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!!?