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Multiple layers of insulation to insulate flat roof?

Postby timbosw » Tue Aug 24, 2021 9:29 am

I have some left over insulation from some other work and a ceiling below a flat roof which has been removed to take out the asbestos.

I have enough insulation to put a layer of 25mm kingspan and then a layer of 50mm kingspan. Am I right in thinking that the best way to do this is to have air gaps either side of each layer of insulation, rather than, say pushing it all up to the top in one 75mm layer? So that the reflective foil can do its job.

Also, if I were to put the 50mm in between the joists and then have the 25mm under the joists would the plasterer I get round to put the ceiling back think I was mad, or (assuming I mark where the joists are) would he be fine to put the plasterboard on and use 25mm longer screws than if it wasn't there?

Thanks,

Tim
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Postby stoneyboy » Wed Aug 25, 2021 8:42 pm

Hi timbosw
Better to leave an air gap above the insulation, fit 50mm between the joists then overboard with 25mm as you have suggested. When fitting the insulation foam fill any gaps so there are not cold spots.
Regards S
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