Converting flat roof from cold to warm roof

Discussion in 'Builders' Talk' started by James Askew, Jun 13, 2018.

  1. James Askew

    James Askew New Member

    One of the many little gems I've come across when renovating my house is that my flat roof is a cold roof - the insulation sits directly upon the plasterboard with a gap above between the insulation and actual roof.

    I'm contemplating converting this to a warm roof as I may as well do it now whilst all the other work is going on and am trying to decide upon the best way to do it.

    Would it work to fit new insulation tight to the underside of the roof instead of removing the felt and going over the top? My concern here is that I'd still be limited to going between the joists which doesn't seem ideal.

    I could theoretically run the vapour barrier under the insulation and under the joists to provide an uninterrupted vapour line, but would have to screw through this to fix the plasterboard which seems like it will defeat the whole point of a vapour seal. Would this make a difference?

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  2. Dam0n

    Dam0n Active Member

    I would have thought you would need a 50mm airgap between the felt and any insulation.

    Not overly sure how much you will gain from this whole endeavour as you have insulation in place anyway.
  3. James Askew

    James Askew New Member

    That'd be a cold roof again which is what I don't want.

    By putting the insulation above the vapour barrier you vastly reduce the possibility of condensation build up internally (i.e. between the insulation and ceiling) which would damage the ceiling, joists, etc. in addition to having a continuous insulated layer, rather than the thermal bridging that is caused by only insulating between rafters.

    I'd have the thermal bridging but I'm hoping that the vapour barrier beneath the joists would reduce the likelihood of condensation build up below.

    Have a look at this if you want to understand more -
  4. rogerk101

    rogerk101 Well-Known Member

    I don't believe that your description above is that of a warm roof. What you have described is the way in which nearly all well insulated modern roofing is done anyway, and this needs to be followed for most building control sign-off.

    Working from the outside downwards to the habitable room space, you should have:
    • waterproof cover (tiles or fibre glass + resin roofing, etc.)
    • battens if pitched tiled roof or plywood if flat roof
    • breathable membrane if pitched tiled roof or nothing if flat roof
    • ventilation air gap of at least 50mm with vents at bottom and top of roof pitch
    • rafters if pitched roof / joists if flat roof
    • bare minimum of 100mm extruded polyurethane insulation panels between rafters/joists (ideally more like 140mm)
    • bare minimum of 20mm extruded polyurethane insulation panels mounted below the rafters/joists (ideally more like 30mm); taped where panels meet
    • vapour barrier (significant overlap between adjacent pieces and gap taped
    • plasterboard
    • skim coats
    • paint
    There will always be a dew point in a roofing system. The dew point in the above system is designed to be in the ventilation air gap for most cold moist weather conditions, and condensation will occur there, but it should be minimal (a) because of your very well sealed vapour barrier, and (b) because of the good air flow in the ventilation air gap. Whatever condensation does build up in there should be 'ventilated' away with good airflow, rather than building up to create any drips or long term damp spots.
  5. James Askew

    James Askew New Member

    You're right, that is essentially what I have, albeit that I don't have a vapour barrier as current. That however is a cold roof, relying on ventilation (which I also don't believe is present due to no openings at end of roof, but need to check) to disperse any water vapour carrying air.

    If the insulation is mounted above the joists, the position of the dew point will be on the outer surface of the insulation, rather than within the ventilation zone, thereby preventing internal condensation. What I'm trying to ascertain is whether fitting the insulation between rafters and vapour barrier below will mitigate the risk enough to not have to re-do the roof...

    Hope that makes sense...
  6. egon999

    egon999 Member

    Warm Deck: Insulation above joists (no ventilation required)
    Cold deck: Insulation between joists (ventilation required)
    The term warm/cold deck is usually limited to flat roofs
    If you have a cold deck roof and want a warm deck one instead then strip off the old felt and use rigid insulation above the old deck (kingspan/celotex etc)
  7. BMC2000

    BMC2000 Active Member

    The correct way to change a cold deck to warm deck.

    1. Block existing ventilation throughout
    2. Take out any insulation that exists below the deck, keeping it here will create interstitial condensation
    3. Fix PiR insulation over the felt. The felt if in good condition will act as VCL. 120mm will meet your regs
    4. Fit waterproof membrane eg single y membrane
    5. Plasterboard and skim to underside of joists.

    This is the industry standard refurb method. If the felt is iffy, remove and fit VCL. If deck is chipboard or iffy, replace with 18mm exterior grade ply or OSB3

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