Position of VCL - warm flat vs warm pitched

[JimExtension]

Hello. When I look at construction details for warm flat roofs I see the VCL is placed above the joists. However for a warm pitched roof the VCL is placed below the rafters.
For example see the attached image from https://www.cncbuildingcontrol.gov.uk/wp-content/uploads/2023/06/CNCD-053-Warm-Roofs-Easy-Guide.pdf

Can anyone explain why the VCL is in a different place depending on whether the warm roof is flat or pitched?

Thank you.

 

[Deluks]

In both cases the vcl is below the insulation, which is correct.
The pitched roof has ventilation over the top of the insulation in any case. Warm roof as terminology usually refers to a flat roof.

 

[Notch7]

However for a warm pitched roof the VCL is placed below the rafters.

pitched warm roofs arent really warm roofs -yes the insulation is above the rafters, but there must be counterbattens over the insulation so there is ventilation between tiles and insulation

what is important is that the VCL is positioned on the warm side of the insulation -it is there to prevent warm air which can hold lots of moisture working its way through the insulation layer and to the cold side.

VCLs onyl work if they are continuous and for that reason they are always positioned on a flat plane -ie they cant wrap around joists or rafters -which is why hybrid roofs arent a good idea

 

[JimExtension]

It looks like adding down lights to a pitched roof is therefore much more problematic than a flat roof as you’d be piercing the VCL for the electrics/lights.
Even if you try to use sealed fittings I’m guessing it would be as fully sealed as you could get with a flat roof?

 

[robinbanks]

No point spending hours meticulously fitting a VCL then bashing a load of holes in it

 

[datarebal]

A pitched warm roof is basically the same as a pitched flat roof . Not ventilated . Vcl warm side of insulation