Routing Wire up a cavity wall

Coaster · 10 Aug 2009

Hi all,
I need to run some cable vertically in a cavity wall, but it has been insulated with rockwool.
Would it be an easy thing to do.. or would I be better off finding an alternative path.

ericmark · 10 Aug 2009

Find an alternative path. Cables need cooling so to run them in insulation is not really a good idea even if you could get them in.

Coaster · 13 Aug 2009

OK ta for the advice

ColJack · 13 Aug 2009

edited to remove incorrect advice.

RF Lighting · 13 Aug 2009

Thought you were not meant to run cables in cavities regardless of wether there is insulation or not.

ColJack · 13 Aug 2009

aw nuts.. getting confused again.. sorry..

last time someone said "cables in a cavity wall" they meant a stud wall...

if it's a regular brick wall construction with a gap between inner and outer wall then you can't run cables in there..
if it's a stud wall that's been insulated then you can run cables in there, you just have to de-rate them..

ban-all-sheds · 13 Aug 2009

RF Lighting said:
Thought you were not meant to run cables in cavities regardless of wether there is insulation or not.

As long as you prevent anyone from putting insulation in later, and as long as you get into the cavity to fix supports for the cables at appropriate intervals, I don't see what the problem is.....

ColJack · 13 Aug 2009

again.. nothing wrong if it does run in insulation.. you just have to derate as aproporiate..
of course spray foam insulation is bad as it would cause chemical problems for the cable..

chapeau · 13 Aug 2009

What's the difference between insulation in a cavity wall and insulation in a stud wall?

ColJack · 13 Aug 2009

that's what I'm saying..

but I think tha BAS was refering to the spray in insulation they retrofit to old houses..

ban-all-sheds · 13 Aug 2009

Yup, that's what I meant.

FYI - a neighbour of a friend had a near-fire start after they got someone in to put more loft insulation in - the installer just chucked it over all the cables in the loft, and the shower cable overheated, the insulation melted, a bit of arcing caused it to char or maybe burn a little before the MCB/RCD tripped...

OK, the cable could have been a bit undersized, (you know how it is with shower upgrades), but it had been OK until it got buried in rockwool.

^woody^ · 13 Aug 2009

Does it make a difference if the cavity wall is now and internal wall - eg after an extension has been added to the exterior of it. Can cable be run within it then?

LiamPope · 13 Aug 2009

Yep - just as long as you squeeze inside to clip the cable!

chapeau · 13 Aug 2009

As long as supported properly at the top, you can have a 5m vertical run of twin and earth in the cavity.

flameport · 13 Aug 2009

The main problems with cables in cavity walls (i.e. between 2 brick walls) are:
1. There is no way to support the cable properly
2. The cavity will contain sharp bits of mortar, metal wall ties and all kinds of other stuff which fell down there during construction. All of which can easily damage the cable.
3. Unless fixed in place for its entire length, water can be drawn along the cable from the outer to the inner wall, resulting in damp areas.
4. Some types of cavity insulation will damage cables.
5. When all of the wiring is in the cavity, and then cavity wall insulation is installed later on, it not only insulates the cavity but also fills the consumer unit and the accessory backboxes with the stuff.

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Routing Wire up a cavity wall

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