Fixing loft boarding *through* insulation and derating cable.

[Mike Hall]

I've been quoted £125 for supply and installation of 350mm loft insulation in the roof of our 3 bed semi, (includes a grant based discount). This seems like a remarkably good deal, and it doesn't seem worth getting my hands dirty for this kind of money, (so much for DIY!) Before I proceed though, I've got a couple of questions that I hope the collective massive can help me with:

The upstairs lighting cable needs replacing, (currently stranded aluminium with no earth). I've seen this question asked before, but have not found the answer as to; whether or not lighting cable fixed to rafters on one side and then covered with loft insulation needs derating. If so then I guess the derating factor is 0.5. My thoughts are 'yes' that this counts as being totally enclosed in insulation, but I have also seen it suggested that having the rafter on one side means that it is *not* totally enclosed in insulation. - which is it?

Secondly, I will wish to board part of the loft after the insulation is applied, and the insulation guy suggests putting the boards on the insulation and screwing *through* the insulation to the joists. Does this seem OK, or should I clear a path through to the joists for a wood-on-wood joint? Also I read a lot of talk about vapour barriers under the insulation, but am still unclear as to whether this is really necessary or not!

[Christian McArdle]

Lighting cable is so overrated anyway, that 50% derating just isn't an issue. Run in 1.5mm if you're really worried.

I don't see how that would work. Surely it would squash the insulation? I'm assuming it is fibre, not Jablite or something, when you would might get away with it, although it might start crushing above the joist lines. This could be solved by insulating upto joist level, boarding, insulating the rest and boarding again. Not cheap though, and I bet you're using fibre, not Jablite.

Definately install one, especially for fibre insulation. Just a sheet of plastic over the joists and plasterboard would be fine.

Christian.

[Set Square]

I'll others to comment on the cable issues.

With regard to the insulation, how tall are the joists? If it's an old house, they're probably no more than 100mm - if that. There's no point whatsoever putting in 350mm of insulation if you are going to compress it into 100mm. 350mm of insulation is only effective if it traps a 350mm layer of air! Admittedly the boarding will add some insulation, but you really need to pack the joists up as high as possible with something solid to support the boards.

Having said that, I'm not sure what to suggest for packing - timber would really be too heavy. Anyone know whether you can get aluminium (or maybe rigid plastic) I-sections which are 50mm wide and maybe 250mm high?

[Alistair Riddell]

The tables in the regs say that the current capacity of standard 1mm2 twin&earth enclosed in insulation is 11 Amps so you should have no problem with normal lighting circuits.

[Mike Hall]

It's a 1950's ex council house - I believe the joists are 100mm, and the insulator guys were going to lay the insulation over the joists (at right angles).

I absolutely agree with the physics of this, however from my point of view I can get the whole insulation done in the next few weeks, and at some point later in the year I can board over only a relatively small portion of the loft, (around the hatch and tanks etc). So, 100mm of compressed insulation is not as effective as 350mm uncompressed, but taking that as given, and going back to my original question I still wonder about the sensibility of screwing the boards to the joists

*through* the insulation, (which I am guessing will be compressed to about 20-30mm at that point). I then end up with 20-30mm compressed insulation *and* a board above a joist which has got to be better than just the board alone????

[Christian McArdle]

If you are only boarding a small section, then just crosslaying some 200mm beams and attaching the boards to these should be pretty easy and will only compress the insulation by 50mm or so. I wouldn't use it for medium-heavy storage though. The self weight of the floor will already be high with the extra wood, and boarding that the ceiling joists were not designed to handle.

Christian.

[Mike Hall]

That's good news, thanks. Can someone who knows better please square the above statement with what I was trying to calculate using the On Site Guide: I've got, (or rather intend to have by the end of today), 9 bayonet fittings on the upstairs lighting circuit, (3 x bed, landing, 2 x bathroom, 3 x loft + bathroom fan). This equates to a design current of well less than 6A, so 6A TypeB MCB should be fine. Now getting to the bit I am a little muddy on! The cable is now protected for 6A, however as it will be under insulation the On Site Guide says it needs derating by 0.5. Using the formula they give this means that although the MCB is 6A the cable needs to be rated for 12A, (right so far??) So I look down the table of 'conventional circuits' and find that the next biggest radial circuit above 10A is 15A which is wired with

2.5mm^2 T&E. Have I gone wrong somewhere with my understanding of the derating process?

[Christian McArdle]

I'm not sure I quite follow you. However, in most circuits cable size is not limited by the current carrying capacity of the cable, but voltage drop or earth loop impedence.

Christian.

[Owain]

"Mike Hall" wrote | That's good news, thanks. Can someone who knows better please square | the above statement with what I was trying to calculate using the On | Site Guide: | Now getting to the bit I am a little muddy on! | The cable is now protected for 6A, however as it will be under | insulation the On Site Guide says it needs derating by 0.5. Using | the formula they give this means that although the MCB is 6A the | cable needs to be rated for 12A, (right so far??)

Yes.

| So I look down the table of 'conventional circuits' and find that the | next biggest radial circuit above 10A is 15A which is wired with | 2.5mm^2 T&E. | Have I gone wrong somewhere with my understanding of the derating | process?

Yes. You don't change the circuit arrangement when you derate a cable. You still run a 6A lighting circuit, you just have to use cable rated at 12A in the applicable method of mounting.

A quick glance at Table 4.7 in Whitfield's electricians guide shows that

1.0mm in wall in thermal insulation has a rating of 11A which is not big enough, so you will have to use 1.5mm at 14A. There should be no problem using that with most lighting fittings.

Incidentally, if you were to use a 15A 2.5mm radial conventional circuit, you would apply derating of 0.5 so your cable would have to be rated at 30A in insulation. 2.5mm is only rated at 18.5A in insulation (compared to 27A clipped direct), 4.0mm is rated at 25A in insulation, so you would have to use 6.0mm at 32A for that radial circuit.

Whitfield has an example where that 15A radial circuit has not only thermal insulation but ambient temperature and grouping factors apply, and ends up with a 15A circuit run in 10mm cable, which would be impractical because the switch terminals would be unlikely to accept the cable.

Owain

[Mike Hall]

Looks like I might need a copy of this Whitfield's as well as the 'On Site Guide' - or is there really not a lot of difference between them?

[Andy Hall]

The Whitfield book has a lot more info in it and commentary on what the issues are. The OSG is more by way of extracts plus a bit of information but not a lot of background info. The assumption it has is that you know most of the stuff and just need tables and other information to hand when on site or planning installations. It's because the main IEE Wiring Regulations book is written in the way of all standards. It is often quite difficult to find things in it quickly.

You can find most of the Whitfield book on line at

in the technical section, but I think it is worth having a copy if you are doing a lot of electrical work, and especially anything a bit unusual.

.andy

To email, substitute .nospam with .gl

[Owain]

"Mike Hall" wrote | "Owain" wrote | > A quick glance at Table 4.7 in Whitfield's electricians guide | Looks like I might need a copy of this Whitfield's as well as the | On Site Guide' - or is there really not a lot of difference between | them?

Whitfield says his book is intended for electricians who have to design their own circuits, whereas the OSG tends to assume there is an electrical engineer in the background doing the design work. Whitfield goes in a deep pocket quite nicely.

Owain