Cold Roof Insulation/Ventilation.

24 May 2019
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United Kingdom
Can someone give me a few pointers please on best practise with insulating/venting a cold flat roof on a new project. The rear single storey extension to a detached house projects 4.8m and is 9m across. I'm proposing 200mm trussed joists running front to back within a parapet wall all round. I'm hoping to put some French doors out onto the flat roof so need to keep the build up as los as possible hence going for a cold roof.

So with regards to insulation and venting. What insulation would work best to go in-between these types of joist and am I ok just with vents through the brickwork to stop condensation build up under the deck.

Thanks in advance, Ernesto
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You will need insulation between and under the joists with this type of roof, normaly about 50mm under, so make sure you have the headroom. Foil faced insulation normally, unless you can calculate the performance of quilt between, and board insulation below

You will need a minimum 50mm air gap below the deck and continuous vents at each end of the joists so that air can flow from end to end. And a vapour barrier at ceiling level to stop moisture getting up into the roof void.
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Trusses rafters

Trussed girders

Trussed joists

Trussed beams

Trussed me.

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