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Flat roof(cold deck) interstitial condensation??

have tacked breathable membrane(waxed side up) on joists and then added firrings(40mm done to 3mm) and then wbp sheets on top then felt and coated in instant waterproof tar.the joists run from front of cabin to rear 4.2 meters long and is 6 meters wide with (5) 75mm vents evenly spaced across the front and directly opposite at rear,to allow air travel between the joists. under membrane there is a 65mm space then 25mm kingspan sheets and then plasterboard(non foil) have i done this correct as i have condensation on under side of membrane which builds up after a few weeks which ends up with collections of water pellets on kingspan which forms little water pools and drip done through led done lighter spotlights.

2 Answers from MyBuilder Roofers

Best Answer

Check the Kingspan website and it details the construction method. Cold deck roof construction is still a current method used (unless this has changed in the last couple of weeks). You should have insulation between the joists and underneath and maintain a 50mm air gap between the top of the insulation that is between the joists and the underside of the ply deck. The downlighters should not breach the insulation between the joists as this is a fire hazard. Sometimes, you have to counterbatten the underside of the joists to get a deep enough void for the downlighters.


Answered 29th Dec 2012

You need to contact tech help @ kingspan, 25mm sounds to thin for a cold roof.

Also current regs state all new coverings should be warm deck (approved doc L) for habitable rooms.

On a warm deck I would use 105mm plus 100 quilt between rafters with 50mm air gap.

Your cold roof should also have a vapour barrier between joists and plasterboard (foil backed).

Hope this helps



Answered 22nd Dec 2012

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