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Hi,
Got a slate roof on my outhouse with a breathable membrane installed. I want to insulate the roof to reduce condensation forming, from what I've read, 10cm on Kingspan or similar is the way to go. However, I've also read that an air gap (usually 5cm) is sometimes be left between the insulation and the breathable membrane. Any ideas under what circumstances the air gap should be left and does it apply to my situation?
Thanks.
In one of our outhouses the insulation sheets had been laid on the beams, a bit like you would in an house - with 'loft' space above.
Not sure if that helps.
50mm air void is the minimum req for ventilation to allow condensation to evaporate. If you insulate under it, you increase the temperature differential and make the risk of condensation greater, thus making the need for the air gap even more important.
If you installed a moisture barrier with the insulation you will mitigate the problem, but you'll still need the air gap. If the outhouse isn't heated, you're not going to see much benefit of insulation though, so I'd consider increasing the ventilation and looking at the overall moisture input into it in the first place.