Warm flat roof two layers of insulation will it work?

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Question answered from a comment posted from a YouTube viewer.
Question was: Can I use two layers of rigid insulation in a warm flat roof construction? Theoretically yes you can, however, don't use insulation that has foil on the face, use tissue-faced flat roof insulation. Tissue-faced flat roof insulation allows more movement of moisture. New research seems to indicate that the vapour resistance of all the materials above the vapour barrier in a flat roof should be equal to, or less than, the vapour barrier. This makes sense, as some vapour will get past the vapour barrier at the bottom of the construction of the warm flat roof. If this vapour cannot get out of the construction above the vapour barrier, it has the potential to cause any timber in that construction to rot. BM TRADA recommends that warm flat roof designs do not include timber materials, such as decking, anywhere between the vapour control layer and waterproof roof covering.
More and more now I am seeing two or three-year-old warm flat roofs looking rotten under the waterproofing membrane, and the only conclusion to reach is that there is a build up of moisture that is migrating through the roof, but not being able to escape through the roofing membrane.

Steven Dickinson
London Flat Roofing
07802300099
[email protected]

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