Cables running within a cavity wall

alans32 · 23 Jun 2010

Hi all
I am considering having my cavity walls insulated, but have recently discovered that all the electric cables from the consumer unit run within the cavity wall, and i was wondering if the extra thermal insulation will have an effect on the cables?

In more detail, I have a brick outhouse/shed attached to the house. The consumer unit is located within the shed. The cables coming out of the consumer unit travel within the cavity up to the first floor where they appear under the floor boards in the house.

The house was built in the 1950's and the wires are PVC with the sockets connected using a radial system.

So i guess i am really asking is it still acceptable to have cables run inside a cavity wall?
Am i looking a re-wire here?
many thanks
Alan

ban-all-sheds · 23 Jun 2010

alans32 said:
i was wondering if the extra thermal insulation will have an effect on the cables?

Yes, it will reduce their current carrying capacity which could make them overheat and polystyrene insulation will damage the PVC sheath of the cables.

Nobody had heard of CWI when your house was built, but now they have, and it's why installing cables in cavities is frowned on.

In more detail, I have a brick outhouse/shed attached to the house. The consumer unit is located within the shed. The cables coming out of the consumer unit travel within the cavity up to the first floor where they appear under the floor boards in the house.

Are they the only ones? Or do all the others in the house drop down the cavity? If they are the only ones it shouldn't be too difficult to re-route them up the surface of the wall in the shed, surely? Does it go up as far as the 1st floor void?

Am i looking a re-wire here?

Lighting cables are usually way over-specced. If the socket circuits really are radials then you might be able to have a lower rated fuse/MCB instead, which might leave only a few circuits (cooker, shower etc) to be rewired.

Best to get an electrician to inspect it - tell him you want a report covering the feasibility of CWI.

alans32 · 23 Jun 2010

Thank you for the detailed reply!
Unfortunately the height of the shed does not reach the first floor of the house. Any ideas on where else the cables might be routed. I guess they could come into the house from the consumer and then up to the first floor buried in the plaster. I guess I would need something indicating that the wires were in the buried in the plaster - perhaps get a 13 amp socket installed vertically in line with the wires..I can feel my wallet hurting already!

alans32 · 23 Jun 2010

It just occurred to me that a cheaper option might be to forget aboutb CWI!!

I did have another thought that doesn't sound too expensive (in my head anyway). Perhaps the cable could be run inside the shed, then using ducting within the cavity for the remaining journey to the first floor.

ban-all-sheds · 23 Jun 2010

That would work from an electrical POV - make the ducts comfortably large.

BUT....

You've then got a direct path from your floor void, which is warm, to the shed, which is not, and you could get condensation running down the ducting into the CU in cold weather.

I don't know how likely that is to happen, but as it would be a Very Bad Thing you should probably not chance it.

What is the room on the ground floor that the shed backs onto, and what's on or against the wall?

alans32 · 23 Jun 2010

thanks again for the reply,
That is a very good point about condensation.
The shed backs onto the living room, and the walls are just plastered. The room above, where all the wires currently appear is a bedroom. There isn't much on the wall (living room side) except for TV, etc.

I have to admit i would rather not get the consumer box moved inside!!
alan

ban-all-sheds · 23 Jun 2010

alans32 said:
The shed backs onto the living room, and the walls are just plastered. ... There isn't much on the wall (living room side) except for TV, etc.

So you wouldn't really want the cables running on the surface of that wall, even boxed in...

I have to admit i would rather not get the consumer box moved inside!!

They tend not to look too good in the living room.

What about boarding out the shed so that it isn't cold?

Or constructing a vapour barrier over the end of the ducting under the floor?

holmslaw · 23 Jun 2010

ban-all-sheds · 23 Jun 2010

Actually, a fireproof barrier is a BGI.

Taylortwocities · 23 Jun 2010

ban-all-sheds said:
Actually, a fireproof barrier is a BGI.

BGI??

WTFIT?

ban-all-sheds · 23 Jun 2010

B****y Good Idea.

SWALK.

Taylortwocities · 23 Jun 2010

holmslaw · 23 Jun 2010

ban-all-sheds said:
B****y Good Idea.

SWALK.

alans32 · 23 Jun 2010

Brilliant. I really appreciate all the info and time taken to reply to my posts. I am a little bit less stressed now i have an idea how to rectify the problem. i will hold fire on the insulation and start looking at the wiring first.
Thanks again
Alan

ColJack · 23 Jun 2010

raise the roof on the shed so you have a direct route into the floor void???

if it's anything like my coal shed then it's a concrete roof?

a sthill saw (sp?), hammer and chisel, and a bit of elbow grease will get you a decent sized hole through that, then put a sloped roof on it in timber and tile..

diynot

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Cables running within a cavity wall

holmslaw

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