Cold flat roof an how to insulate

Discussion in 'Getting Started FAQ' started by JOHN BEDDING, Feb 27, 2021.

  1. JOHN BEDDING

    JOHN BEDDING New Member

    I have an existing bitumen flat roof maybe 30 years old but still serviceable. I suspect it must be a cold roof but has had no insulation on the underside. I have taken down the ceilings and there is no evidence of past condensation although it is impossible to adequately ventilat, as the joists run to a blank wall. I was intending to put in 100mm celotex, can this increase the possibility of condensation and sweating problems, logically to me it seems to make it less likely.
     
  2. Jord86

    Jord86 Screwfix Select

    No as long as you use a vapour barrier or board the ceiling with foil backed plasterboard.
     
  3. JOHN BEDDING

    JOHN BEDDING New Member

    Thank you Jord I was intending to put in a vapour barrier you think that will do the trick? There are so many opinions out there it's difficult to get an idea of the risks.
     
  4. Jord86

    Jord86 Screwfix Select

    Is there any way of fitting soffit vents or drilling a hole through the wall for airflow?
     
  5. JOHN BEDDING

    JOHN BEDDING New Member

    Yes I can fit soffit vents on one end but then it hits a blank wall the other end which is a party wall
     
  6. Dan Parkinson

    Dan Parkinson Active Member

    You need to create a vented upstand where the joist meet the wall.....
     

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  7. JOHN BEDDING

    JOHN BEDDING New Member

    Do you mean vent each joist void, out through the flat roof.
     
  8. Dan Parkinson

    Dan Parkinson Active Member

    Yeah.

    Cut the deck back, sit vertical batons on top of joists and fix to wall. Plywood up the batons then cap off with a length of tilt fillet.
     
  9. JOHN BEDDING

    JOHN BEDDING New Member

    Thanks for that Dan, but I think I have decided as the roof is not in the full flush of youth to invest in a warm roof and not insulate the underside.
     
  10. Dan Parkinson

    Dan Parkinson Active Member

    A wise decision
     
  11. John9857

    John9857 Member

    If you insulate above the flat roof part and use Kingspan TR27 or Celotex Crown-Up of crown-Fix or something on to that. Use a good Phenolic insulation like Kingspan K110 or Kingspan TP10 maybe. Both of which will give great U-values
     

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