Raised Tie Trusses - Insulation + Airtightness

[BadgerBadger]

We've been a little restricted on both ridge height and plan depth, so are using raised tie trusses to achieve internal ceiling height.  So from the wall plate there is a short section of sloping ceiling, before it flattens off across the main ceiling level.  Above this there is a small, but very usable, loft space which is also intended to house our MVHR unit. I'm current debating how best to insulate this detail and make it airtight?

 

Our architect detail has rigid insulation between rafters on the sloping section (with a thinner layer underneath to limit thermal bridging) and then switches to a thick layer of rockwool above the flat ceiling.  So our airtightness layer follows the internal ceiling.  This detail leaves our MVHR unit in an uninsulated space, the thick layer of rockwool reduces headroom/crawl space further, and all the upstairs light fittings and MVHR terminals will be crossing the airtightness layer.

 

So it crossed my mind if a better solution might be to run the rigid insulation all the way up the ridge, therefore bringing the loft space into the thermal/airtight envelope?  There would be some fiddling sealing required around the ceiling ties, and general trickiness of installing the insulation but we have some vaulted space elsewhere in the build so it would be consistent with this.  We would likely still want some acoustic insulation at ceiling level, but it overall might reduce the amount of space lost in the loft and make the MVHR detailing easier.

 

[Jenki]

On my planned build I was thinking similar, but I see massive issues with getting this airtight ( planned camber truss and a few attic truss for storage) I'm now leaning to moving to standard truss and cold roof.

The MVHR ducts will be below the insulation, and I was thinking of adding a 75mm service void at the ceiling to run pipes/ cables minimising the penetrations to the cold loft and airtight membrane.