Keeping cavity injection foam away from cables?

[Jeff D]

From my electrical fuse box on the ground floor, some cables run upward inside the cavity wall to the first floor. I am planning to have the cavities injected with insulation foam soon, and want to keep the foam away from the cables, to avoid overheating of the cables.

I am thinking I could remove the cables from the wall, then feed some kind of flexible 3" bore conduit up the cavity and then rout the cables inside that.

Alternatively, I could re-route the cables up the inside of the house (in a corner of the hall, for example). Is rectangular-section plastic trunking suitable for this purpose? There are two 6mm cables, two

2.5mm cables and one 1mm cable.

What are my options, and their merits?

Thank you,

Jeff D

[Lobster]

AIUI you aren't supposed to route cables inside cavity walls; apart from the possible overheating thing, isn't there the potential issue of the foam attacking the PVC cable sheath?

Don't know how you'd get the conduit into the cavity, up the cavity, and then out of the cavity at the other end?

Either that or chase out the walls and bury the cables

David

[Bob]

Why use foam? There is an alternative that most cavity wall firms use like strands of cotton wool. very effective, doesn't attack PVC or wall ties - or emerge from any cracks or holes. I would love to see you try the conduit idea! Cables will not overheat providing they are the correct size for the load after they are derated by a factor relating to the type of insulation.

[David Hansen]

On Sat, 28 Apr 2007 14:06:25 +0100 someone who may be Jeff D wrote this:-

They shouldn't do. Leaving aside external cabling, they should either be on the surface or buried in the wall. Who installed these cables?

It is unlikely you will find many people to do it, due to the problems foam insulation causes. However, you will find plenty of people who will blow what looks a bit like chopped up fibreglass into the cavity.

Good luck trying. Anyway, the heat will probably still have nowhere to go as the conduit will be surrounded by insulation.

Yes it can be used for this purpose. provided the cables are adequately supported inside it. Alternatively they can be clipped to the wall and then perhaps covered over. Check the size of the cables is adequate in the group.

Remember Mr Prescott if you are in England or Wales.

[Jeff D]

I was envisaging dangling a line down the cavity, and hauling the imagined flexible pipe up through. Should be easy enough. But if one isn't supposed to route cables through the cavity I'd better go for the internal routing option.

If I opt for the latter, do the cables have to be inside steel conduit

- or is it legal to simply plaster over them?

Thanks,

Jeff

[Jeff D]

The cables are the right size in normal circumstances. What do they use to clip the cables to the wall (when inside trunking or in a chasing)? Is there any trunking available with some kind of easy clipping system built-in? Anyone provide a link?

Thanks,

Jeff

[Lobster]

Providing the cable runs are in acceptable locations you can just plaster over them in shallow chases (most people use plastic capping or conduit within the chases to protect the cables during plastering and/or hold them in place.

David

[Lobster]

Have a look at:

The oval stuff is what's normally used when you chase out a wall; the capping is normally for new builds (easier because you don't need to thread the cables though, but is much wider than the equivalent oval conduit, so you'd need to chase a much wider channel. The capping will normally wedge itself in place within an appropriately-sized chased channel without need for fixing; use masonry nails or similar to fix capping.

For surface mounted trunking (yuck) look at:

The 'backs' of these fix to the wall however you want (self adhesive pads, screws or whatever; put your cables in and then the 'front' snaps on.

David

[Jeff D]

Thank you very much for the help. I'm wondering if I should remove the cables from the cavity and channel them down an inside wall now. What is the official objection to running cables up a cavity? I would have thought that would be the safest place one could have them. Mine have been that way for about 15-20 years. But if I'm going to be ordered to have them chased into an inside wall at some stage in the future, I'd rather do it now, rather than after I have laid laminate flooring upstairs when they will be harder to access.

Jeff

[Lobster]

Well the objections are as pointed out earlier, but personally I don't know whether it's a 'best practice' issue or whether it's a complete no-no given that it's already been installed this way... I expect one of the several pro electricians who frequent this newsgroup will be along soon to set you right!

The issue has certainly been raised in this newsgroup several times before in the past so it would definitely be worth a google.

David

[AlanC]

Does this apply to ALL cables or just power? I have dropped a number of TV/Sat cables and ethernet CAT5 cables down the cavilty of my house. We dont have cavilty insulation but I have been thinking about it. From what I have read/heard, modern insulation doesnt attack the cable sheath.

Alan