Hi everyone.
Putting cable in contact with expanded polystyrene is a bad idea, because it causes the PVC insulation on the cable to become brittle.
Can this, or any other problem, occur when running cable next to other thermal insulants?
Thanks, David Pearson.
Polystyrene (not just the expanded variety) somehow causes the plasticiser to leach out of PVC (turning it into uPVC?). This is what causes it to go brittle. It's a chemical thing and I'm no chemist.
There are other issues to worry about when running cable next to or in thermal insulation, the most obvious being the thermal capacity and hence current-carrying capacity of the cable. There are regulations and guidelines about this, but to get a sensible answer you're going to need to provide us with a few more details of your problem. For example, what size cable on which circuit for how long over/under/in/through what kind of insulation in what kind of installation.
Hwyl!
M.
The plasticizer leaches out into the polystyrene. It shouldn't be a problem as the plasticizer causes the polystyrene to shrink away from the cable.
There is no reaction with Kingspan, Celotex, Rockwool or fibre wool.
However bear in mind all of these insulate far better than polystyrene and could cause the cable to overheat if laid poorly.
OK, thanks everyone.
The regs about putting cable next to insulators are explained in Newnes, but I will have enough room in the ventilation space between the insulation and sarking for cable.
Hmmm..... can I put junction boxes in that space? It will be venilated at eaves and a through-roof ventilator, so condensation should not be a problem, but it is one more thing to worry about...
Thanks, David P.
yes, I've seen various warning about the effects of polystyrene on pvc insulation, a few weeks ago I came across an offcut of T+E that had been sitting under a polystyrene slab under my bath, certainly been there 15 years, probably 30. I was interested to see that the PVC seemed to have had more effect on the poystyrene, the outer skin of the insulation had gone a little scasly and flaky, but underneath that it seemed OK, the polystyrene had as you say shrunk away from the cable, just like it does with solvent based glues.
Of course I'm not saying ignore the issue and lay cables over polystyrene, just that the damage to the cable that I found was faitly minimal ...
If I have understood you correctly you are putting insulation in the plane of the roof, and intend to run cables on top of that insulation? What is the detail below the insulation? There is a big warning about putting things like junction boxes (i.e. with screw terminals) in places where they can't be inspected. Under floorboards is generally thought ok as it isn't too difficult to lift a bit of carpet and lever up a floorboard, but the classic no-no example is within a plaster/boarded wall because you'd have to destroy the wall to find the JB.
Your over-the-insulation idea sounds a bit like the in-the-wall problem I'm afraid.
Hwyl!
M.
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