Loft insulation and electrics

Yes, IF the derated capacity of the cable running in insulation is still sufficient for the circuit protective device. It usually will be (amply) for lighting circuits, it will NOT be for electric showers.

Provided the junction boxes remain accessible, are of non-flammable construction and the insulation is also non-flammable, there should not be a problem. Downlighters should not be covered though.

Along the top of the joist it could get damaged or crushed, my preference is for the sides of the joists, accessible but out of the way.

Owain

I am also in eth position of adding 280mm of insulation to a bare loft. I can run the lighting wiring above the insulation; there is enough slack. But this still leaves the 280mm of cable that is completely enclosed in insulation. Is that acceptable? the wiring has just been done.

Also, are you required to uprate the loft insulation just becuase you are reroofing, or is it voluntary? It might be important if you want to use the aded insulation as mitigation to some other work in the house that reduces the thermal insulation. You can only play the "improved loft insulation" card once. In my case I am delaying teh loft insulation until we do the kitchen extension in case I want to use it in mitigation for putting more than the standard amount of window in the extension.

Robert

"RobertL" wrote

Also, are you required to uprate the loft insulation just becuase you are reroofing, or is it voluntary? It might be important if you want to use the aded insulation as mitigation to some other work in the house that reduces the thermal insulation. You can only play the "improved loft insulation" card once. In my case I am delaying teh loft insulation until we do the kitchen extension in case I want to use it in mitigation for putting more than the standard amount of window in the extension.

Robert

Are building control involved in the roof replacement? If not then I guess you can upgrade the insulation when you like. If they are, then they will want to see all elements upto current regs IME.

Phil

Apparently, a re-roof of more than 25% of the roof area required building regs. And, because of that, you're required to increase the insulation 'where practical'. Which I clearly can, since I've currently got none.

So, I'm expected to put 280mm (270mm?) of insulation in, otherwise the works won't get signed off and I won't get a completion certificate.

So it's 25% limit re roof repairs? Thanks for that information.

First cable derating...

- A cable has a quoted Current Carrying Capacity (CCC) in "free air" clipped to a non-insulating surface. An example would be a cable clipped to a wall stud in an otherwise insulated wall, or clipped to a joist in an otherwise insulated roof. For 1.5mm FTE cable its CCC is

- A cable is derated to a lower Corrected-CCC when it is run in conduit or surrounded by insulation. An example would be a cable surrounded in insulation for more than 500mm, correction factor would be 0.5 (IIRC). For 1.5mm FTE whilst free-air CCC is 19.5Amps the Corrected-CCC is just 9.75Amps.

Second "fuse" type...

- A rewireable fuse takes longer to disconnect an Over Current (OC) fault than a cartridge fuse or type-B circuit breaker (MCB, RCBO). For rewireable fuses a second correction factor of 0.735 must be applied - that is, Corrected-CCC needs to be 1/0.735 higher than the fuse rating

After applying cable factors the Corrected-CCC of the cable must STILL exceed the CPD rating. For 1.5mm FTE run >500mm through insulation with 0.5 factor creating C- CCC of 9.75A protected by a 6A Type-B CPD there is no problem. Conversely there would be for 0.7mm (if I recall) imperial Flat-Twin (no Earth CPC), the figure would be somewhat less.

So you need to find out...

- 1 - what type of overcurrent protection you have on the upstairs lighting final circuit

- 2 - what size & type of cable you have currently installed

In new build I think they are using BS7211 XLPE LSOH which is higher CCC rated to 90oC not BS6004 PVC cable which is lower CCC to 70oC. The argument is that whilst the wiring accessories can not withstand

surrounded by insulation. BS7211 is more expensive, about =A345 vs =A325 at Screwfix, and much harder to work with re stiffness & stripping (forget the simple hand twist & pull tools).

If your upstairs lighting drops are in oval conduit, changing the upstairs lighting circuit is trivial - about =A325/45 in branded cable and no more than =A38-10 for new ceiling roses. Additionally you can rid yourself of those pesky junction boxes which would otherwise be hidden under insulation. If your lighting drops are not in conduit, things get more tricky - you end up with "loop-in" junction boxes directly above each light drop relying on the old cable (which runs in plaster) for the lighting drop and thicker cable to each ceiling rose. You may need Part L compliant ceiling roses. Other issues might be supplementary bonding in bathrooms.

If a reroof >25% requires a Building Notice then you can add the wiring onto that at no extra cost? Not looked.

There are a lot of houses now with 200mm insulation and very slim cables over the top. In theory as long as the cable is touching an uninsulating material such as plasterboard or clipped to a joist that is ok. In practice I guess we will know in about 5-30yrs time - and I bet they don't provide a second grant to fix it (perhaps that's Gordon Brown's plan re downstream employment & tax creation device :-)

Do not have the 17th regs to hand so can't be more specific re figures, do have imperial data from 13th if the house is old.

Just to add to that excellent description... the 17th edition allows for installation method 101 which is defined as: "above plasterboard ceiling covered by thermal insulation, insulation thickness >100 mm". This allows for the fact that the cable is probably sat on a ceiling and hence is able to dissipate more heat than would be the case for a cable completely surrounded with insulation. Hence there is a little more margin in the figures.

For the cable ratings, see column E of the table here:

Note that the rewireable (BS3036) fuse de-rating would need to be applied on top of these figures if that is what you have, however unless there are additional de-rating factors to include, it should not drop the rating below the typical 6A breaker size.