Wiring options for shed - advice required

[imageel]

I am in the process of building a garden office/workshop and although I intend to get an electrician to install the power and light circuits, I would like your advice on what options are permissible for concealed power circuits -
The studwork is 44x97 sheathed outside with 12mm ply + cedar shingles and will eventually be rockwall insulated and lined with probably 8mm ply, so I can't see how 2.5mm T/E (or whatever after derating) would satisfy the 50mm clearance requirement (safely inside and out).
As I do not want to have any surface conduit I'm concerned that whilst I may know where the cables run, others may not and certainly from the outside the risk of e.g. nail penetration high.
Would SWA within the void satisfy the 'mirror image' issue raised in http://web.archive.org/web/20061110111450/http://www.niceic.org.uk/downloads/concealedcables.pdf

i.e. I am assuming that the location of the 'accessory/switchgear' would not be possible (unless you measured really careful) from the outside wall

Ultimately it will be my electricians decision on how to address this, but am interested to understand what other options there may be,
Cheers
Ed

 

[imageel]

Thanks for the suggestion however Dado trunking isn't really suitable - I want to use this mainly as a workshop for woodworking - so am thinking that most sockets will be 6+ inches above bench level for ease of access and safety against me e.g. hand planing a dual gang socket off the wall...

Yes meant rockwool (not rockwall) - I know Celotex is better, but it's pricey too - workshop is 4x5m so large area to cover...

Not sure I understand the issue re: vapour barrier -layers from outside to in are -
Cedar Shingles->Permeable membrane->12mm ply->97mm studs infilled with 'something'->8mm ply

Do you think that is going to create a condensation problem somewhere? -I thought that was the correct way to clad a stud wall shed..
but grateful for advice if I've done something wrong - better to correct now rather than watch it rot over the years ;-(
Cheers
Ed

 

[ban-all-sheds]

I don't see what the problem is.

97mm framing + 12mm ply + shingles puts cables run just under the interior ply at >50mm from the outside surface. You'll need to use surface mounted accessories, but then you would anyway in order to avoid perforating the vapour control layer.

I would have thought the materials would be

Cedar Shingles -> 12mm ply -> 97mm studs infilled with insulation boards (e.g. PIR foam) with a vapour control layer on the warm side and a bead of sealant around the edges, small void for cables ->8mm ply ?

 

[imageel]

Thanks BAS- however you have highlighted the very point I am not understanding -
My vapour barrier is on the outside of the sheathing, underneath the shingles - why can I not fit recessed (flush) accessories - there is no vapour control layer between the inside ply and the insulation?

...well there isn't now, but depending on your reply there might be

..I thought I had covered this in my 'fag packet' design, but maybe I have missed something vital... must admit I would rather be making cabinets than this workshop but needs must ...and lots to learn!
Cheers
Ed

 

[EngStudent]

Can I ask why you don't want to use conduit on the surface - it's only a workshop!

 

[imageel]

It's hormonal - I also think it looks butt ugly

 

[flameport]

My vapour barrier is on the outside of the sheathing, underneath the shingles

If you intend to heat the building and actually spend any time in there, the vapour barrier must be on the inside, between the insulation and the inner wall surface (plywood in this instance).

This ensures both surfaces of the vapour barrier are at the same temperature of the room.
If the barrier is on the outside, it will be much cooler than the inside air. Warm moist air will therefore travel through the insulation and condensate on the cold barrier, where it will eventually lead to the insulation being soaking wet and causing rot/mould.

Just to be clear everyone is talking about the same things:
Vapour barrier is a plastic sheet such as this: http://www.screwfix.com/prods/12869/
Totally waterproof.

If you want something on the outside (under the shingles) to prevent rain penetration, then a breathable membrane should be used: http://www.screwfix.com/prods/55379/
which although prevents rainwater entering, does allow any moisture underneath to escape.

 

[imageel]

Thanks for the clarification - I had to be honest forgotten about the need for an internal vapourc control layer!
Will have to consider the options - for sure surface mounted outlets would be simplest....

Many thanks to all who have offerred advice
Ed

 

[OwainDIYer]

Yes meant rockwool (not rockwall) - I know Celotex is better, but it's pricey too - workshop is 4x5m so large area to cover...

It's cheaper if you buy seconds with cosmetic faults.

http://www.secondsandco.co.uk/

For a workshop I would always recommend high level trunking with conduit drops. Makes it much easier to add new points as required each time a new toy arrives.

Also consider:

- emergency switching
- calling for help in an emergency (phone/intercom)
- emergency lighting so you're not left in darkness with a power tool spinning down