Condensation on the underside of my flat conctete roof


Postby brent heathcote » Tue Mar 13, 2007 4:07 pm

I have built a shower room under flat concret roof. The false roof consists
of wooden battens to which plaster board has been attached. there is
three to four inches of polystyrene insulation blocks on top of plasterboard. The polystyrene has been cut away in three areas to allow
for 3 sunken lights into plasterboard. So there are 3 circular areas where
there is no insulation (fire regulations - due to heat from lights).
The roof space between the insulation and underside of concrete is approx 10 inches. The problem is that the underside of the concret, due to being a cold surface is absolutely dripping with condensation causing plaster-board ceiling to go mouldy and is starting to 'soften'. Would be
grateful for a solution. I have received conflicting advise !!
One is to put two air bricks into cavity space (leading to outside)
Second is definately do not put air bricks in but to insulate the top side of
the concret roof with special insulation board of 0.20 therm unit value??
if so how can it be fixed. The roof felt is new so can this be left on??
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Postby thedoctor » Fri Mar 16, 2007 1:03 pm

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Postby Gryfiss » Tue Oct 23, 2007 8:10 pm

This type of roof construction is known as a cold roof. The void between the insulation board and your roof deck MUST be ventilated. Install either trickle vents (leading to the outside) or some form of mechanical ventilation in the roof. This should sort your problem.
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