Routing cables under insulation board

[shor7736]

Hi, I’m currently in the process of converting a garage into a garden room and am looking for some advice on where to run the cables for sockets and lighting. The room has a flat roof with 150mm deep rafters. I have fitted 100mm celotex in between the rafters so there a 50mm void between the top of insulation and the bottom of the roof deck. My plan was to run the cables within this gap, however after cutting a hole in the insulation, I noticed that this void gets extremely hot, especially when the sun is shining on the roof. Is it still safe to run cables in this void?
My other option is to batten under the rafters (which I was going to do anyway as the distances between the rafters are odd) and run the cables in the void between the bottom of the insulation and the top of the plaster boarded ceiling. This void will be 25mm. This would be the much easier option as it would avoid removing sections of the celotex. Is it safe to run cables in the smaller void under the insulation?
Thanks in advance

 

[Bruspark]

Standard PVC cable insulations are generally rated for reaching 65C (70 max) in free air under their full load so if the environment is already 65c they can't be asked to carry much current at all

It's the wrong time of the year to 'stick a thermometer up there' and see what 'extremely' hot actually measures but an enclosed section under a roof could easily reach 50C in summer and you'd have to assume it could be much higher in some places.

So It's tricky and getting it designed and done professionally would be best  (there are also some other Earthing/safety issues to consider in an outbuilding)

FYI only, Your second idea is indeed better than the first and any through ventilation of that space could also help. 
BUT   For the socket circuits in particular you really need to do some calculations as the combination of hot ambient /surface  temperatures and insulation around  /above could easily halve (or more ) the marked current
carrying capacity of the cables which is most likely to create a major problem for the power cables  
(With LEDs lighting etc it can be perhaps be addressed by over sizing the cables or reducing the protective breaker rating ).    A qualified electrician could do some calculations for you  IF the temperatures are actually known..

Working with what you've said and no certainty over temperatures I'd possibly be looking instead at running the cables in generously sized white plastic trunking around the edges and across the ceiling (so below the insulation and the plasterboard ) leaving a significant air gap at ends/corners etc to help cool the trunking.
That gives the cable the chance to benefit from a 'room temperature' cool surface and also lets you have a peek next summer to make sure it's looking ok.  Also fit a smoke alarm . .

However with the supply from the house and Earthing to think about too I recommend you get an electrician in to do the worrying and the work for you . .