the temperature of the decking and lower the due point thus bringing it closer to the danger point of being in the building. That said, I have been given calculations in the past that specify exactly what you are thinking of doing. From my understanding, the calculations work as long as the mass of insulation is above the vapour control layer. Twice now we have had specifications that show 100mm on the outside and 50mm on the inside. I only ever pass on my experience and suggest you get calculations done before you?
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    23rd November 2017 at 7:57 pm
  • Sean Reply

    Great videos – Thanks for sharing

    This Quetion is to do with warm roofs and cold roofs,presumably, the 50mm of insulation on the underside of joists would act as the vapour barrier if taped/sealed correctly or if insulation between joists a vapour barrier (i.e. poly) to the underside of joists? Furthermore would the void between the insulation layers (between joists) now need ventilation due to possibility of bringing this void temperature closer to the dew point?

    8th December 2017 at 10:38 pm
    • Steve @ London Flat Roofing
      Steve @ London Flat Roofing Reply

      Yes, the underside of the insulation on a cold roof can be used as a vapour barrier, make sure its airtight as the transfer of air transfers moister also.
      Yes in a cold flat roof you need to have through ventilation above the insulation.

      11th December 2017 at 8:14 am

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