Discuss Thermal insulation covering cable in the Electrical Forum area at ElectriciansForums.net
The gap may be warm and you'll need to factor this in to your cable sizing calculations, but you need to avoid putting holes through the insulation as much as possible to avoid compromising it so in the gap is probably the best place for the cables.This is on the walls so feeding everything.
ELEC htrs, shower, water htrs, lighting power
. Just think that gap will be the warmest place?