Flat roof insulation

25 Aug 2014
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United Kingdom
We have a flat roof extension which is about 25 years old and has a cold-deck roof. There is some rockwool type insulation between the joists which I can see trough the ceiling rose hole but I would like to improve on this and bring the roof up to current standards.
Rather than converting it to a warm deck, could I add a layer of thermal plasterboard to the ceiling, and if so what thickness would be needed to make a worthwhile difference, or would I be creating condensation problems within the roof void?
Thanks for any info.
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if the roof is cold deck then it should be ventilated already. regardless-adding insulated plaster board isnt going to cause any detrimental effect. probably cheaper to put insulation on top and re surface your roof maybe?

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