Junction boxes, cables and 250mm Rockwool

[Lobster]

Hi

I'm busy rolling out Rockwool as per the BCO's instructions, 100mm between the joists and then another 150mm transversely over the top. God what a job. Haven't done this in 20 years, but it's as bad as I remember it. No, I'm lying, it's worse, because (a) my joints are 20 years older and (b) back in them days you only needed 50mm insulation or something.

So I'm now building up a veritable sea of glass fibre up there. No way of telling where the joists are now, so heaven help me if I ever need to access it again.

The Rockwool instructions helpfully tell you to avoid covering electrical fittings, junction boxes etc - but how the hell are you supposed to do that? I've been attaching my cables and any jct boxes to joists, but these have now vanished. I have visions of attaching little timber masts to the joists, to attach the jct boxes etc just so I can see them above the Rockwool! This can't be an unusual problem...

David

[Andrew Gabriel]

When running cable inside thermal insulation, you have to derate it by 50%. The free air rating of 1.0mm² is around 14A. In lighting circuits, it's normally protected at 5A or 6A (and the design loading maybe even less). This means it's already derated by more than 50%, so you can cover it safely. Another issue is damaging a covered cable by stepping on it and not noticing, or something similar.

If you have any other circuits up there, you will have do do a similar calculation, and you might have to reroute the cable or increase the conductor size. (Electric shower would be a particular concern, and ring circuit would need sorting too.)