I have a flat roof above a bedroom over a dormer window - the roof is about
2m x 3m and was completely uninsulated. I have put 100mm rockwool insulation between the joists (pushing it in from inside the loftspace - a bit of a surgical job) - which has made a big difference. There is about 50mm space between the rockwool and the underside of the roof.The question is, how should I ventilate this space? It would be very difficult to add vents to the barge board or soffit. The building is victorian, therefore the "inherent ventilation" is generally good and the location is very windy - the loft space itself is well ventilated.
My plan is to find some lightweight 50mm ducting and lay it on top of the rockwool, then channel air from under the tiles into the duct.
I realise insulating on top of the roof would probably be the preferred way of insulating the room, but as the room is on the 4th floor, that becomes a very big job.
thanks for any advice, Andrew