Insulating a flat roof extension?

Discussion in 'Builders' Talk' started by EddieEdmunds, Jul 20, 2020.

  1. EddieEdmunds

    EddieEdmunds New Member

    Hi all, I have just taken down the ceiling in my kitchen extension and discovered there is no insulation in it at all. I've seen various comments re creating a vapour control layer to control condensation and moisture but not exactly sure of the best way to go about it. It is being reboarded and skimmed next week so need to get something done in readiness. 2020-07-19 10.01.50.jpg 2020-07-19 10.02.58.jpg

    Cheers, Edd
  2. DevilDamo

    DevilDamo Member

    It’s not unusual for the older roofs to not have any insulation. Insulating below the deck, i.e. cold roof would normally require venting and you may also find yourself in the realms of notifying Building Control as it’s an elemental change.
  3. rogerk101

    rogerk101 Screwfix Select

    If it were my roof, I'd postpone the plasterer and take the time and effort to do the job properly in a holistic manner, rather than the botch job you'd have to do if you tried a minimal fix.
    To do that, you'll have to accept that the tar paper and the t&g planks have to come off, so you'll need good weather or a good tarpaulin.
    Rip it all off, but leave the existing joists in place. They look like they're 100x50, which isn't deep enough for both the required insulation and the air gap for venting. If it were mine, I'd double them up, so nail another set of 100x50s on top of the existing ones. That gives you 200mm space which is enough for 170mm of insulation and 30mm air gap (or 150mm insulation and 50mm air gap).
    The air gap needs venting on both the higher side and the lower side, so the air can flow and get rid of any moisture build up.
    I would cover the whole lot with OSB or plywood, and fibreglass the top. That'll ensure that it's all there and fully waterproof for your great grandchildren!
    So from the top down ...
    • fibreglass top coat
    • fibreglass main coat
    • 22mm (or 18mm) OSB or plywood
    • 30mm air gap
    • 170mm rigid insulation between joists
    • 20mm rigid insulation below joists
    • vapour barrier
    • plasterboard
    • skim
    • paint
    What I've outlined above is for a cold flat roof. Even better is a warm flat roof, and there's plenty of info about those in this forum and all over the web. There are also some excellent videos on fibreglassing flat roofs and on installing cold and warm flat roofs.

    Forget building control ... whatever you do is going to be better that what's there today. Just take lots of photos while you're doing it so if ever anyone wants to do an EPC on the building, you can show them what's hidden behind the ceiling without having them drill inspection holes to verify it all.
  4. Jimbo

    Jimbo Screwfix Select

    You might be consider insulating below, ie between plasterboard and joists. Eg 50mm or even 25mm of celotex would be a tremendous improvement.

    If you are getting building control they will want a lot more.
  5. Frutbunn

    Frutbunn Member

    Not necessarily there is a certain amount of discretion within the approved document what is practical etc
  6. EddieEdmunds

    EddieEdmunds New Member

    Thanks for the replies folks. The boarder came yesterday as he was doing the whole bungalow so I couldn't put it back but did stop him doing the flat roof part so that is now left as is.

    Rogerk101 option is pretty in depth and bearing in mind I am refurbing the whole bungalow costs do come into it. In a couple of years I may be looking at extending out the back with another flat roof extension so could get them to sort it properly, but in the mean time.......

    My boarder mentioned as per jimbo to use celotex. Leave a 50mm gap from roof, then celotex followed by polystyrene sheets and then battens followed by plaster board. What do I do from an air flow point of view? There is obviously a gap from the flat roof through to the main loft but does it need venting at the front as well?
  7. Jimbo

    Jimbo Screwfix Select

    Yes, vent on soffit. TBH it would likely be better (assuming the timbers are 100mm) to do a heavy-weight 50mm rock wool within the joists (like 50mm cavity batts for example) then use a either celotex and plaster board across the joists inside (no need for battens) or an insulated plaster board, which just saves a step. Use long plasterboard screws to go right through the celotex/foam into the joists.

    The reason for the rock wool is two-fold - first, that celotex is only effective if it's completely sealed. If there is any air flow at all around it, it will have almost no insulating properties. So fitting between the joists is difficult to guarantee any performance.

    Second, the rock wool will also provide some level of acoustic insulation, where celotex is almost acoustically transparent.
    Last edited: Aug 1, 2020
    EddieEdmunds likes this.
  8. rogerk101

    rogerk101 Screwfix Select

    Bottom line is that there needs to be a flow of air ... otherwise there's no point in the air gap. On my flat roofs I somehow create an opening in the lower part and another in the upper part and doing that has always cured any buildup of condensation. Without it, there WILL be condensation which WILL accumulate over time, and WILL eventually rot your timbers or cause problems with your plasterboard/paint.
    EddieEdmunds likes this.

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