Cable direct clipped to wall behind PIR insulated plasterboard

[jwilf]

I'm in the process of insulating a room located above the garage in my house. It has 3 solid brick external walls so gets very cold - I'm installing insulated plasterboard with 50mm PIR (celotex) bonded to 12.5mm board.

My plan is to fix the boards using Soudal PB adhesive foam, direct to the walls (onto the existing plaster). There are some sockets around the room and I'd like to change the locations and add a couple of new ones. Currently the cabling goes under the floor, however for ease of installation I would like to run new cable horizontally around the room between the new socket locations.

My thinking is to clip 2.5mm T&E direct to the walls (which are solid masonry external walls so they are cold) then chase a channel in the back of the PIR to accommodate the cable when I stick the insulated PB to the walls.

My question is do I need to consider any derating factor given the cable will be behind the PIR insulation? The load on the circuit will not be high (it's an upstairs ring main) and the cable will be in contact with the cold wall, and not directly in contact with the insulation (I will cut the chase in the insulation sufficiently large to create some space around the cable) so my thinking is it will be OK, but would like to get some thoughts from others.

Thanks

 

[EFLImpudence]

I would think from your description that it will be alright.

However, to be certain, you could use 4mm² T&E in the affected areas

or

Reduce the MCB to 20A which will be adequate for upstairs.

 

[flameport]

then chase a channel in the back of the PIR to accommodate the cable when I stick the insulated PB to the walls.

Not necessary - just clip the cable to the wall and fix the insulation directly over it.

 

[jwilf]

Fixing the insulation without cutting a channel isn't going to work, I'm attaching it directly to the wall using foam so there won't be any gap.

So the alternative would be to chase the cable into the wall first, but I'd rather not do that if it's not necessary.