Posi joist wiring ref

J.C.E · Nov 1, 2022

Evening

What ref method would you give t/e running the web of a posi joist?

100mm insulation below the posi joist then a MF and plasterboard ceiling below that

Cable not touch insulation

Cable touching:clipped to posi joist

Vortigern · Nov 1, 2022

Essentially this is to determine the derating factor. I might say 103, although not surrounded by insulation of >100mm the UFH might cause the ambient temperature to be equivalent. Would need to know more about expected UFH temperature for derating first. What is the cable for sockets or lighting? But then if I found out the insulation below the UFH was such that it did not raise the ambient temperature it may be a different conclusion i.e.101. It does need a bit of thinking about from the idea do we use ref. method to derate or ambient temp. to further derate. Meaning a RFC would need to be done in 6mm² or worse, which aint happening. Needs more information I would say.

Pretty Mouth · Nov 2, 2022

I'd use method C. Not in contact with insulation, deep void with open webbed joists, easy for heat to disperse

brianmoooore · Nov 2, 2022

If there is UFH of the floor above, the thermal insulation should be at the top of the joists, keeping the void cool.

cireland · Nov 2, 2022

I'd probably go with method C too.

davesparks · Nov 2, 2022

brianmoooore said:
If there is UFH of the floor above, the thermal insulation should be at the top of the joists, keeping the void cool.

Why? Insulation between floors is normally fitted for acoustics rather than any thermal consideration, this is backed up by the specified double layer of acoustic plasterboard. So to achieve the best acoustic performance it would be at the bottom of the void tight against the plasterboard.