Service void in vaulted ceiling insulation question

[Sniffer1977]

Hi all
currently having my vaulted ceiling built.
Roof construction is 150 x 50 rafters .
Breathable membrane then rafters will have 120 mm foil faced pir and foil faced 60 mm pir underneath .(maybe 50 mm if building regs flex)
Then it will be battoned and plasterboarded creating a void for cables , but mainly for downlights to be installed without cutting into PIR.

we have to achieve 0.13 u rating which above just about does .

Should this void be kept free of insulation , or should I stick some more pir in between the battons (just not around the downlights)

The additional pir will be if anything left in the bank

thanks all for any thoughts/guidance

 

[^woody^]

There should be no air voids in this type of ceiling except the ventilation void if that is the design, and a service void is unnecessary.

 

[Sniffer1977]

There should be no air voids in this type of ceiling except the ventilation void if that is the design, and a service void is unnecessary.

Hi Woody
Thanks for the reply.
I thought I had seen some posts on this forum that the void was the way to go rather than cut into the insulation and reDuce the U value .
Could you possibly offer a suggestion as to alternative way to go ?
Thicker insulation , then cut out for downlights ? I’m not sure if that the more awkward for the builder to attach to rafters e.g if I go to 90mm under rafter rather than 60mm Plus separate plasterboard ?

im just looking for the best way forward here and appreciate any ideas or thoughts , but I do want to keep the downlights as I like the look ( I know not everyone’s taste )

Thanks

 

[^woody^]

You could fit the cable in the ventilation void, and bring it through the insulation where the spotlight is.

That may be best as you should be using sealed housings for the lamps and the connectors are typically on the back of the cans.

Alternatively, channel a cable in the underside face of the insulation board, and foil tape over it to keep it in place while you plasterboard. As far as I know PVC cable does not react with PIR/PUR insulation like it does with EPS, but if you are worried, invert some plastic cable capping in the groove, put the cable in that and then tape over. Dover worry about any heat loss from the insulation as its negligible, and there would be no heat concern with the cable as its on the surface.