Loft Insulation Advice

[loziv]

Hi

British Gas are due to lay loft insulation for me today - at present we have none between the joists but have some between the rafters in the roof.

My question is should they lay the loft insulation underneath the electrical cable in the loft - the house has just been reqired and there is some slack on the cables to allow this. If they lay the insulation and it covers the electrical cables would this cause a fire?

Same with downlighters / light fittings - should loft insulation be fitted over where the lights are in the ceiling?

Finally when they lay the insulation should they not put any in the eaves to help with ventilation of the loft?

Sorry for all the questions I just want to make sure they carry out a professional job.

Many Thanks

Lauren

 

[PrenticeBoyofDerry]

The standard thickness for insulation is now 270mm.
Electric cables are effected when covered by thermal insulation as it reduces the current that the cable can safely carry as heat can not dissipated as easy. There is a de-rating that can be given in accordance to the way the cables are fitted within the insulation, I will explain later if you wish, if you think you may have a problem with some of your circuits.
Normally lighting circuits being protected by MCBs or fuses rated at 6A or less will be okay, as the current capacity of the standard 1.00mm cable even when maximum de-rating calculation are made will still carry the current safely. Some lighting circuits are protected by 10A devices with the cable size being 1.5mm again this will be okay. But if the the device is 10A and cable is 1.00mm you will need to get the cable above the insulation.

Now power circuits are different as they are specially designed for the load
and the cable size is normally quite close the the current carry capacity of the cable already. So any de-rating because of thermal insulation will likely have a major effect, so any other power circuits, such as sockets, shower, heaters etc.... should be more closely looked at.
If you have any please reply with details and I can explain what you have and what can be done.
Again try get these cables above the insulation.
Down lights should also be protected from insulation either by cover/caps
http://www.tlc-direct.co.uk/Main_In...nlights_Index/Fire_Cover_Downlight/index.html
or by keeping the insulation at least 150mm from the fittings.

The eaves should be left clear to allow air to circulate but if you are insulating the rafters this could be pointless, what will the loft be used for, if for nothing or light storage, insulating the rafters is not really needed.