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Insulated ceiling ring final nightmare.

Former Community Member
Former Community Member
I am having a bit of a nightmare trying to adequately size cable for a ring final. The existing circuit is 2.5mm2 T&E which runs in the ceiling void between ground floor and first floor (this is a residential property). Currently there is no ceiling insulation installed. An extension means a new ceiling and significant modification to the ring final, and also (to meet regs) sound proofing insulation installed in the ceiling. This has a similar U value to thermal insulation in most cases.


100mm of insulation is to be installed throughout the new ceiling - which is 50% old circuit.


My first thought were to re-wire the whole circuit in 4mm T&E as with 2.5mm2 and the 18th ed tables its method of installation in all places wont give me 20A on each leg of the ring.


Great I thought, problem solved with plenty of overhead. More of a pain at 2nd fix, but not the end of the world.


Except now when I am looking through it seems like 4mm2 might not be enough if its going to run the risk of being covered with insulation. Joists are 170mm, so while the cable will not be covered completely the insulation will tend to balloon around it, so it looks like I will have to de-rate by a factor of 0.5 which would mean needing 6mm2 cable which seems crazy.


Any thoughts? Where am I going wrong in my assumptions / calcs?


Thanks in advance


James
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    Ah the building is going to be flats, is that why you have the acoustic insulation? You are making an assumption about the acoustic insulation, using standard Rockwool insulation does not give much soundproofing. The acoustic product comes in panels about 50mm thick, which have considerable density. This is necessary to absorb low frequencies which are acoustically much more troubling, but they offer good high-frequency attenuation too. Commonly such is carried out with the slabs fixed to the ceiling with very tight joins, and then covered with 2 layers of high-density plasterboard, joins all staggered. All this has quite a high acoustic attenuation, and also the necessary level of fire control. The slabs just between the joist are difficult to fit and lose a considerable proportion of the sound attenuation. All this should be in the hands of an acoustics designer, and if done in this manner you still have an insulation-free ceiling void. It would stop any fire damage above unless there is a very considerable fire, burning for a long period, hours, as the acoustic insulation is very effective against heat transfer too.
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  • 0
    Ah the building is going to be flats, is that why you have the acoustic insulation? You are making an assumption about the acoustic insulation, using standard Rockwool insulation does not give much soundproofing. The acoustic product comes in panels about 50mm thick, which have considerable density. This is necessary to absorb low frequencies which are acoustically much more troubling, but they offer good high-frequency attenuation too. Commonly such is carried out with the slabs fixed to the ceiling with very tight joins, and then covered with 2 layers of high-density plasterboard, joins all staggered. All this has quite a high acoustic attenuation, and also the necessary level of fire control. The slabs just between the joist are difficult to fit and lose a considerable proportion of the sound attenuation. All this should be in the hands of an acoustics designer, and if done in this manner you still have an insulation-free ceiling void. It would stop any fire damage above unless there is a very considerable fire, burning for a long period, hours, as the acoustic insulation is very effective against heat transfer too.
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