Locating mains wiring in external walls with foil-backed insulation behind plasterboard

[GRC]

I want to put some deep wall fittings/fixings below the wall lights in one of my rooms. This is an external wall.

Logic suggests that since the room has ceiling lights too, that the electricians/builders would have run the cable in the pitched roof void above the ceiling, and dropped them down vertically to the wall lights, but I want to make sure of this. I have no access to the roof void, no hatch

I have a Draper stud/metal/voltage detector, but all it's responding to is the metal in the insulation. Run it over the wall on metal/volt setting, and it beeps between the studwork, where the insulation is, and goes quiet when I pass it over a stud (and beeps where there are plasterboard nails in the studwork). All I'm getting is the continuous tone which indicates metal, no intermittent tone, which should indicate live wires.

Run it over an internal wall (no foil) on metal/volt setting, and it only responds to live wiring and nails. Hold it next to a loose cable (vacuum cleaner) and I get live wire tone. So I'm happy the detector is working fine. It's brand new.

I can't get a reading off the ceiling for live wires, probably because the device has a depth limit, and the cables (if there) will be running atop the joists, out of range

How would you proceed?

(How do I know the insulation has foil? I saw the studwork in other external walls when bathrooms were being redone)

 

[EFLImpudence]

These detectors are, in my experience, useless even if you didn't have insulation.

Look in the switch, if the cable enters from the top then all you can assume (never assume) is that it runs vertically.

As the wall is plasterboard then you can make holes without drilling and then look and feel around for cables.

 

[flameport]

Remove the wall light and see which way the cables go.

 

[GRC]

Thanks, I reached the point where, although I was trying to avoid it, I removed the wall light, which was a bit of a fiddle, but that showed me the supply cable leading straight upward, and with nothing else in the vicinity, that gave me the go-ahead.

EDIT - and I've just seen the previous post suggesting I remove the wall light!

Done, and sorted. Thanks all.