So, I have just finished internally insulating the walls of a 200 year old house. I remove lath and plaster from the internal walls, left the sound original plaster on hard on the walls, and stuck PIR then plasterboard over.

The pic shows the one section I have yet to finish as I am not sure what to do around this mass of electrical cables, which was originally run behind the lath and plaster. I have left a space around the cables and my choice now is either to stick the plasterboard over the top and leave this section completely uninsulated, or fix 10mm PIR over the top before the plasterboard to reduce the cold bridge.

Its really difficult to find anything definitive about insulation and cables, on one side its clear that burying cables in lots of insulation is a fire risk, but how much insulation is not so clear. My gut tells me that a channel like this with a cold outside wall on one side of the cables and 10mm PIR on the other is probably not going to be a problem, but not really clear on what’s allowed/sensible in this situation.

You say there is "a cold outside wall on one side of the cables" but as revor says there seems to be another room or cupboard behind the wall. Clarification, perhaps more pictures, would be helpful.

Posted By: revorYou are correct to be concerned. What is on the other side of the door can see some meters is it a cupboard?. Is it brick behind the cables? You mention an outside wall. The cables in themselves are bunched to some extent and that in itself is a derating situation. Is there a consumer unit on the other side of the door? How much thickness of insulation is there there. A possible solution would be to cover cables with a metal mesh then cover this with insulated plaster board or foil type insulation and 9mm plaster board. The mesh will keep the insulation off of the cables. Probably quite some time has elapsed since wiring was done don't think that would comply today.

Posted By: djhYou say there is "a cold outside wall on one side of the cables" but as revor says there seems to be another room or cupboard behind the wall. Clarification, perhaps more pictures, would be helpful.

Behind the cables is lime plaster, probably about 200 years old as it was behind the lath and plaster, that lime plaster is straight on to an external stone wall. Below the cables is a shallow cupboard on the same external wall and this contains the incomer, meter, consumer unit etc. The cables above are all of the circuits in the house, coming out of the CU below.

Posted By: philedgeYou can derate each circuit by reducing the fuse/MCB rating. If you can't or don't want to derate, you could cut a wide channel in the brick at the back of the cables then wet plaster/render them in situ in the channel and insulate over the top of the plaster/render. Any heat rise in the cabling will be conducted into the brickwork.

If you take a realistic look at circuit loads you might surprise yourself about how little load there is on most circuits that would allow you to significantly derate in line with the regs

So its ok to have insulation on the inside if the cables are buried in plaster, am I understanding that correctly? Is it just that the cables are not allowed to touch against the insulation? I had been thinking that if I put the 10mm PIR accross the gap then plasterboard over, there would be an air space behind to conduct away any heat, but maybe I am not understanding the regulations on this.

Posted By: Peter_in_HungaryThe aim is to carry away the heat created when the cables are running at or near their rated capacity. As such (still) air is not a good conductor of heat which is why philedge suggested plastering them in to give a good method of conducting away and heat. So I would second the suggestion of plastering them in ensuring that all cables are within and in contact with the plaster then insulate over.

Ah, I am with you now. So, ideally if I was able to carefully chisel away the old lime plaster behind those cables and push them in tight against the stone wall, then bury them in plaster, I could then just cover with the same 40mm PIR I have on the rest of the wall or whatever lower thickness would fit on top of the plaster?

Or, if the above didn't work out, just plaster over them where they are sitting now and stick 10mm PIR over this section, as long as the cables are buried enough that the don't touch against the insulation?

Is there any particular depth they need to be in the plaster or is the main thing that they are in contact with it?


P.S. I had thought about derating before but discounted this because I would have to derate every circuit in the house, and while I could easily derate the upstairs ring, lighting etc, its probably not an option for the electric shower and kitchen/cooker circuits.

Excellent. That should be do-able. I'll err on the side of caution with the insulation, make sure there is plenty of plaster around the cables.

Thanks philedge, and all who responded, very helpful.

Thanks again.

Presume it doesn't matter what kind of plaster is used, just easy-fill, multifinish or whatever I have available?

Brilliant. Thanks for this.

I searched for ages on the internet, and other forums trying to make sense of this and everything was very vague, so great to get clarity on this. Love this forum.