Evening All,
I have an extension to be built with a warm flat roof, likely format, 18mm deck, VCL, 120mm Roof Board / PIR type insulation like Kingspan TR27 and a built up high performance felt membrane, probably 3 layer.
There is also a false pitch where things look a bit more tricky to try and get continuity of insulation.
What is the normal accepted approach to try to get similar levels of insulation / air tightness in that perimeter corridor, beneath the false pitch, around 3 of 4 sides of the extension area?
The false pitch is pitched off struts from the flat roof joists 400mm back from the walls.
The struts and the false pitch roof joists are 100mm deep.
The designer appears to be showing insulation bats between rafters, nothing actually written in text and in addition, 165mm insulation [edit: boards] sitting just above plaster ceiling level between flat roof joists just in that false pitch corridor. Not in rest of flat roof due to warm roof being present.
Worries me that corridor area, (the outer ceiling / roof cavity), could become trapped with warm / moist air with tight insulation between rafters below the warm deck level.
The ceiling plaster board is specified foil backed but won't stop everything, that is what the VCL is for?
Would it be better to fit insulation board between false pitch rafters, like 50mm or 75mm between the 100mm deep rafters of false pitch and then another 50mm or 75mm underneath the false pitch rafters? Aim would be to try and get similar performance to the 120mm insulation on rest of roof to avoid issues, edit: Cold spots, condensation, drafts...
If I am on right track, could you or would you put a vapour control layer on the underside of that insulation below rafters to help seal for airtightness or better let moisture vent through the breather membrane and plain tiles?
Image attached.
Appreciate any help. Thanks
I have an extension to be built with a warm flat roof, likely format, 18mm deck, VCL, 120mm Roof Board / PIR type insulation like Kingspan TR27 and a built up high performance felt membrane, probably 3 layer.
There is also a false pitch where things look a bit more tricky to try and get continuity of insulation.
What is the normal accepted approach to try to get similar levels of insulation / air tightness in that perimeter corridor, beneath the false pitch, around 3 of 4 sides of the extension area?
The false pitch is pitched off struts from the flat roof joists 400mm back from the walls.
The struts and the false pitch roof joists are 100mm deep.
The designer appears to be showing insulation bats between rafters, nothing actually written in text and in addition, 165mm insulation [edit: boards] sitting just above plaster ceiling level between flat roof joists just in that false pitch corridor. Not in rest of flat roof due to warm roof being present.
Worries me that corridor area, (the outer ceiling / roof cavity), could become trapped with warm / moist air with tight insulation between rafters below the warm deck level.
The ceiling plaster board is specified foil backed but won't stop everything, that is what the VCL is for?
Would it be better to fit insulation board between false pitch rafters, like 50mm or 75mm between the 100mm deep rafters of false pitch and then another 50mm or 75mm underneath the false pitch rafters? Aim would be to try and get similar performance to the 120mm insulation on rest of roof to avoid issues, edit: Cold spots, condensation, drafts...
If I am on right track, could you or would you put a vapour control layer on the underside of that insulation below rafters to help seal for airtightness or better let moisture vent through the breather membrane and plain tiles?
Image attached.
Appreciate any help. Thanks
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