Insulating around cables

[WiringPeter]

I need to insulate under my suspended floor which has many cables running through the joists (consumer unit is below floor). My main concern is causing the cables to overheat etc. Here's a quick overview:
Joist height: 140mm
Cables: Mixture of circuits but the 2.5mm 32A rings are my main concern (lights etc. would be fine)
Insulation: 2 x 50mm sheeps wool slab (100mm total)

If I do a 50mm slab under the cables then it fits quite snuggly below the cables (just touches but no stress on cables). My thoughts are:
Second slab on top of cables - cables sandwiched between insulation - overheat?
Second slab forced below cables - would probably put stress on the cables (if I can even get it under there)
Put both slabs (100mm) above the cables - this will be compressed around the cables and put some weight on the cables.
Have a gap around the cables with second slab - can I rely on these slabs not moving during the next few decades (probably not)?
Put wood (approx 25mm thick) each side of the cables and lay insulation over the top (leaving a small gap around the cables). This would be hard to do well and the insulation may even rest on the cables

Any ideas how I can do this? I really want to get 100mm insulation as the room does get cold. I don't really like any of my suggestions above but I'm sure others will have had the same problem.

 

[David Prosser]

I have suspended floor and have 100mm Kingspan insulation fixed underneath, this does not bother the cables underneath the floor because I rewired and ran the cables across the sub-floor not through joists. If you can do this it would removed most of the cable worries you may have. If this is not possible you could try 50mm Kingspan type material and then 5mm sonic gold underlay taped down. This keep draft to a minimum through the floor boards and skirting whilst still allowing good ventilation under the property.

 

[BroadReach]

Personally I would put as much insulation underneath as practical without compromising the wiring or the ventilation.

Much more important to eliminate draughts too.

Not too much heat is lost through the floor, if draughts are excluded, compared to walls windows and roof as heat rises.

Some insulation is better than none though.

I am unqualified to answer re the cables, mechanical engineer not a spark.

 

[Marvo]

Could you not install trunking and put the cables in it before you insulate? If you oversizesize it a little so it ends up less than 50% full it should allow more than sufficient cooling.

 

[WiringPeter]

Could you not install trunking and put the cables in it before you insulate? If you oversizesize it a little so it ends up less than 50% full it should allow more than sufficient cooling.

That sounds like a good idea. Can anybody comment on whether they think this would be a good idea? I'm guessing this would be reference method A, which would give me 20A for the rings and 32A for my 6mm oven, which is just enough. I guess I'd need to put a batten in there to screw the conduit to to avoid extra weight on the cables.

My floor will be air tight. I have expanding foam, an airtight membrane, air tightness tape and an air tightness sealant. The room was draughty before as airtightness was very bad. The extra 50mm insulation does make a difference. 50mm is currently about 0.5C warmer than no insulation and 100mm is about 1C warmer than no insulation. Add 18mm ply and carpet and it's probably the difference between a comfortable 18C and 17C but on a cold day the difference would be far greater.

One of the problems with rigid insulation (PIR etc.) is that it can be hard to get a good fit, it can't be squeezed in like wool and I'd also need to cut and join where the cables are. I decided to go for 100mm wool as the performance would be better than 50mm PIR and doesn't have the same condensation risk.