Loft conversion with sloping roof.

[trying my best]

Afternoon all! Situation is old sloping tiled roof above ongoing loft conversion. 50mm air gap to be maintained under tiles between rafters then 80mm of solid foam type Kingspan insulation between rafters, then Thinsulex multi-foil insulation across underside of rafters at right angle followed by 25mm battens nailed under this to take plasterboards. Depth of rafers means that apart from the 50mm undertile gap all the space is taken up by insulation.

Yes I know regards derating the 2.5mm T&E that I'll be using for downlighting but would prefer not to run through all this insulation so was wondering about something like using small trunking without the capping on, open side up with just the odd short piece of cpping to retain the cable in the trunkingfixed to rafters where necessary just below the 50mm air gap and cutting a rebate in the Kingsmill to fit up to this. This would mean the cable was not in insulation and it would be possible to rewire in future without too much hassle. It would also mean that the Kingsmill would only loose a 25mm x 25mm channel down one side to accommodate the trunking.

Or even better, your suggestions.

Also trying to make my mind up as to wether to run underfloor cables beneath the chicken wire and fibreglass loose on the ceiling below or clip to joists, 50mm + down, above the insulation? But again has implication if rewiring.

Just want to here any good practical best practice ways etc. Thanks for any inputs, back soon.

 

[Hiram]

Why not pvc tube it??

 

[trying my best]

Thanks Hiram, any input appreciated, Can do PVC conduit but wanted to avoid enclosing cable as much as possible , main thing is above all the insulation any cable would be within 50mm of slate laths and tiles though would not really interupt air flow but would be liable to damage if anyone working on tiles etc. Rebated down one top corner of Kingspan would mean below the 50mm with just bottom edge and one side edge close to but not touching The Kingspan. Suppose the question is would anyone bother putting anything in to enable ease of future rewire and hold cable away from insulation or would you just pass cable between insulation and derate, though OSG suggests avoiding enclosing in insulation. Probable I'm over complicating and too cautious, but just wondered what others would do with 50mm gap on top and then all insulation?

 

[Jono Pashley]

Perhaps Tray/Basket fixed around the roof space?

 

[Evans Electric]

I wouldn't worry about rewiring ,Besty, although the trunking sounds good.

 

[green-hornet]

I would not worry about the method I have done a few of these and there is ample space between battens to comply with the ventilation regs, this helps to keep all cable together and the ventilation means the cables do not overheat as there is a natural airflow.

 

[binky]

Definetly overworrying - future re-wire if you do your job properly, is 40 to 50 years away. By then it will probaly need complete refurb anyway. Plus how many previous installers make your job easy by thinking of the future - NONE:^O. Most people are too busy thinking of today's profit margin.

Have done several installations like this and I normally run cables clipped to rafters in air gap, with plenty of slack to second fit after insulation has been fitted. The important thing is to get the postions clearly measured out on plans so that when cutting through insulation and plasterboard, you can find your cable again. As for running through insulation this is not a problem - you just derate the cable by half, so go check the numbers for 1mm and 1.5mm and ask you self 'do I need 2.5mm' (the answer is no) you'll find it hard to terminate to downlighters anyway.

 

[binky]

Definetly overworrying - future re-wire if you do your job properly, is 40 to 50 years away. By then it will probaly need complete refurb anyway. Plus how many previous installers make your job easy by thinking of the future - NONE:^O. Most people are too busy thinking of today's profit margin. Have done several installations like this and I normally run cables clipped to rafters in air gap, with plenty of slack to second fit after insulation has been fitted. The important thing is to get the postions clearly measured out on plans so that when cutting through insulation and plasterboard, you can find your cable again. As for running through insulation this is not a problem - you just derate the cable by half, so go check the numbers for 1mm and 1.5mm and ask you self 'do I need 2.5mm' (the answer is no) you'll find it hard to terminate to downlighters anyway.

Forgot to mention - the only insulation that really causes problems in polystyrene as this attacks the plasticisers in PVC

 

[Evans Electric]

Definetly overworrying - future re-wire if you do your job properly, is 40 to 50 years away. By then it will probaly need complete refurb anyway. Plus how many previous installers make your job easy by thinking of the future - NONE:^O. Most people are too busy thinking of today's profit margin. Have done several installations like this and I normally run cables clipped to rafters in air gap, with plenty of slack to second fit after insulation has been fitted. The important thing is to get the postions clearly measured out on plans so that when cutting through insulation and plasterboard, you can find your cable again. As for running through insulation this is not a problem - you just derate the cable by half, so go check the numbers for 1mm and 1.5mm and ask you self 'do I need 2.5mm' (the answer is no) you'll find it hard to terminate to downlighters anyway.

I'll go with what Binky says , house bashing isn't rewirable. Also go with the 2.5 its OTT !!

 

[trying my best]

Sorry! I put 2.5mm at one point, my mistake as I only meant 1.5mm. Seems generally then to clip the cable to the spars with the 50mm ventilation gap above all the insulation to each fire rated downlight. All is to be 30mA RCD protected so no problems suggested regards cable being fixed so near to underside of slate laths, (about 40mm away)? Confirm if you have time please. By the way, only thinking of you lot regards trying to make rewiring easier, you may think is this the one mentioned in the forum when struggling with clipped cables in twenty years time. I'll be too old to do it then.

 

[green-hornet]

The general idea of the 50mm depth, has been helped by the new regulations as regards to 30mA rcd additional protection.

Whilst cables should be run in "safe zones" the fact that the cable is so close to the roof tiles will not impair on the cable or its position.

I have found that cables installed in the ventilation void are very well aired by the act of passing air, so there is no problems, and like the others have said as for rewirable! not likely in a domestic, and should outlast the effectiveness of the insulation.

 

[batty]

I always put plenty of circuits in that way they will never get overloaded.

Batty

 

[trying my best]

Thanks to you all, a pleasure to see good responses to what may seem obvious to you but I cannot be too careful for peace of mind and safety of others despite staying within the regs. Think I'll get that cup of tea.

Thanks again till next time.

 

[trying my best]

Yes Batty, plans state energy efficient fittings that will not take tungsten etc so lighting circuit will not be overloaded as such and the ciruit is just for the one loft space, probable about 14 x 11W downlights, and an extractor in the shower area ceiling.