Insulation between joists

16 May 2011
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United Kingdom
Hi, I have removed the plasterboard ceiling to install new materials for better sound insulation. I am planning to install Rockwool between the wooden joists, then Tecsound50 sound barrier mat, then resilient bars with 15 mm sound resistant plasterboard.

I wanted to add 100 mm thick Rockwool for better insulation but the electric cable drilling through the joists is 5 mm from the bottom of the joists. Shall I install two layers of 5 mm Rockwool and sandwich the cable in between or just I install 5 mm thick Rockwool so the cable lays on top?

Any other idea on how to better insulate between the joists?
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When electrical cable is installed within thermal insulation (or/and containment such as trunking/conduit)
Correction factors to the components of the circuit (cable & fuse raitng) require to be calculated, as the heat dissipation from the cable when under electrical load becomes prohibited. This then decreases the safe current carry capacity of the cable (CCC). This then requires de-rating factors to be applied, meaning if the cable can no longer safely carry the current, either cable size (CSA) needs to be increased or maximum current reduced by decreasing fuse rating (this is not always possible, as certain circuit loads and appliances require a minimum fuse rating to function).

I have attached a link that offers some guidance to the above:

Normally the lighting circuits with cable CSA equal to or greater than 1.00mm2 twin and earth, on a 5A or 6A device (fuse,MCB,RCBO) are not effected to a degree where alteration is required.
But it is very likely that your socket and other power circuits (cooking appliances, shower, heating, outbuildings etc..) are.
This is the last room of the flat and the cables are serving the ceiling light, two wall sockets and a safety light in the garden. Can I make a cage with chicken-wire around the cables so as to not get in contact with the insulation. Or any other tip?
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The cables need ventilation, although you are separating from contact, it is still enclosed and de-ratings will still be applied. Ideally resting the cable above insulation would be my advise if circuit re-design is impracticable.

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