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!

 

[Cid]

I don't think plastic boxes will work with dot dab. Use conventional metal boxes. Can't see why that will be a problem. Get deeper ones if you have to.

 

[telectrix]

how are you going to dot and dab to the brick when you've covered it in 3" of kingspan?

 

[DPG]

Glad it's not just me Tel. Thought I was missing something there.

 

[telectrix]

forget the kingspan, spend the several hundreds of pounds saved on the few quid a year increase in your gas bill.

 

[Jamie Harper]

Okay.. So the Kingspan is insulated plasterboard, so the whole thing can be dot and dab onto brick (its K118, which is dot and dab suitable). I wish i could just forget the kingspan, but its the best way of achieving the u-value I need without taking up 15% of the area of the room, building control involved for a number of reasons so not avoidable.
If i use metal boxes, they will need to be offset from the wall, or I could use longer screws for the front panels?

 

[Cid]

On second thought You maybe able to use the plasterboard back boxes by chipping away some of the insulation for the grip.

Deep metal box with long screws should be fine (I can't see why it wouldnt be).

 

[Jamie Harper]

On second thought You maybe able to use the plasterboard back boxes by chipping away some of the insulation for the grip.

Deep metal box with long screws should be fine (I can't see why it wouldnt be).

Thats my thought at the moment, Ive never used the plaster boxes, are the sturdy? Also would I need additional trunking to stop the cable touching the insulation? Ive read this in a few places, but dont really understand why its needed?

 

[Cid]

They are as sturdy as the plasterboard. Nothing wrong with cable touching the insulation. But you will obviously have to take this into consideration when doing the maths.

 

[Jamie Harper]

They are as sturdy as the plasterboard. Nothing wrong with cable touching the insulation. But you will obviously have to take this into consideration when doing the maths.

May still go for the metal boxes and longer screws. This then allows me to set everything out ready for plasterboard to go up.

By "Doing the maths" do you mean the additional length of cable to come out of the plasterboard?

 

[Cid]

No - it may reduce the current carrying capacity of the cable

 

[Jamie Harper]

No - it may reduce the current carrying capacity of the cable

Could you elaborate? Are you thinking of the thermal heating element of touching the insulation?

 

[Cid]

I don't know if you're an electrician or not but I'm talking about the effect insulation has on cables. Some brief information here TLC Electrical Supplies - https://www.tlc-direct.co.uk/Book/4.3.6.htm

 

[Jamie Harper]

I don't know if you're an electrician or not but I'm talking about the effect insulation has on cables. Some brief information here TLC Electrical Supplies - https://www.tlc-direct.co.uk/Book/4.3.6.htm

Thanks for that, I am not an electrician no, electronic engineer mind, so not too far away! Had a feeling it was to do with that, will have a read.
I will be capping the cable against the solid brick wall, and then its protecting the cable from there really. Maybe plastic boxes are better, possible to rout the cable through conduit through the insulation.

 

[DPG]

How are you going test and certify this installation?

 

[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.

 

[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.