Back Boxes for 57.5mm Dab n Dot Insulated Plaster Board

[Jamie Harper]

Hi all,

About to begin my first fix rewire. Due to regs, and the house being a 1908 solid wall construction, I need to include 57.5mm Kingspan to all external walls. To try and save some room, I will be Dab and Dotting the plaster board to the exposed brick. I will be installing the plasterboard myself, but will have a plasterer to do a final skim coat.
Now i have read mixed things regarding which back boxes to use for this.
I understand the I can still use metal boxes, but this will require an offset using some wood or the sorts to bring the box flush, but this seem very time consuming for every single external wall socket (roughly 20 sockets).
Now, again, mixed reviews on plastic back boxes. A lot of people saying they are great and work perfect for this sort of job, but may require some PVC piping to avoid contact with the insulation? Other people saying they are rubbish and not worth using. Why is this?

Any help on this would be great!

 

[EalingBadger]

I'm not sure if the mention of metal back boxes was in relation to my post or another, but to be clear, dry line back boxes are plastic. Furthermore they sit in front of the insulation.

No, it was in the context of the OP's original post.

I know that dry line back boxes are plastic.

Plastic back boxes without insulation behind them will also suffer from condensation as well. Although not quite as badly, and they won't rust. It is still not a good idea though.

 

[Deleted member 26818]

Can’t say that I’ve noticed condensation causing that much rusting of metal back boxes.
Seen a lot of rusting caused by rising damp, and a few cases where rain has penetrated an outside wall then used the cable chase to run down the wall.

 

[EalingBadger]

Can’t say that I’ve noticed condensation causing that much rusting of metal back boxes.
Seen a lot of rusting caused by rising damp, and a few cases where rain has penetrated an outside wall then used the cable chase to run down the wall.

It is a phenomenon that is uniquely acute in the context of a well insulated wall.

Technically, it's called cold bridging, and the effects can be ruinous.

The nice warm air inside your house is loaded with water vapour. When it hits the cold metal box connected to the exterior brickwork it dumps this substantial load pronto there and then.

One of the tricks to keep this effect to a minimum is to prevent air circulation hence the emphasis on sealing ALL punctures in the wall and ceiling fabric of new build well insulated houses with fancy expensive butyl tapes. The now mandatory pressure testing of new buildings is intended to expose flaws in this sealing.

The same thing can occur in other contexts as well and cold bridging in roof structures has caused massive and very expensive damage - this time with the condensation rotting out the roof covering (typically OSB in this context).