There are known issues with placing PVC sheathed cabling in direct contact with polystyrene - would the same risk apply if we wanted to place a PVC duct in direct contact with polystyrene? This would be a cooker hood extract taken through the ground floor straight to outside.

The duct manufacturer says they've never heard of problems with PVC & polystyrene, but that if I have I should use Aluminium....

<blockquote><cite>Posted By: Ed Davies</cite>I don't know.

However, the problem with PVC cable in contact with polystyrene is that the polystyrene leeches the plasticizer out of the cable making it stiff and brittle and liable to crack letting the electrons out. Hard PVC without plasticizer (the 'u' in uPVC stands for 'unplasticized') ought not to suffer from the same problem. That's just a guess, though.</blockquote>

You are probably right, Ed, as PVC pipework is often in contact with EPS/XPS where it goes through walls with bead CWI and also where it runs through insulated slabs. If there was a problem then I'm pretty sure we'd have heard about it.

or wrap the duct with a polythene bag. Thats what they do to stop mains leads supplied with electronic equipment reacting with the polystyrene packaging.

Just for the benefit of anyone else like me contemplating the installation of polystyrene bead cavity fill insulation and concerned that 1960's / `1970's electricians might have used the cavity as an easy way of re-wiring the property, here are the instructions taken from the Knauf web site:
Compatibility with other materials:
PVC insulated electrical cables
should not be allowed to come into
direct contact with extruded
polystyrene as the plasticiser which
keeps the PVC cables flexible can
migrate from the cabling into the
insulation. Plasticiser migration has
no effect on extruded polystyrene
but it can cause embrittlement of the
electric cabling which should be
avoided for safety reasons. Electric
cables should be run in trunking or
conduit or wrapped in aluminium
foil in order to avoid plasticiser
migration.

Fortunately in my situation there was only one such cable that dropped 500mm down the open top cavity to get to a bi-pole switch in the bathroom (washing machine & dryer) I was able to ram in a chunk of "Celotex" off cut to keep the styrene beads away from the cable. I was sealing the cavity with these off cuts to prevent styrene balls showering into the soffit behind the gutter board..

I wonder just how much of a risk a length of brittle cable enased in styrene bead would actually be in practice, provided it was left undisturbed.

Posted By: harryhoundI wonder just how much of a risk a length of brittle cable enased in styrene bead would actually be in practice, provided it was left undisturbed.

When I was a student, I lived in a house with a basement. The basement ceiling was covered with the house wiring. It all looked like original stuff from when the house was built in the 1930s. The insulation cracked off it if you so much as breathed hard. But so long as the lights, TV and fridge kept working, we didn't much care.

Posted By: harryhoundJust for the benefit of anyone else like me contemplating the installation of polystyrene bead cavity fill insulation and concerned that 1960's / `1970's electricians might have used the cavity as an easy way of re-wiring the property, here are the instructions taken from the Knauf web site:
Compatibility with other materials:
PVC insulated electrical cables
should not be allowed to come into
direct contact with extruded
polystyrene as the plasticiser which
keeps the PVC cables flexible can
migrate from the cabling into the
insulation. Plasticiser migration has
no effect on extruded polystyrene
but it can cause embrittlement of the
electric cabling which should be
avoided for safety reasons. Electric
cables should be run in trunking or
conduit or wrapped in aluminium
foil in order to avoid plasticiser
migration.

Fortunately in my situation there was only one such cable that dropped 500mm down the open top cavity to get to a bi-pole switch in the bathroom (washing machine & dryer) I was able to ram in a chunk of "Celotex" off cut to keep the styrene beads away from the cable. I was sealing the cavity with these off cuts to prevent styrene balls showering into the soffit behind the gutter board..

I wonder just how much of a risk a length of brittle cable enased in styrene bead would actually be in practice, provided it was left undisturbed.

Probably not to much if the cable never moved, but of this you can not be sure but best to play it safe. However in my experience I have ripped out PVC cabled from a loft that was full of eps 'quavers' and apart from them sticking to it and showing some small markings they cables appeared to be fine, I would estimate them to only be 20 years old frm the type of consumer unit, did not at the time but now wish I had done an insulation resistance test on them to see what difference it would make

Posted By: nikhoward

Posted By: djh

Posted By: harryhoundI wonder just how much of a risk a length of brittle cable enased in styrene bead would actually be in practice, provided it was left undisturbed.

When I was a student, I lived in a house with a basement. The basement ceiling was covered with the house wiring. It all looked like original stuff from when the house was built in the 1930s. The insulation cracked off it if you so much as breathed hard. But so long as the lights, TV and fridge kept working, we didn't much care.

It would have been rubber, as a spark it is easy to replace (just pull or sneeze or breathe on it) but as not copper core (and only csa of about half a mm) not worth as much when you weigh it in

Thankfully you almost never find it any more, very unsafe