Wiring in insulated stud wall

[dan1w]

I'm doing the wiring for our garage conversion extending an existing radial circuit.

The studwork is filled with rigid insulation which has about 20mm total clearance between brickwork and plasterboard, to keep an air gap behind the insulation this will be reduced slightly.

How is this usually wired? I understand that the best way to avoid derating the cable is to run it touching the plasterboard, but then to clip the cable I'd have to chop into the insulation to make room for the second cable or fix an additional vertical stud for the second cable to run back up.

Or is it acceptable to notch the noggins? If so how far would they have to be apart to support the wiring?

The other option is running the cable behind the insulation but from what I can see this would derate 2.5mm cable to 13.5A.

 

[John D v2.0]

The studwork is filled with rigid insulation which has about 20mm total clearance between brickwork and plasterboard, to keep an air gap behind the insulation this will be reduced slightly.

So you already only have 20mm space and you need to install insulation? So the insulation will be only 10mm?

the best way to avoid derating the cable is to run it touching the plasterboard, but then to clip the cable I'd have to chop into the insulation to make room for the second cable or fix an additional vertical stud for the second cable to run back up.

Or you can run a single cable down, and put a maintenance free box at the top

The other option is running the cable behind the insulation but from what I can see this would derate 2.5mm cable to 13.5A.

As long as it's touching the brick it would be ok

 

[ban-all-sheds]

Less OK are all the holes in the vapour barrier that you get if you put cables there.