PU Foam and T&E cable

[DIYinBerks]

I have read some info that standard PU foam is not compatible with PVC-insulated cable; and elsewhere contradictions that intumescent foam IS and IS NOT compatible. Anyone shed any light on this subject? I need to seal as best I can where cables go through some fairly big holes in the wall between under the (first) floor which I want to keep warm, and the garage which is very cold. Or if neither of these foams is advisable, is there another product?

 

[Chri5]

Use Kingspan or celotex and keep the runs to the edge, still some derating is required for the cable capacity.

If you have used 1.5 mm TE for lights that will be fine, ring in 2.5mm or direct circuits such as shower could be an issue, however if you put the cables in trunking it will help. Although that will bring into play grouping factors.

You don't say if there's metres and metres of cable or if its just through a hole so it's hard to say (without more info from you).

 

[PrenticeBoyofDerry]

Is this question regarding the breakdown of PVC cables due the chemical reaction of the plasticisers and certain thermal insulation material,
rather than de-rating of cables?

 

[DIYinBerks]

Exactly so. I am just dealing with holes in the brickwork as the cables go through the wall: I just want to seal them so a few cm of foam is all that is needed - I didn't think that this will have a major effect on heating and mean the cables need derating.
I am more worried about reports I have read that PU foam makes cables go brittle and fail. (But some others say it doesn't!)

 

[Chri5]

4 to 1 sand and cement mix.

Solid enough to prevent ingress of beasties and air, good enough as a fire break and soft enough to break should you ever need to add more cable.

Some use rockwool and a simple 20mm skim of mortar on either side, that gives even better further re use potential.

 

[bernardgreen]

Some preformed foam does cause dis-colouration of the PVC on cables. What effect this has deeper into the PVC I do not know.

Other preformed foam has no apparent effect on PVC.

Expanding foam does have an effect on PVC as the foam produces chemicals as it cures. Again different types / makes of foam will have different effects but best advice is to keep expanding foam away from cables

 

[DIYinBerks]

My reserve solution was going to be plaster but maybe a weak sand/cement mix would be better and even easier - drying time is not an issue. Not as quick & easy as foam though...
I'm still a bit curious because there are many posts saying keep expanding foam away from wiring - but there are a few authorative sounding ones (found one quoting a manufacturer) that say the reverse. Will probably check with the manufacturer on Monday.

 

[xdave]

Can you wrap some plastic tightly around the cables where they pass through the hole, spray with water and then inject the foam around them? Something like pallet wrap would let you get a tight fit to the cables, but you'd need to test if it was a barrier to the chemicals in the PU foam.

 

[JohnW2]

I'm still a bit curious because there are many posts saying keep expanding foam away from wiring - but there are a few authorative sounding ones (found one quoting a manufacturer) that say the reverse. Will probably check with the manufacturer on Monday.

A few years ago, I asked several manufacturers of expanding PU foam about this, and they were all surprisingly adamant that there was no problem with PVC cables. I'd be interested to hear if manufacturers still take that same position. I've certainly 'done it' frequently over the years and (two or three decades on,in many cases) have not (yet!) seen any evidence of any resultant damage to PVC.

Kind Regards, John

 

[Chri5]

Have you ever tried to reopen a pu filled hole or void to add in cable?

It's an absolute monster, pig, mare job. Pu is permanent and when used with loose bunched cable means you have no way of adding cabling unless you are prepared to blind stab and hope you don't damage existing cabling.

I hate foam, as a product I find it hard to see why anyone other than cowboys would use it. A good chippie, brickie, window fitted and the like should never ever need it.

 

[DIYinBerks]

I take your point, Chri5, but not sure sand and cement would be that much easier to get out...
Personally I think there are places where foam provides a good alternative to traditional methods - but it is true that the Cowboys of this world often use it as a bodge - and that gets it a bad name maybe. I have seen window frames held in *only* with foam for example: personally I would fix first with proper fixings but then use foam to improve thermal and accoustic insulation - hard to do well in a retrofit. I have also seen foam used to support plumbing. Perhaps I should start a thread for the worst application of foam?

 

[mfarrow]

We use expanding foam extensively for filling cable ducts to prevent vermin coming in. Never heard of any problems.

The worst application I've seen is securing CTs onto singles in an LV panel. It looked an absolute mess.

 

[DIYinBerks]

Checked with the manufacturer today - and the foam poses no problems for PVC house wiring.
This answer might vary from other manufacturers - I am using SIROFLEX Trade Strength Expanding Foam, as sold by Toolstation amongst others.

 

[Chri5]

We use expanding foam extensively for filling cable ducts to prevent vermin coming in. Never heard of any problems.

The worst application I've seen is securing CTs onto singles in an LV panel. It looked an absolute mess.

Have you ever had to remove it to add in more cabling?

Not even sure in the damn stuff is fire rated.

 

[bernardgreen]

Just been told of PVC cable that turned almost black where it was in contact with some sort of foam sealing the end of a underground duct. Probably 30 years old.

( PVC triple+E outdoors, it will be replaced )