u values for new warmroof constructed flat roof

dilly · 28 Nov 2009

hi got to construct a new flat roof,this is not a refurbishment it is completely new.does anyone know what u value i need to achive.It will be used as a balcony as well therefore what will be the most suitable insulation for it,ideally the insulation wants to be of minium height .
thanks

noseall · 28 Nov 2009

We are currently having to install 97mm pre-insulated plywood deck (Kingspan) on top of the joists then a further 25mm between and fitted tight to the underside of the 97mm deck.

dilly · 29 Nov 2009

thanks for that but u value are you getting,also are you having to allow for cross ventilation as your putting in insulation betwwen joists

freddiemercurystwin · 29 Nov 2009

The U value required for a roof on a domestic extension is 0.20. If it is on a new build house it is 0.25.

Only a call to Kingspan Technical department can confirm the exact insulation thickness required. It depends on the joists spacings. It is not clear from the online literature.

No need to ventilate a 'warm roof' where the insulation is above the joists as there is no condensation risk as the joists will be the same temp as the room below.

noseall · 29 Nov 2009

dilly said:
thanks for that but u value are you getting,also are you having to allow for cross ventilation as your putting in insulation betwwen joists

As Freddy has said, no need for a warm roof.

Agreed, the 25mm is between joists but fitted tight against the deck so as not to generate an air gap. Foil tape the joints.

dilly · 29 Nov 2009

thank you chaps.

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u values for new warmroof constructed flat roof

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