Roof insulation

I intend to have "Kingspan" insulating foam fitted between the beams (joists, rafters or whatever) supporting the workshop roof. There are several places where the foam will be in contact with the 1.5mm T&E cable used for the lighting. Is there any risk of the two plastics reacting and the insulation of the cable being damaged?

I intend fitting uPVC cladding below the insulation, unless anyone here can suggest a cheaper, light weight alternative. I don't really want to use plaster board because of damp problems in the winter.

edited for typo

Edited By Anthony Knights on 02/07/2021 14:39:57

I would doubt it. The number of places where that sort of insulation comes into contact with household wiring will be enormous.

My roof is lined with 6mm ply. Not sure of the relative cost, but it sure is handy to be able to screw hooks etc in to suspend things for painting, drying etc.

Anthony

Thats what I did years ago and have seen no cable to insulation reactions. No cladding tho'.

But

With 20/20 hindsight I'd have put the wiring in accessible trunking round the eaves rather than pinning to the roof structure. But my workshop is large enough to have a flat ceiling with attic space above. I also insulated the ceiling.

I'm rather partial to the tongue and groove wall and ceiling cladding. Did the whole house with cladding from IPSL as a quicker, less messy solution to less than cosmetically acceptable 65 year old plaster on plasterboard ceilings. Polystyrene tiles held it for 30 years but I got fed up with painting. Stripped the tiles and stapled the cladding to the old ceiling then called it good. Super great in kitchen and bathroom 'cos its wipe clean.

Clive

Anthony

Due consider that the cable will need to have a current de-rating factor applied because it is in contact with the insulating material, further de-rating needs to be applied if it is surrounded by insulation.
Not usually a problem in a small workshop especially in your case where 1.5mm cable has been used for the lighting circuit, but in an installation where cables are running close to maximum rating it could render them non compliant for the circuit.

Emgee

In other words the cable will overheat, PVC is good for 50 to 60C after that it starts to weep fluid. If your lighting cables are touching the roof in summer the insulation could tip them over the edge.

Edited By Dave Halford on 02/07/2021 16:07:19

Might pay to read the relevant parts of this from the IET:-

Foam & cables.

Edited By V8Eng on 02/07/2021 17:10:10

Not convinced that weeping cables was fixed back then. It was still a concern at least 15 years ago in hot locations with new good quality cables..

If indeed the ambient temperature in the roof void rises above 30 deg C you get hit with another rating factor, for 35 deg C multiply the current carrying capacity of the cable by .94, that doesn't seem too bad but it is .35 at 65 deg C for PVC cable rated for max 70 deg C working temperature.

Group several final circuits together and you get further de-rating of all conductors in the group so when designing your installation planning cable routes and cable sizing is an important factor.

Emgee

Edited By Emgee on 02/07/2021 19:32:39

Have a look at Oakwood Garden Rooms on you tube. He did a video a while back that covered foam panels and electrics in roof.

No affiliation just interesting videos.

I ran the cabling in plastic conduit below the ceiling so I can get to it if ever needed.

Another consideration is that rats and mice seem to like chewing certain modern cable insulation. I believe that it's because of soy-based (or other vegetable) materials used to replace mineral oil-based materials, either in the insulation itself or as cable lubricant in manufacture. Automotive manufacturers discovered this to their cost: non-palatable wraps were/are available as a 'remedy'. Probably little comfort to someone whose vehicle's wiring loom has been chewed up. Does anyone know more about this? How real is the risk with domestic wiring?

The cable is clipped to the side of the roof beams, about 1 inch from the bottom edge and therefore not near the roof. There are two cables, each feeding 3 standard BC lamp holders, fitted with 9 watt (60 watt equivalent) LED lamps. Each cable is only carrying about 1/4 amp so not likely to overheat.

I may price up 6mm ply as an alternative to the uPVC cladding. I need just under 18 sq. metres which if my maths is correct is about 5 1/2 8ft x 4ft sheets

Thank you for the replies gentlemen.

Anthony

You might try OSB as a lining material. Cheaper than ply and more water resistant (unless you go for marine ply which is seriously pricey). Travis Perkins for example ** Stock This **

John