Electric Cables / Loft & Insulation

[Mart_]

I'm not sure what to do here but I've recently lifted and binned 270mm of insulation and relaid 100mm with the intentions of putting a further 200mm down shortly.

Previously my electric cables laid on top of the plaster board (running over the joists) and covered by the 270mm insulation. I've read that doing this carries a low level of risk.

When I relaid my new 100mm I pulled the cables up and over. But now i'm thinking theres not enough slack to do this again when I put 200mm down.

So I either sandwich the cables between 100mm and 200mm or put the cables back on top of the plaster board under neath 300mm.

What's best?

Thanks

 

[EFLImpudence]

In between the layers of thermal insulation is worse than it was before - necessitating a derating of the current carrying capacity of the cable of 50%.


As it was before, 38%.
This does not matter with lighting circuits (not the lights) as they are hugely over-engineered.
Socket circuits - borderline
Shower and Cooker circuits - definitely not.

 

[Mart_]

Thanks EFLImpudence. I've finally got around to boarding so have put the electric cables underneath.

Cheers

 

[PrenticeBoyofDerry]

As EFLI has already touched on. It is important that cables are routed correctly and when they placed within voids that contain thermal insulation this would affect the dissipation of heat given from the cables.
If cables are not derated correctly for the method of install, then they can melt and start fires.
Generally lighting circuits on devices of 6A or less will be fine. But any circuit above the 6A rating would need to be assessed and tabulated in accordance to the reference method used to install and cables derated as required or cables should be placed outside the thermal envelope.

 

[ban-all-sheds]

Thanks EFLImpudence. I've finally got around to boarding so have put the electric cables underneath.

So Reference Method 101?

 

[ban-all-sheds]

Just curious...

I've recently lifted and binned 270mm of insulation and relaid 100mm with the intentions of putting a further 200mm down shortly.

Was it worth all the effort and expense to get an extra 30mm?

 

[John D v2.0]

Just curious...

​

Was it worth all the effort and expense to get an extra 30mm?

Agreed, although our builder randomly did that, but didn't replace it, and when I complained he said normally people want him to do that!
However if finances allow he could do what I did and do 100mm wool with 75mm celotex on top then board it directly. The cavity boards are best because they're easy to fit up the loft hatch.
Regarding wiring I just clipped to the purlins and rafters.
Edit-purlins

 

[Mart_]

Just curious...

Was it worth all the effort and expense to get an extra 30mm?

It was a lot of effort and definately worth it.

Not long after moving in we heard movement in the loft. Not surprising with the holes in the soffits.

So as soon as we had the soffits and guttering done I hit the loft. I Found three dead rats, droppings and poison everywhere. So I ditched all the old stuff, bought a cheap vacuum from Argos and vacuumed up every bit of s@@t and then put the new stuff down opting for the extra 30mm at the same time

In saying that, it's not a task I'd like to do again any time soon!!!

 

[securespark]

Regarding wiring I just clipped to the purloins

These presumably are the roofing timbers that fell off the back of a lorry?

vacuumed up every bit of s@@t

It's not just the s@@t you need to be bothered about, rat's urine carries Weil's disease and that would have been all over your loft as they are incontinent.

 

[ban-all-sheds]

they are incontinent.

But we voted in June last year to send them back, didn't we?