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!

 

[Andy78]

When I had this situation the builder kindly cut an area of the insulation off of the plasterboard where the back box was going to be. He cut quite a large area so that the wings of the plasterboard box easily pushed out. He didn't cut into the plasterboard.

I keep 2 types of plasterboard boxes on the van. The appleby ones as they seem sturdy and a different variety for when the plasterboard is too thick for the appleby (which happens quite a lot). I've never had any problems using them and can't see why anyone would.

Seeing as though you are the builder you can be kind to yourself and cut away some of the insulation before you put the boards up so that you aren't using all sorts of strange implements to chip away at the insulation through the box hole.

The trick with Appleby boxes is to trim back the white rails at the side of the box with a knife. This lets the yellow tab sit further back in its final position.

 

[HappyHippyDad]

The trick with Appleby boxes is to trim back the white rails at the side of the box with a knife. This lets the yellow tab sit further back in its final position.

Cheers Andy, I've got about 100 of them in the shed so I'll have a little practise with that.

 

[stevethesparks]

All great answers at last! Thanks very much!

A mix bag but its good to know my options and different methods. I agree that cutting a big hole out seems a waste but it may be the best option, strange only 1 person mentioned the skeleton boxes, i thought this would have been the "correct" way to do it?
As also advised i will have a chat with the electrician who will be signing it off.

It is the correct way to do it, they are also called extension boxes.

 

[Bellendian]

Just looking at it from an electrical perspective, 35mm galv boxes spaced off the wall with offcuts of copper tube cut to length would give a decent fixing, combined with a good bead of adhesive around boxes as you're boarding the wall.
From an insulation side though, this does compromise the vapour barrier and may lead to condensation.
Last solid wall internally insulated refurb I worked on, used a different detail.
Walls were lined with celotex, and this was fixed using 2x1 batten secured to wall at 600centres on warm side celotex.
Services run in this batten cavity, p/b screwed to battens, dryliners fitted to p/b for accessories.
Total build up was 60+25+12.5mm though...took up at lot of space.

 

[Michael Monaghan]

Screw the 35mm deep metal box to the brick slab the wall then on second fix cut the box hole neatly and use an extension box . They come in 25 or 35 mm deep so with 2 x 35 mm boxes you'll be about just right
I'll try upload a photo

 

[dksanders]

I have an local electrician who has agreed to sign off. He will monitor the work throughout. Just wanted to get everything right first time, know as much as possible and make sure the installation goes smoothly.

Is he Third Party Certification registered?

 

[Loky11]

Make sure you mechanically fix the p/board if you dabbing insulated boards, they can pop off during fires and block fire escapes.

 

[Michaelwgroves]

I don’t like the idea of dot n’ dab on a solid external wall. I’ve had this dilema on a number of projects. I prefer to use a stud system. Take a look at Tradeline from Travis Perkins, it’s same system as British Gypsum but much cheaper.
http://www.tradelinedrywall.com/products/metal-solutions/wall-liner/
It only adds 18mm to your solution, less thickness of dot n’ dab, so maybe total extra 10 mm. But you can also plumb wall and run wires and pipes in the 18mm void. No risk of damp pushing it off.
You can put OSB in void for fixings, so no problem if installing kitchen units etc.
Then just install 25mm dry line back boxes and no need to cut insulation. But if you need deeper boxes, Celotex 60mm is 3 * 20mm so just scrap off top layer.
In reality the loss of 10mm is unnoticeable, but a stud system gives you so many benefits.

 

[EalingBadger]

A bit late with this comment and the problem has already been alluded to by a couple of other posts, but if you do not have any insulation behind those metal back boxes then you will get condensation and you will get the boxes rusting out over time. The only way to avoid this would be to hermetically seal the sockets - something that is simply not practical, and that's without even considering the fact that every single socket has three great big holes where the plug pins go on the front of them.

 

[Michaelwgroves]

A bit late with this comment and the problem has already been alluded to by a couple of other posts, but if you do not have any insulation behind those metal back boxes then you will get condensation and you will get the boxes rusting out over time. The only way to avoid this would be to hermetically seal the sockets - something that is simply not practical, and that's without even considering the fact that every single socket has three great big holes where the plug pins go on the front of them.

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.

 

[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).