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 (9)

  • 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 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
  • Robert Westwood Reply

    Hi Steve fantastic video, I have a client who is losing a lot of heat through his warm deck roof, he is requesting we also insulate below. Whether it’s between the roof joists or below with insulated plasterboard I am not sure if this would create other issues. Is there any advice you could pass on? Thank you very much.

    31st December 2022 at 10:56 am
    • Steven Dickinson Reply

      If you have a good warm roof, then you should not be losing any heat through it. Adding insulation under it can be problematic and is not something that is recommended. Generally, if you have to add insulation under a warm roof, you do not add more than one-third of the thermal performance of the insulation that is above it. This is because adding insulation under the decking that has the air vapour control layer on it, lowers the temperature of the decking that bringing the dewpoint lower in the roof and closer to the critical area where you do not want to get condensation or any kind of sweating (interstitial condensation).

      1st January 2023 at 4:14 pm
    • Wilson Kelly Reply

      Hi Steve, you’ve videos are really informative. I’ve done a cold roof on my flat roof loft conversion and it hasn’t been vented properly. I’ve spoken to a local roofer and I have just enough room to fit a warm roof on with a lead ridge fitted. Would I need to remove the insulation between the joists? It has a 50mm air gap with a 100mm celotex between the joists and 25mm across them.

      14th June 2023 at 6:16 pm
      • Steven Dickinson Reply

        sorry for the late response I get so much spam on this and I only have to go through it once every two or three months but if you still need my help please just get in touch with me via my email address. Speak soon

        19th August 2023 at 8:51 pm
  • Rob Wootton Reply

    Hi Steve, all great videos. Very good point on insulating behind the fascias, and I can see the problem. However, I would like to know how this is done, as how do you get a proper vapour seal on that part of the insulation?
    I’m presuming the fascia is removed and plugs of foil faced PIR are placed between the joists and layer of OSB that sits on top of the joists (vapour barrier foil orientated towards the fascia). But how is sealed properly from the inside? i.e. If the vapour control layer (VCL) is laid over the OSB to start the Warm Roof, and brought down over the fascia PIR, it will be on the ‘outside’ not ‘inside’ of the fascia insulation. Or if the OSB on top of the joists is lifted up and the inside of the fascia PIR taped at gaps with foil tape, how is sealed to the OSB when it is put back down?
    I’m a bit confused with that. If you could please advise the process? Thanks very much.

    24th March 2023 at 2:26 pm
    • Steven Dickinson Reply

      sorry for the late reply I get so much spam on this that is really hard to find the real people who need help. However I’m here now if you need help still please just email email me speak soon

      19th August 2023 at 8:56 pm

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