Shed roof insulation and electrics question...

D

danroach

Hello there.

I'm insulating a shed roof in my garden and have a question regards the wires which feed the lights.

I've attached an image which will hopefully help get the idea across.


My electrician has fitted lights in the shed and routed the cable through the roof joists at about half the total depth of the joists themselves. I was planning on leaving the roof un-insulated but, stupid me, didn't realise just how warm it'd get in the summer and how cold in the winter. So, I've stared insulating the roof with 50mm polystyrene deep sheets. Each compartment (between the joists) has two layers of 50mm polystyrene insulation with the wires sitting roughly in-between the two.

As I've started to do this, I began to think about possible overheating once the underside of the ceiling is boarded in. I'm hoping that the polystyrene will actually reduce the possibility of overheating but wondered if I could possibly get a few opinions - kind of make sure I'm not heading straight into some blatantly obvious pitfall...

Many thanks in advance,

Dan
 
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There are calculations that can be applied

I'm not sure, are you actually covering the cable with insulation or is it just near it?

How is the light supplied? From a fused spur or an mcb in a consumer unit? If so, what rating is on the mcb?

Do you know what size the cable is?

I'm pretty sure you will be fine but it's worth checking
 
D

danroach

Hiya,

Thanks for the reply.

Well, the cable is cable tied to the joists and the insulation will, I suppose cover the wire.

We have a spur coming from our main consumer unit in the house out to the shed (via armoured cable) and the lights are supplied from a separate small consumer unit that our sparkie fitted in the shed. Not sure what rating is on the shed mcb I'm afraid.

The cable looks like standard mains cable that, for example, feeds the back of the electric sockets in our house.

Not sure if any of that helps??!

Thanks for your input.

D
 
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It doesn't really.

Assuming the lights are on a 6a mcb, the cable is 1.00mm T+E (worst case) you will more than likely be fine.

Without knowing specific details it's impossible to say.

How many lights have you got in your shed? And what size are they?
 
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Assuming a "standard" setup with a B6 MCB and 1.0mm cable, then I wouldn't worry too much about the overheating effect. I'd be more concerned about the reaction between the PVC and the polystyrene, I'd want to put some sort of barrier between the two...
 
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And talking of barriers....

If inside the shed is warm and cosy and has people in it exhaling all the time, and then there's insulation to stop heat reaching the roof, which is therefore cold, what happens when warm moist air gets to the cold roof timbers?
 
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Assuming a "standard" setup with a B6 MCB and 1.0mm cable, then I wouldn't worry too much about the overheating effect. I'd be more concerned about the reaction between the PVC and the polystyrene, I'd want to put some sort of barrier between the two...
I would also be concerned about leaching of polymers for lights the beams will likely conduct away enough heat. I also thought using polystyrene roof tiles was banned because of fire risks they were used a lot at one time but in the main now all gone.
 
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Polystyrene and PVC cables don't mix well. Many people say "I have seen issues". I have on a couple jobs where these sheets where placed on PVC cables. the cables "sink" into the polystyrene, and become brittle.
 
D

danroach

Thanks for all of the replies. Very much appreciated.

OK, so if I was to perhaps clad the wires with a sheathing, like this for example...

http://www.screwfix.com/p/tower-pvc-channel-25mm-x-2m-40m-pack-of-20/46474

...ensuring the polystyrene doesn't come into contact with the cable, perhaps that might be some solution?

I've plenty of ventilation in there so I don't foresee damp or condensation to be a problem, but thanks for pointing it out.

I'm not sure what is meant by leaching of polymers though...
 
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I'm not sure what is meant by leaching of polymers though...
Polystyrene causes the plasticisers in the PVC in cables (the chemicals which make it soft and flexible, unlike the Unplasticised PVC - UPVC - used for windows, for example) to separate and migrate out of the cable. The polystrene turns into a sticky mess, and the cable insulation shrinks and becomes stiff and brittle.
 

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