1st floor overhang (external below) detail - help!

[sean1933]

Hi all. I'm just in the process of laying the upstairs floor joists and this detail is giving me a headache.

 

Buildup is  blown EPC beads in cavity. @ overhang between joists /steels will be glass fibre/wool with 75mm sheet of PIR underneath (under boarded with cement board probably). Note the red airtightness/vcl membrane (ignore where it laps into tray - no need for this as wet plastered wall), whilst this is fine I wonder whether there should be a secondary external membrane such as protect vp400 to protect against windwashing and vapour release, or is this unnecessary in my scenario (drawn in blue).

 

The attached link whilst for a timber suspended floor suggests there should be, but is this unnecessary where I have impermeable PIR underneath that I can tape?

Note I would run the VP400/Pro Clima Solitex Plus straight under rather than draped to pickup insulation under the 1st storey cavity wall.

A Best Practice Approach To Insulating Suspended Timber Floors | Ecological Building Systems

 

Thanks in advance.

 

Edited March 18, 2021 by sean1933