Electric cable and polystyrene insulation ?

by **gjbiker68** » Fri Dec 07, 2007 3:46 am

I've just been told that polystyrene insulation and PVC covered electric cable can chemically react with each other causing deterioration of the PVC sleeving ? I had planned using polystyrene to insulate some new stud work I've just put up but now I'm not so sure. Even when the insulation is in place there will still be a free flowing air gap between it and the stone wall (approx 100mm gap) where the cable will be run vertically from the loft.
Any advice or should I just splash out and use Kingspan or equivalent ?

Thanks in advance

GJ :D [/code]

by **kbrownie** » Fri Dec 07, 2007 12:56 pm

hi gjbiker68, never heard of this reaction, but there's calculation to be made when cable is installed inside insulation, because of the heat reaction it changes the current values, and a larger diameter cable may be needed.
regards
kb

by **sparx** » Fri Dec 07, 2007 6:19 pm

Hi, 2 points, yes they do react badly, the plasticiser goes hard making the cable brittle and liable to crack and absorb moisture, and cause over heating.
second is it is not allowed to use cavities for wiring, it was stopped years ago due to 'bridging' of inner to outer skin, allowing damp to track across from outer to inner 'leaf' of wall, & it can run down cable and into sockets ect. see bldg regs,
regards SPARX

by **gjbiker68** » Fri Dec 07, 2007 9:57 pm

Thanks Sparx, thats answered the polystyrene query but opened up another.
All I'm doing is replacing some stud work and insulation, the cable run is as it was - between the stone wall (old house, not cavity wall) and the stud work. I understand what you're saying but what are my options here ? Sleeve the cable with conduit ?

Cheers,

GJ :)

by **sparx** » Sat Dec 08, 2007 12:07 pm

Hi, yup I think even PVC conduit would do as not a prob. if it goes brittle, cable still clear of insulation, make sure cables are 50mm below surface of inner skin to comply with regs, should be ok as insul. prob more than that, best wishes,SPARX

by **gjbiker68** » Sun Dec 09, 2007 1:51 am

Thanks again Sparx, shouldn't be too much of a job to slide the conduit up (famous last words). I'll get my order in before xmas !

GJ :shock: