PV cables through cellulose?

[Stratman]

Hi there!

 

Getting ready for roofing and in roof PV panels on our MBC build. PV installer says he'll run cables from the PV panels through laps in the roof felt, through the 400mm blown cellulose void between rafters (going in after) then through the vapour barrier below the rafters.

 

Do you have any thoughts about this as I am a bit worried about cables overheating and causing a fire. Is this an accepted approach to cabling?

[SimonD]

I'm not an electrician but my electrician was always reluctant to run cable through insulation like that. Instead all cabling was run in ducting installed through the insulation so there's at least some air gap between the cable and duct.

[Onoff]

Have a look here at 4.3.6. Gives the gist.

 

https://www.tlc-direct.co.uk/Book/4.3.6.htm

[Dillsue]

13 hours ago, Stratman said:

Do you have any thoughts about this as I am a bit worried about cables overheating and causing a fire. Is this an accepted approach to cabling?

How is any other cabling being routed in and around the insulation??

[Stratman]

Thanks - the TLC guide explains the issue well and how to overcome it. Discussed with installer and he has already compensated for the derating by increasing cable sizes from the panels to a rotary isolator in the loft space.

[MikeSharp01]

10 hours ago, Stratman said:

Thanks - the TLC guide explains the issue well and how to overcome it. Discussed with installer and he has already compensated for the derating by increasing cable sizes from the panels to a rotary isolator in the loft space.

Then all will be well - except! You need to think about how the cables come through the airtight layer as you will need to make that interface airtight as well. I used some squares  of EPDM with hole punched through smaller than the cable then pushed the cable through and sealed the EPDM to the airtight membrane once it was installed - it does mean pushing the cable through the airtight membrane so you need the end free to do it.

[Alan Ambrose]

Btw dc isolators are frowned upon these days unless the inverter doesn’t have its own dc isolation.

[Nickfromwales]

On 28/05/2026 at 19:28, Stratman said:

Thanks - the TLC guide explains the issue well and how to overcome it. Discussed with installer and he has already compensated for the derating by increasing cable sizes from the panels to a rotary isolator in the loft space.

I assume he’s running them them in 25mm flexi conduit, not directly sat in the Warmcell?

 

FYI, we’ve done this on every MBC build, conduit and cables through the cellulose, so zero issue to worry about other than weather and airtight sealing.   

[Stratman]

14 hours ago, Alan Ambrose said:

Btw dc isolators are frowned upon these days unless the inverter doesn’t have its own dc isolation.

He explained that the cable remains live until it gets to the inverter so it is to isolate the cables as close to the panels as possible for safety

[Stratman]

On 29/05/2026 at 06:15, MikeSharp01 said:

...how the cables come through the airtight layer as you will need to make that interface airtight as well. I used some squares  of EPDM...

Good idea. He is all over the airtightness aspect fortunately.

[Stratman]

15 minutes ago, Nickfromwales said:

...25mm flexi conduit, not directly sat in the Warmcell?

Another good idea. I'll discuss with him. 

[Nickfromwales]

47 minutes ago, Stratman said:

Another good idea. I'll discuss with him. 

Trying to airtight single cables is a PITA, plus the conduit acts as an insulator giving the cable some separation from the insulation.

 

I would not consider this optional, more of a must!

 

Derating for 400mm is OTT btw, but not much cost to go from 4mm to 6mm so may as well, just a non-issue afaic. 

[SimonD]

1 hour ago, Nickfromwales said:

Trying to airtight single cables is a PITA, plus the conduit acts as an insulator giving the cable some separation from the insulation.

 

I did try to say! 🙄

 

On 27/05/2026 at 20:49, SimonD said:

I'm not an electrician but my electrician was always reluctant to run cable through insulation like that. Instead all cabling was run in ducting installed through the insulation so there's at least some air gap between the cable and duct.

Sensible sparky the one I work with, and he is hot on the solar and ev cabling and derating even for less than 400mm.

 

[MikeSharp01]

4 hours ago, SimonD said:

6 hours ago, Nickfromwales said:

Trying to airtight single cables is a PITA, plus the conduit acts as an insulator giving the cable some separation from the insulation.

 

I did try to say! 🙄

No, not the way I discussed above - for single cables, much tougher for multiple. Just cut a square of EPDM (0.7mm) punch a hole just smaller than the diameter of the cable (or its smallest dimension for the flat stuff - invention of the devil) then just pull the cable through the hole and tape the square of EPDM to the airtight layer