DIY Doctor

Flat Roof Causing Condensation to Freeze

Postby monkeyno2 » Fri Sep 28, 2018 12:27 pm

Hello, I have an issue with my flat roof. I have a 3 foot cavity between my ceilings and a concrete flat roof which is about 2 foot thick. Last winter we found that our ceilings were becoming damp. On inspection of the loft cavity we found that the loft insulation was sodden through. It was that cold up there that any condensation was freezing then melting and dripping down onto the insulation. Our bathroom ceiling eventually caved in due to the weight of the insulation that had become completely saturated with water. We stripped the insulation out and got two dehumdifiers up there and to be fair they are doing a good job at taking the moisture out of the air and over the summer the loft has been dried out but it's leaving cracks in the plasterboard in the ceilings now. It's getting to the same time of the year now and we need a solution to insulate.We fear that the same thing will happen. Ventilation isn't great in the loft. We paid a company to drill some holes and fit vents which they did to a substandard level we think. The exterior walls are over a foot thick which caused them great issues. So there are a serious of drill holes at 6 points in the side walls. Is there anyone out there with experience of an issue similar to ours? Should we just insulate with traditional insulation again or look at spray insulation? Or get some sort of fan in the exterior walls to help airflow?
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Postby collectors » Sun Sep 30, 2018 3:17 pm

Depends on the size of loft area, but it sounds like you want 2 to 3 x 6" vents at either end & prerebly 2 at low level & 1 at high level to get the best cross flow of air. A good drill with a clutch & a masonry diamond tank cutter would work fine for this similar to what a plumber uses for soil pipes.
I would also check you have no leaks 1st. Get a hose on the roof & give it a good soaking & see if anything comes through.
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