Fixing Metal back box to a steel box column

[Spinny]

So I am looking to position a metal back box for a double socket just in front of a steel box column. Not sure of the thickness of the steel - I guess maybe 8mm or 6mm.

As it is a steel column rising from foundation level it is or feels cold to the touch, so I plan to insert a layer of insulation e.g. 10mm XPS board between the back box and the steel to heat insulate the back box.

What size Techscrew should I use to screw the back box into the steel column ?

How do I insulate the screw to stop it cold bridging the XPS insulation to make the back box cold ?

 

[Temp]

Id probably glue it in place.

[Nickfromwales]

Yup. 
 

CT1 to hold both the XPS to the steel and the box to the XPS.
 

Don’t use bare XPS as it’s quite friable, get something like Jackoboard or tile backer board, the type with the grey gritty surface coating to accent adhesives, and that’ll hold up much better. 
 

I’d use a pvc conduit back box here, and defo not a metal one here. You can still bury that and plaster to the edges of it, so zero chance of cold getting any further from the steel than you want. The conduit boxes aren’t as fragile as the regular surface mountable back boxes, as they’re made from a different, softer plastic. Link  

 

Tbh the entire rising faces of this steel should be clad with the insulation material, not just where the socket box is.

[saveasteading]

8 hours ago, Spinny said:

How do I insulate the screw

You don't worry about it. The heat loss will be miniscule.

For perspective, you could  counter it by closing the front door a second more quickly or turning a light off for 10 seconds... I'm guessing.

 

Depending how much space you have, it makes it easier to screw or glue a timber or ply plate to the steel, and then the box fitting is by wood screws.

[JohnMo]

8 hours ago, Spinny said:

How do I insulate the screw

I would be more concerned that you actually discovered the steel and you can put your hand on it. It should be fully insulated not hidden behind the plasterboard uninsulated.

[Spinny]

Ta for comments.

Steel is not boarded over yet. So yes will be insulating between it and the p/board. It supports an RSJ across the inner leaf above my bifold opening. So in theory it sits in the warm zone inside my wall insulation, and underneath my warm roof. But of course it has cold rising up the column from foundation level and nobody thought about insulating pads and stuff when it went in, so an unfortunate vertical thermal bridge I suppose.

 

Have a light switch to go in too and will be using a 47mm box to accomodate a smart home relay module, probably got about 60mm between steel and back of P/B.

 

Can you get surface pattress boxes with an adjustable lug ? (Just in case my spirit level isn't working that day.)

 

 

 

[JohnMo]

48 minutes ago, Spinny said:

Ta for comments.

Steel is not boarded over yet. So yes will be insulating between it and the p/board. It supports an RSJ across the inner leaf above my bifold opening. So in theory it sits in the warm zone inside my wall insulation, and underneath my warm roof. But of course it has cold rising up the column from foundation level and nobody thought about insulating pads and stuff when it went in, so an unfortunate vertical thermal bridge I suppose.

 

Have a light switch to go in too and will be using a 47mm box to accomodate a smart home relay module, probably got about 60mm between steel and back of P/B.

 

Can you get surface pattress boxes with an adjustable lug ? (Just in case my spirit level isn't working that day.)

 

 

 

So obvious question, can you not locate the switch away from the steelwork?

 

Wrap steel in 25mm PIR (or more if you have space) and seal all joints with aluminium tape - cold bridge gone.

 

Or use a dry lining pattress box, instead of the normal ones for skimmed walls?

[Nickfromwales]

1 hour ago, JohnMo said:

So obvious question, can you not locate the switch away from the steelwork?

 

Wrap steel in 25mm PIR (or more if you have space) and seal all joints with aluminium tape - cold bridge gone.

 

Or use a dry lining pattress box, instead of the normal ones for skimmed walls?

PIR tends to need to be mechanically fixed though, whereas insulated XPS backer boards can be bonded on and won’t pull away.
 

With PIR you’re reliant on the foil staying ‘fully stuck’ to the core, and that’s not always great after cutting it into small sections and the foils began to tear away. 
 

Any box would be better than metal here afaic, so if there’s > 60mm to play with then a fast fix box would work well too. 

[Mattg4321]

7 hours ago, Spinny said:

Ta for comments.

Steel is not boarded over yet. So yes will be insulating between it and the p/board. It supports an RSJ across the inner leaf above my bifold opening. So in theory it sits in the warm zone inside my wall insulation, and underneath my warm roof. But of course it has cold rising up the column from foundation level and nobody thought about insulating pads and stuff when it went in, so an unfortunate vertical thermal bridge I suppose.

 

Have a light switch to go in too and will be using a 47mm box to accomodate a smart home relay module, probably got about 60mm between steel and back of P/B.

 

Can you get surface pattress boxes with an adjustable lug ? (Just in case my spirit level isn't working that day.)

 

 

 

 

If going for PVC box, then don't fit a surface pattress as it's not only the wrong size (the socket face won't overlap the edges). It doesn't have an adjustable lug.

 

I use these sometimes where walls are very slightly damp in old houses and eventually rot out the metal boxes. No such problem with these. Probably have to order in, nobody keeps them in stock round here at least.

 

https://www.cef.co.uk/catalogue/products/1063228-2-gang-35mm-pvc-flush-pattress-box-round-corners-with-20mm-conduit-ko-white?gclsrc=aw.ds&gad_source=1&gad_campaignid=23051364679&gbraid=0AAAAADOkMAZCou3JXOW_WiNJ2r4F_Fg3c&gclid=Cj0KCQiA9OnJBhD-ARIsAPV51xNG2MYKTdf6T2rR14bNj17wnkbY5gyAYeFQLZOSGdyOnJgqzd1ZN0UaAoK4EALw_wcB

[Nickfromwales]

7 minutes ago, Mattg4321 said:

If going for PVC box, then don't fit a surface pattress as it's not only the wrong size (the socket face won't overlap the edges). It doesn't have an adjustable lug.

It can be set in behind the PB here as there's 60mm to play with to the back of the PB as above  

 

As for the adjustable lug, good point, but if you use a spirit level sat across some long plate screws you can get it spot on with near zero effort.