DIY Doctor

Installing Ceiling Lighting and running cable through insulated Cavity

Postby andyt1801 » Tue Mar 30, 2021 7:42 am

Hi everyone,

I’m looking for a bit of advice on fitting 2 ceiling downlights above a bar in a room of my house that’s being converted into a games room. I am planning on wiring these 2 lights to a fused spur switch next to the bar, this will then be connected to a nearby plug socket circuit. It’s not really practical to connect to the existing lighting circuit for the room as the wiring is all at the opposite end of the room, and the property is 140 years old which is making things challenging enough. I don't have a cavity wall, so all insulation is between the exterior stone wall and the plasterboard inside the room. As a result I'm keen to ensure I do not compromise the insulation or vapour barrier unless absolutely necessary.

I’m firstly looking to confirm if my electrical circuit plans are suitable? I have attached an annotated schematic. The Twin & Earth between the double plug socket and Fused Spur Switch is something I’m not 100% sure on due to it potentially passing through insulation. Would this need to be derated? Or is it okay for it to stay as 2.5mm since the cable is only going to a Fused Spur which is then being used for lighting? Otherwise, should I be using 4mm wire?

The next thing I need help with is the best method to route the cable around the insulated cavity (both around the corner between the socket and switch, and also from the switch up the wall into the ceiling space). Again I've made an image of this for reference. It is going to be extremely hard not to break the vapour barrier in places. I would either need to route the cable within the insulation and vapour barrier between the double socket and fused spur, or I would need to drill through the studs and feed the cable through the holes and then repair the vapour barrier. What would be the best option here? I’m keen to avoid too many holes in the wall when doing this, but I understand that some will be necessary.


Wall Cross Section.jpg
Wall Cross Section
Electrical Schematic.jpg
Electrical Schematic
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Simply Build It

Postby stoneyboy » Wed Mar 31, 2021 9:17 pm

Hi andyt1801
Your proposal to perforate the vapour barrier behind the plasterboard is not a good idea, even if you were able to make some sort of re-instatement. Suggest you extend the existing lighting circuit running cables in the ceiling space instead.
Regards S
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