Flat Roof

[hellow]

Question I had a flat roof designed wrong/build and trying to find the "correct" answer to fix it.
Currently have the roof build as follows

Joist 200mm @400 Centers
75x50 firings
18 mm OSB
Membrane
50mm PIR insulations
18mm OSB
oprema 3kg per
5Kg per cap sheet

No this doesnt meet the Uvalues
can i just do the following:

3mm skin
12.5 plasterboard
20mm PIR under joist for no bridging
100mm PIR between Joist
Joist 200mm @400 Centers
75x50 firings
18 mm OSB
Membrane
50mm PIR insulations
18mm OSB
Soprema 3kg per M
5Kg per cap sheet

50MM ventilation gap between first 18mm OSB and 100 PIR insulations between the joist

or do we just put 100mm PIR ontop and refelt ect

 

[noseall]

Question I had a flat roof designed wrong/build and trying to find the "correct" answer to fix it.
Currently have the roof build as follows

Joist 200mm @400 Centers
75x50 firings
18 mm OSB
Membrane
50mm PIR insulations
18mm OSB
oprema 3kg per
5Kg per cap sheet

No this doesnt meet the Uvalues
can i just do the following:

3mm skin
12.5 plasterboard
20mm PIR under joist for no bridging
100mm PIR between Joist
Joist 200mm @400 Centers
75x50 firings
18 mm OSB
Membrane
50mm PIR insulations
18mm OSB
Soprema 3kg per M
5Kg per cap sheet

50MM ventilation gap between first 18mm OSB and 100 PIR insulations between the joist

or do we just put 100mm PIR ontop and refelt ect

Put 100mm on top. Yiu dont want to create a cold roof - thus creating the need for additional troublesome venting.

 

[freddiemercurystwin]

Or fill the joist/firing void beneath with rockwool.

 

[hellow]

Or fill the joist/firing void beneath with rockwool.

will building control approve this? will I get the U value that's needed ? they keep talking about condensation risk assessment

 

[freddiemercurystwin]

I would ask them see if they'll accept it, bit of a hybrid, not too disimilar to what is sometimes done on a garage conversion where the existing roof finish has stayed on and ventilation would have been dificult to achieve.

 

[noseall]

Or fill the joist/firing void beneath with rockwool.

Can moisture laden air not pass through quilt type insulation?