Running lighting cables in vaulted insulated ceiling

Hello,

I have an extension being built with a vaulted ceiling that has 125 * 50 rafters. The ceiling has a breathable membrane. 25mm air gap is left from top of rafters to then 100mm celotex between rafters and then 52.5 celotex insulation backed plasterboard beneath.

I have read with interest the posts and debates about downlighters in celotex etc. We will be using ceiling mounted lights but I am interested then in what is standard practice for the running of the lighting cable to the lights.

Would cabling be ok run in the air gap between the back of the insulation and then what is standard where it needs to come down to the ceiling height. Needless to say I am not doing electrics myself but am interested in what is standard practice in these situations? some form of conduit?

I am hoping to fit the insulation myself to save cost but want to know where and how cables can be run so I can leave out sections for the spark.

Thanks

 

I would run the cables in the air gap and then where you require a lighting point, either poke the wire through a small hole in the celotex or better still, clip the wire to the edge of the joist then you'll have something solid to screw the light to, and you can just cut a small notch out of the celotex to let the wire past.

There is no need for conduit or anything like that.

Ideally, you would wait untill your electrician has completed his first fix work before you start to install the insulation.

 

As far as possible, the cables should be run in the air gap. They will normally be supported by being clipped to the joists, etc.

Cables should not be run through the insulating material as this will affect the current-carrying capacity. The thermal effects can be looked at by your electrician but lighting cables are not carrying anything like their maximum current so its not such an issue as a cooker/shower circuit for instance.

As the cables pass from the air space to the ceiling face, they also should be clipped to the joists. This means that the cable is not totally surrounded by the Celotex, etc.

 

Thanks both. I did see another pst where someone mentioned about going next to joists to dissipate the heat.

Many thanks for prompt responses.

 

Wouldn't an impermeable one be better, to stop warm moist air from the room getting to the cold roof?

 

Hi B.A.S.,

Would welcome some advice on this as I have to admit I was confused by the distinction between warm/cold and ventilated unventilated roofs.

The membrane is the klober permo air - which from manufacturers site says following:

"Permo air is the most breathable air open low resistance underlay on the
market. It is the ideal solution where there is a high risk of condensation
forming in the roofspace."

My extension will have a kitchen in it so increased chance of moisture in my opinion.

Also from manufacturer:

"Being both air open & vapour permeable further minimises the
risk of condensation forming, particularly during the drying out
period of a building. No ventilation required. Suitable for cold and warm roof applications."

Any advice very welcome. The BCO has been and did not raise any issues but that in my opinion doesn't necessarily mean they can't and wont exist!

Also forgot to mention that I would have thought I would need to put a VCL layer o the warmside of the roof i.e. inside underneath the insulation but that this would probably mean I couldn't use the celotex plasterboard lined insulation?

 

Try the roofing forum.

My understanding, which is non-expert, is that you have a waterproof but breathable membrane in the cold roof, but that has nothing to do with needing a VCL in the element which separates the warm and the cold spaces.

You can get foil-backed insulation, and foil backed plasterboard. Whether you can get composite boards with a VCL between the insulation and the plasterboard I don't know.

 

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