DIY Doctor

Cabling and Loft Insulation

Postby martynfogg » Mon Jan 07, 2008 9:29 am


I want to increase my loft insulation by adding Space Blanket over the existing mineral wool.

The current insulation already covers cables for the lighting circuit and I assume that I can't just add insulation on top because the cables may overheat. However, the cables are too tightly laid to get them over even the existing insulation.

Is there a way to run the cables under insulation, maybe within a conduit, or do I have to rewire?

Also, once the insulation is added, I'd like to lay loft boards over the top in some places. Again, can the cables run under the boards?

Many thanks,

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Simply Build It

Postby kbrownie » Mon Jan 07, 2008 1:15 pm

Hi martynfogg,
When running cable through thermal insulation, there are certian derating calculation which could mean in your case that the cable's current carrying capacity would be changrd and a larger cable (csa) be installed, but this depends what is already there it may be okay if it was installed knowing thermal insulation would be surrounding the cable.
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Postby ericmark » Mon Jan 07, 2008 1:25 pm

It would be foolhardy to say yes without seeing your loft and what is involved but lights in the main draw very little current and in the main are not a problem but any power cable will be a problem i.e. shower. You need advice from someone who can see what you are going to do. It would not matter how many questions we ask we could not be 100% sure unless we visited the job. 1mm cable is good for about 14.5 amp and lights normally will have a maximum of 6 amp fuse so I would expect it to be OK. But ring mains are very close to limit as are shower or immersion heater supplies. In Wales only registered installers can get the grants so it is always laid by trained people cheaper to get it done than DIY with grants so we don't have your problem.

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