Really need the advice of a knowledgeable roofer having consulted 2 local 'flat roof experts' and not been impressed.
We plan to retro fit a warm flat roof on our 1930's semi. The present roof of 82 m2 is board and bitumen felt, although 20 years old has not been subject of water ingress and is in good order. It is well suited to insulate with a pre boarded product and waterproof single membrane. There is a parapet wall to the front aspect and two chimney stacks on the boundary between the houses. Our neighbour has no plans to upgrade his roof.
My first question involves the cavity between ceiling and deck which is vented. Clearly if I proceed as planned it would be a waters of money as the warm air will disappear out of the air bricks. Is it a simple matter of sealing the air bricks ?
My next dilemma is the party wall. I don't know if the wall goes to ceiling height or deck height. If it is ceiling height then even if I seal my own air brick I will lose heat through my neighbour's . If the party wall goes to deck height then I would have a cold interface at the brick. Would I need to cut away a section of the roof along the party wall to insulate between the joists at this point.
Going from cold to warm roof required theb cavity to be full of insulation Bunker. If there is any air in the cavity then condensation will occur if there is no air flow (hence the vents) condensation in a sealed roof will eventually cause more damage that the rain ever will.
There is absolutely no point in placing insulation of any kind over a vented cold roof. The only way to effectively insulate this space is to insulate under the ceiling which will obviously reduce ceiling height or to pull of the roof covering (or the ceing below) then fill the void with slab insulation ensuring minimal air gaps and inviting intersticial condensation. Close the vents and recover-preferably in rubber. The party wall problem will then be solved also
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