Loft insulation over? cables

Hello all, I intend to spend some quality time adding knauf insulation to my attic this weekend, the rolls state that I can't run it over/next to electric cables in any circumstances! However, I have many light fittings (recessed halogens in my bathroom which poke through into loft), and also lighting cables and shower cabling etc. If I avoid all of these, there is going to be lots of un-insulated attic, seems a bit of a waste of time!!!

Any suggestions,

Thanks

Andy

put cables above insulation

1) run all cables above the insulation

2) cut a 9 inch circle out of the insulation where the downlighters are - they need space to disipate the heat they generate

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read this post first

Also, see these:


http://homepage.ntlworld.com/okecokey/tophat.jpg

http://homepage.ntlworld.com/okecokey/tophat1.jpg

What exactly is this insulation made of?

Don't think it's anything special - Knauf is just a make, they do fiberglass, rockwool, membranes, rolls, slabs, floor insulation, wallboards that sort of stuff....

The stuff they warn about is probably polysytyrene slab or polyeurathane. The risk there is that the plasticiser chemicals can leach out over many years and attack the cable, denaturing the plastic. In practice, in a well ventilated roof this is unlikely to be a problem, but it has been seen occasionally, so the makers are understandably wary.
The only other problem is with cables very close to max rating, which is unusual in a lighting circuit, although shower cables need some thought.
If in doubt, clip or cable tie cables out of the way of the insulation, and then where they really have to drop through it, which with cunning is usually at most a few 10 inch lengths, slip a bit of oval conduit around it - as we are not really needing the mechanical properties, it can be slit lengthways on a thin edge, and zipped over the cable (saves disconnecting the ends), but use the large (20mm stuff) the smaller sizes don't do the unzipping trick wide enough to be comfortable, and you can nick the cable jacket if not careful. If need be such pieces of conduit can be cable tied into final position, if they seem wayward !
regards M.

Some plastics do indeed attack PVC insulation. I've certainly seen it with polystyrene whether ventilated or not. It's a contact problem and I suspect it works both ways. The PVC plasticizer appears to eat into the polystyrene then the resulting chemical cocktail attacks the PVC. This can also happen if PVC is left in contact with emulsion painted walls for prolonged periods.

You won't have any such problems with 100% inorganic insulation. That just leaves thermal considerations as mapj1 points out.

My loft is insulated in mineral wool and fibreglass, so no cable worries. I ran my shower cable in a plywood "tray" fixed to the side of the joist to keep the insulation at bay.