Checks prior to updating insulation

[Vicario]

My loft is about to be insulated with that stuff that they spray in. The surveyor mentioned something volcanic - not sure what he meant. Anyway, all the electrics up there will be covered and I was wondering what it would be sensible to check before the work is done. There are the usual lighting cables, junction boxes and two downlighter hoods, which they intend to 'cone'. Also a very short electric shower cable (uses about 40A) linking the on/off cord to the shower. It's 10mm and runs mostly through the main house up the walls into the loft space for a very short run before going to the pull cord and out again for about 1m.
Does this blown stuff add or detract from the fire safety of a loft?

 

[ban-all-sheds]

Insulation is not flammable.

All the makers and professional installers of it will advise that the electrical cables be re-routed so that they remain above the insulation.

All cables have their current carrying capacity reduced when they run through insulation. With lighting that's not a problem, as the worst case even for 1mm² is 8A.

Your 10mm² shower cable is a different story though - if that's lying on a ceiling or clipped to a joist, and ends up with >100mm of insulation over it its capacity will drop to 36A. If it ends up completely surrounded by insulation it will be only 32A.

I suggest you get a qualified electrician to sort out the cables before the insulation is blown in.

 

[Vicario]

Thanks for the very informative answer. The 10mm cable is actually just 80cm from where it enters the loft to the pull cord and then about the same from the pull cord to where it descends into the bathroom. Are the load capacities you gave the point at which things start to get hot when insulation is covering? I have a 9.8KW shower, by the way, protected by a 50A MCB and an RCD. Does the fact that only a short length of the cable run is affected not change the calculation?

 

[ban-all-sheds]

It's not short enough - once over 500mm then the full de-rating factor applies - in fact it's as good as full (0.51 vs 0.50) at 400mm. You need to get the cable re-routed so that it is above the insulation.

I don't know how thick the insulation will be, but you will also need to take account of where the cable runs vertically through it - anything over 200mm will make it unsuitable for your current shower, let alone a more powerful one.

 

[davelx]

There are the usual lighting cables, junction boxes and two downlighter hoods, which they intend to 'cone'.

I hope you mean they are going to "cone" the junction boxes - these MUST remain accessible for inspection and test, as screws tend to loosen over time and you do NOT want them sealed up under foam.

 

[flameport]

If this is the spray foam, you don't want that over any cables. Unless your house was rewired last week, the cables will need replacing long before the insulation does.
This means that when the inevitable rewire is done, the insulation has to be destroyed to get the new cables in. Same applies when you want to change anything such as removing the downlighters and installing proper lighting.

 

[ban-all-sheds]

Surely it's blown loose-fill, not foam? Foam gets sprayed onto the underside of tiles, not over joists.

 

[timtheenchanter]

volcanic leads me to believe its the blow rockwool stuff, so not sealed but a PIA to get at anything under it (worse that rolls by a long chalk)

 

[ban-all-sheds]

Tell me about it - I spent a few hours in my Mum's loft a couple of weekends ago burrowing through vermiculite trying to find a fault in a lighting circuit...

 

[Chri5]

Tell me about it - I spent a few hours in my Mum's loft a couple of weekends ago burrowing through vermiculite trying to find a fault in a lighting circuit...

Must have been like a field trip / fun day out for you