Condensation in steel socket back boxes

A.Chapman

Hi All, any of your knowledge or know how would be very much appreciated with my concern. 

Have recently had thermal backed (Polystrene) plasterboard battened onto the inside walls of an outside cavity wall in a 1920’s semi. I noticed a week or so ago during more seasonal weather that the steel back boxes that are cut out of the plasterboard and exposed had ‘droplets’ of water and the wood batten around them was damp. This worried me immensely. The plaster dried a while ago and the house is well ventilated. The steel back boxes on all other walls in the house are fine, it is just the ones that have been cut out of the thermal board. Can anyone please give advice on why this is occurring and/or how to prevent it. Would plastic back boxes stop this? And why is it happening in the first place? I am considering asking the electrician to cap the cables in the sockets in question as I am concerned the moisture in the boxes will cause issues. Thanks for reading and your attention in this matter. 

Bendy_House

Were the rectangles made in the insul p'board for these boxes cut right through into the battened void? If so, I suspect that the warm moist air from the house is being 'sucked' constantly through there, via the plug's pin holes for example, and condensing out where it's obviously the coldest part of the wall. If the boxes aren't insulated behind them, AND air is able to seep through constantly.
If this is the issue, then ideally some insulation should be added to the backs of the boxes so there's a continuous, unbroken layer (tho' it'll likely be thinner behind the boxes), coupled with the cable entry points - and any other holes in the box should be blocked. Whether plastic boxes are better sealed, I dunno.
This must surely be a known issue, with an obvious solution?! When t'inLaws had problems with their interweb a few years back, in a near-new Persimmons home, I found corrosion in the back box of their extension telephone socket. There were open cable paths that led to behind the p'board, and I could felt the movement of air. Almost certainly that was the cause - warm moist air from the house seeping through and hitting a cold spot = condensation.

FreeBear

How thick is the insulation and how deep are the back boxes ?

Plastic dry wall back boxes will not rust, but they will take a bit of fiddling to fit (they don't generally have 20mm knockouts like metal boxes do). It would also be worth plugging the end of the conduit where the cables enter the back boxes.

A.Chapman

Sorry his late reply. Yes, cut right through and now sit in a battened frame. (Reason being, initially they were chased into a rendered wall, however then we subsequently decided to batten and thermal board the wall with 30mm (total) insulated plasterboard. Which means the steel back boxes are quite deep back behind the battens and polystyrene backed plasterboard). 

Bendy_house, your comments make total sense re the warm air in the room being ATTRACTED to the ‘cut out’ area. It has caused me much worry and I have been thinking of ways to resolve. My electrician says moving the back box will cause damage to the plaster. I would prefer them removed tbh. 

He has currently Capped the cables, but I don’t want to cover the gap and leave live wires behind. I have spare insulated board to fill gap and was hoping this would stop the air getting to the cold wall behind. 

I have thought about putting Rockwool in the boxes in the hope it will keep the metal back box surfaces warmer? What’s your opinion please? Is that safe or expanding foam? 

Again, thank you so much for your interest in this

Bendy_House

I think, if you can simply stop the draft, the issue should go away. So use sealant (one that won't attack the wire's insulation) to seal around the cable entries, screw holes, etc. Just seal off the back-box.

Perhaps line the inside of the BB with Duct tape, even? And then a wrap around the cable as it comes through, to seal it against this. (But, again, you'd need to be sure the adhesive doesn't affect the cable insulation).

What are you currently proposing - to REMOVE your sockets? That must surely be a bind?

Bendy_House

There are products out there, tho' I don;t know how good they are.

Schneider make air-tight back boxes, I understand, or I found these - do they fit inside the existing BBs? If so, that could be a simple solution: https://www.phstore.co.uk/instaabox

Other solutions suggested by sparkies - some do seal around the cable entry points using acrylic sealant.

A.Chapman

The instaabox image looks interesting. I’ll check out. I think if I could eliminate the condensation I would have piece of mind. The first photo below shows the moisture and it was wet throughout. The other photo is after the electrician capped the cables. I think I will reinstate the sockets and also use plug blockers to prevent air getting through the plug holes. 

FreeBear

What numpty fitted those back boxes ?

They should be flush with the plaster, not recessed so deep into the wall. That far back, you'd have to use extra long screws as the standard ones would be too short..

Bendy_House

It does seem as tho' as couple of tweaks could have helped. One is as FB says, to have the boxes mounted further forwards which would also allow a layer of insulation to be snugly placed behind it.
Then to seal any air gaps. On that point, it's hard to see where the passage of air could be, as the box appears to be fully surrounded by battens. Unless the cable runs allow that air route.
And plastic boxes would, in themselves, be better insulated.

I am assuming - certainly don't know for sure - that the cause IS a passage of air, since if the BB was sealed, then very little air would pass in there. But, that's not to say that some cond wouldn't take place in any case. I don't know.
The extent of cond makes me think there's a lot of new, moist air passing through.
I'd certainly be loath to actually lose these sockets - that would be too inconvenient. I wonder if it's worth carefully slicing around the metal BBS - avoiding the cables... - to allow them to come out with little surrounding damage, fitting insulation (rigid stuff), and then refitting them (with plastic equivs if poss, or with inserts if not)?
There HAS to be a solution!

fenwick458

I'm not sure what the problem is, when plaster is drying out the air is humid and the metal boxes are exposed so you will get condensation there. once the plaster is dried out, and the socket fronts are fitted, this won't a be a problem anymore.

Bendy_House

fenwick458 said:

I'm not sure what the problem is, when plaster is drying out the air is humid and the metal boxes are exposed so you will get condensation there. once the plaster is dried out, and the socket fronts are fitted, this won't a be a problem anymore.

Argh, I'd assume the sockets had been added...

The OP did mention, tho', that the plaster had dried out a while before, and the house was well ventilated.

Hmm, I don't know if there is an issue or not :-(