Wiring and polystyrene loft insulation

[JayDee158]

I'm getting mixed views about the safety/risk of the reaction of PVC wiring and polystyrene insulation. Some say no problem, it will take years for the cables to deteriorate to present a risk, others say the cables that are affected should be replaced and kept out of polystyrene. The house was built in early 1970s with lighting cables running through polystyrene loft insulation. When I moved in I lifted the cables out resting them on top of some ordinary mineral wool insulation. However the cables are sticky and although are still flexible do seem brittle.

I now want to top up loft insulation. I plan to keep the polystyrene and top up over it with mineral fibre. I don't really want to go to the time and cost of re-wiring, so I thought I could house the cables in plastic conduit. But again I seem to get contrary advice: some say cables should be kept open to the air in lofts not covered in insulation others say plastic conduit is fine since that's what conduit was made for anyway.

So I think the cables might be ok without replacing and I think I can run them through conduit to keep them clear of the polystyrene. But is that right and safe? And can I cover them with mineral wool top up insulation? Grateful for any advice please

 

[ricicle]

If your cable is showing signs of deterioration then it will need to be replaced sooner rather than later. New pvc cable will need to be kept away from polystyrene, but lighting cables, generally, will be fine under insulation. If you cannot avoid the polystyrene then conduit would be fine but as you will need to rewire then choose routes to avoid it.

 

[JayDee158]

Thanks, that's the most helpful advice I've had so far. In fact one electrician said that in 30 years he'd never heard of the problem of poly affecting PVC cables!

Any ideas how best I can judge how urgent cable replacement is?

 

[Taylortwocities]

In fact one electrician said that in 30 years he'd never heard of the problem of poly affecting PVC cables!

Any ideas how best I can judge how urgent cable replacement is?

That eklectrciian should get out more!

Check out this DIYNOT topic.
//www.diynot.com/forums/viewtopic.php?t=134950&start=0
Some juicy pictures on there.

You should bear in mind that PVC cables will last for 50 years+ if treated properly. Being in contact with polystyrene for only a few years will migrate the plasticiser and make the cables brittle. After a few more years the insulation resistance will be compromised and a rewire will be be needed for the affected areas.

 

[JayDee158]

Grateful for the advice, thanks.

But just out of interest does anyone know why building regs or whatever would have permitted polystyrene loft insulation to be used in conjunction with PVC cables? And does anyone know whether current regulations still permit it?

 

[chivers67]

It may have been permitted by someone without knowledge of the problem and they may not have realised the electrican ran cables in direct contact or through it.

 

[EFLImpudence]

Or conversely, the thermal insulation installers just do it without thinking/caring/knowing about the cabling.

Do cavity wall insulation installers think/care/know about cables in the cavity?

 

[17thman]

Do cavity wall insulation installers think/care/know about cables in the cavity?

I should hope that is why all through cavity cable runs are protected.

 

[EFLImpudence]

I should hope that is why all through cavity cable runs are protected.

What about older properties when it was common practice?

 

[17thman]

I should hope that is why all through cavity cable runs are protected.

What about older properties when it was common practice?

that cannot be controlled by me or you as as far as old installs, i see many, where unprotected cables have been run through cavities. Security and garden installations normally. i am fully in union with your comments about insulation installers.

 

[chivers67]

Or conversely, the thermal insulation installers just do it without thinking/caring/knowing about the cabling.

Do cavity wall insulation installers think/care/know about cables in the cavity?

Most use fibre glass not polystyrene anyway don't they?

 

[seasickstevie]

Most use fibre glass not polystyrene anyway don't they?

Not an expert on the matter, but I believe it's still the norm to use polystyrene for 'retro' cavity insulation.