Under-floor insulation and cables

[jgh]

I've been collecting solid sheets of insulation to insulate the cellar ceiling under my kitchen floor, and have got enough to start fitting it. However, I have cables running between and through the ceiling/floor joists, 32A ring main and 5A lighting. How should I pack the insulation between the joists to avoid negatively impacting on the installed cable carrying capacity?

Here is a picture of a bit I've been experimenting with:

I've been thinking of leaving about an two-inch gap in the insulation where the cables pass through, or would the resulting installation method leave the CCC ok if the cables ran through the insulation?

If the cables are within the insulation they've have 90mm above and 90mm below them.

jgh

[Bill Wright]

Don't let any sort of polystyrene touch the pvc covered cables.

Bill

[Tim Watts]

On Monday 10 February 2014 20:30 Bill Wright wrote in uk.d-i-y:

True - but that's PIR foam, not polystyrene.

I would leave the gap in the top layer of PIR and lay the lower layer as a single piece with cable sitting on top of the foil. The heat will dissipate upwards and through the floor surface.

It will have some effect on the cable rating, but not as much as burying completely in insulation.

I'd also run some bright yellow or red insulation tape along the bottom foam board and mark "electric cable" in Sharpie along the length at intervals.

[fred]

In article , Tim Watts writes

If you do it the other way up ie have continuous insulation nearest the floorboards and gap the insulation below that around the cables then you have not changed the cable installation method at all, it is still in free air and requires no derating.

The other method is some kind of intermediate undefined state and so cannot be relied upon as one method or the other.

I'm liking the underfloor tube light btw :-)

[Andrew Gabriel]

Can you unthread from the holes, and rerun them under the joists?

If the back of the fluorescent batten gets hot, I would keep it away from that.

[jgh]

No, that would require hacking them out of the ground floor walls everywhere. I think I'll go with the suggestion of one layer above the cables flush against the underside of the floor, and another layer underneath with with gaps to expose the cables so they're "in free air". It's a cellar, so the ambient temperature is rarely above 10 C.

I have some runs that run along the side of joists. I can't quite work it out from Table 4A, but I seem to remember some body mentioning that runs clipped to the vertical face of timber with insulation covering the cable run has the same derating as not being covered because it's running along timber joists, not crossing the filled void.

The notes to "in a thermally insulated wall in contact with a thermally conductive wall" defines "wall" as including timber, but is describes it as in conduit, not clipped direct. That one cable run is re-routable, so I may move it.

BTW, that underfloor tube light is otherwise known as my cellar ceiling light. It's just low enough that I occasionally brush my head on it. ;)

jgh