Cold draughty roof

[tobydog]

Well, I finally managed to get to the previously unseen gable ends of my roof in the loft this weekend. New loft hatches helped!!

Although the main part of the roof was replaced in 1987, the ends of the roof still retain the original timbers. As I mentioned before, the loft is quite small because to the vaulted ceilings, however, I was very pleased to see that the end walls are actually lime render on battens. This probably explains why the roof retains no heat despite the thatch because the wall sections are no doubt quite thin. Where the lime has been applied it's broken a few of the battens, but the odd thing is that it's come through looking as though it's gone through wire mesh. The is good news because it's possible that it was applied during the renovation in 87. I really hope the builder didn't apply cement on top of old lime because we do have some cement on the front section.

There are loads of wires in the roof because all the sockets are fed from up there. I know from earlier responses that you shouldn't really cover the wires with insulation, however, does this apply to all wiring? The reason I ask is that I can see what are clearly wires for sockets and light switches (low consumption) and the feed to the electric shower (high consumption). Some of the cables are a bit tight against the joists but the shower one isn't. Can I safely let some of the tight low comsumption wires lie under the second coat of insulation or is this a bad idea, especially with thatch?

 

[Flyfisher]

Covering cables with insulation isn't a universally bad thing to do but it does affect the design and performance of an electrical system.

Cables have resistance and will therefore warm up when current flows through them. The higher the current, the warmer they will get. It is mainly this characteristic that is used to determine the 'rating' of a cable, i.e. the amount of current that the cable can carry without becoming dangerously hot.

In 'free air' a cable will be able to safely conduct its 'standard' rated current while remaining at a safe temperature. When covered by thermal insulation, the cable will be less able to dissipate heat and must be 'de-rated' to carry a lower current so that it doesn't overheat. Cable manufacturer's datasheets will contain information about the current ratings for different environmental conditions where the cable is installed.

In general, few domestic wiring systems run anywhere near their maximum capacity and one simple method to de-rate already-installed cables is to fit a lower value fuse in the consumer unit, so moving cables or even re-wiring need not always be necessary to ensure a perfectly safe installation, even when covered by extensive insulation.

A suitably qualified electrician should be able to inspect an electrical installation and determine if the cables, and the way they have been installed, provide a safe system. They should then certify the installation accordingly.

 

[tobydog]

Thanks Flyfisher. I do need to get an electrician in to run an extension from the light at the other end of the loft. I'll talk to him.

The joists are at least 150mm high so I can get a fair amount of insulation down (first layer will be 170mm) before worrying about where the cables run. The fuse box (yes a fuse box) to the old part of the house is in the utility room at the end of the house and ALL cables lead into the loft between two rafters of the vaulted ceiling. There must be about 6 to 8 cables in there. I might compromise here and slide an insulating batt under them all.