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!

 

[Jamie Harper]

How are you going test and certify this installation?

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.

 

[DPG]

Fair enough. Is he involved already to advise on cable sizes and routes though?

 

[Flanders]

You could try these, not used them my self but may leave a better finish. The problem with the plastic dry lining boxes is that they bring the edge of accessory out a tinny bit from the wall from which may be a problem if you are looking for a high standard of finish or fitting flat plate accessories .

2 Gang 35mm Fire & Acoustic Metal Dry Lining Plasterboard Box - https://www.tlc-direct.co.uk/Products/FB4235.html?source=adwords&ad_position=&ad_id=315107931576&placement=&kw=&network=u&matchtype=&ad_type=&product_id=FB4235&product_partition_id=297072075353&version=finalurl_v3&gclid=EAIaIQobChMI65CioqPX4AIVTDPTCh2OqAh4EAQYAiABEgIwT_D_BwE

 

[Jamie Harper]

Fair enough. Is he involved already to advise on cable sizes and routes though?

He has been involved yes, just dont want to constantly be pestering him!
I have done some calculations for cable sizes, but I forgot to take into account the insulation. Luckily not bought all the cable yet!

In regards to routing through, I have planned it all out ready.

 

[Jamie Harper]

You could try these, not used them my self but may leave a better finish. The problem with the plastic dry lining boxes is that they bring the edge of accessory out a tinny bit from the wall from which may be a problem if you are looking for a high standard of finish or fitting flat plate accessories .

2 Gang 35mm Fire & Acoustic Metal Dry Lining Plasterboard Box - https://www.tlc-direct.co.uk/Products/FB4235.html?source=adwords&ad_position=&ad_id=315107931576&placement=&kw=&network=u&matchtype=&ad_type=&product_id=FB4235&product_partition_id=297072075353&version=finalurl_v3&gclid=EAIaIQobChMI65CioqPX4AIVTDPTCh2OqAh4EAQYAiABEgIwT_D_BwE

Nice thought, guess ill be paying for the high quality finish. Would still need some conduit to run the cable through the insulation? Or do I just use the de-rating factor in calcs? Or Both!!?

 

[MJPD29]

47mm metal boxes and long screws

 

[westward10]

If you have an electrician signing this off he is the best person to ask as he can see the installation and give correct advice. If you feel you are pestering him then compensate him for his time.

 

[Silly Sausage]

Due to regs, and the house being a 1908 solid wall construction, I need to include 57.5mm Kingspan to all external walls.

What are your Building Inspector's thoughts on you putting 20 odd holes in the wall insulation? Surely that's going to create cold spots and compromise the specified U value (whatever it is) of the walls.

 

[Grant webber]

Cut plaster board boxes in after the plasterboardings done. Put the bit of insulation u cut out behind the back box so there's no holes in insulation. Bring cables into box. Job done.

 

[telectrix]

do it right. 35mm back boxes onto brick then 25mm skeleton boxes to make up the space. that way the cables are all enclosed.

 

[HappyHippyDad]

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.

 

[123]

Tend to find on this kind of job that the socket back boxes are where all the condensation etc ends up. I'd give the dry lining boxes, with some form of insulation stuck behind them the most thought.

Metal back boxes over time will rot, and if you cut a huge wack of insulation out for plaster board boxes, it kinda defeats the purpose of installing it to begin with.

 

[stevethesparks]

Extension boxes if necessary

 

[Deleted member 26818]

If you know where you intend to site the sockets and accessories, you can fix the wiring in place before dotting and dabbing the plasterboard, then cut out the holes for the back boxes and just pull the cables through into the back of the boxes.
Plastic dry lining back boxes would be ideal and any lip would be hidden by the skim of plaster.

 

[Jamie Harper]

If you know where you intend to site the sockets and accessories, you can fix the wiring in place before dotting and dabbing the plasterboard, then cut out the holes for the back boxes and just pull the cables through into the back of the boxes.
Plastic dry lining back boxes would be ideal and any lip would be hidden by the skim of plaster.

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.