I am converting my loft with the following build up: 50mm ventilation gap, mineral wool between rafters then pir then vcl.
I am considering whether to use a 25mm service void for wiring to speed up installation and reduce the number of times the vcl is punctured. I am installing an mvhr.
I would be grateful for some views on the pros and cons of putting some insulation in the service void and if I do go down this route what material would be best.
Thanks!

Thanks, my preference is mineral wool but I'm not sure how well the will friction fit between battens on the flat ceiling sections. I reckon a polyester roll would fit better but not so good on fire rating..

The ducts will go between joists and in the eaves. The eaves will be insulated at roof level but it's not going to be cost effective to make the eaves airtight so we will tape around the duct entry points to the living space at the ashlar walls

thanks, I have friction fitted thicker rockwool "flexislabs" overhead before but 25mm may not be as easy as 140mm..
It's a good point about the eaves, it's something I have been wondering about also so will start a separate thread

thanks, the eaves space won't be used. there is ventilation at the eaves via soffit vents and the ridge is ventilated. the roof sarking felt is in reasonable condition.
I'm keen to extend some insulation to the top of the cavity wall to prevent thermal bridging here (whilst maintaining the 50mm air gap above).
I will be using mineral wool between and above the roof floor joists in the eaves and to cover the MVHR ducting.
It won't take much more mineral wool to extend the insulation between rafters down to the eaves and I had thought this would provide some additional acoustic benefit (close to main road) and might help a bit with keeping the eaves warmer in winter and colder in summer (with less heat loss through the ashlar walls and from the ceiling in the living area below). I could just focus on insulating the loft floor and ashlar walls but given it won't be much more work to do the whole roof I thought it would be worth it but perhaps not!

A service void is a narrow space behind the surface material (e.g. plasterboard) that is used to run services such as electric cables. The space is usually created by using thin battens in front of the insulation/membrane/whatever.

I always worry about and often find that service voids are unintentionally draughty being connected all be it accidentally to outdoor air, even from inside a floor void etc

Hi Greenpaddy, that's a good question and one I discussed with my builder yesterday. I think the better solution will in fact be to fix the membrane on the eaves side of the steel to the flat ceiling. That way we can avoid all the posi joists and hangers and just have to tape around the old ceiling joists.