Opinions on Install Method

[Hefferl]

I have to install t&e cables in the following environments so i'm interested in other's opinions as to appropriate Installation methods 

Firstly, stud wall constructed as follows:

a) 47mm x 140mm Stud

b) 12mm OSB external to stud

c) 100mm celotex between studs pushed up against OSB

d) 12mm celotex inside face of stud

e) 12.5mm plasterboard on top

If cables are installed in the 40mm or so void between c & d is this comparable to Installation  Method 2 (Ref Method A) ? - Ok, there is no conduit but the solid insulation will create a similar  consistent air gap. 

Or is it better to install cables between d & e - which I see as method 102 

Secondly, cables in capping against existing masonry with 25mm + 12.5mm celotex backed plasterboard dot & dabbed over. - can this be considered to be 102?

Thanks all.

 

[Evans Electric]

If its just switch drops  ..in my opinion ...it doesn't matter what you want to call it in a  Domestic situation .  The load is , what ...0.2 A    Just call it 102 .

 

[Hefferl]

If its just switch drops  ..in my opinion ...it doesn't matter what you want to call it in a  Domestic situation .  The load is , what ...0.2 A    Just call it 102 .

No it will be sockets too...

 

[ProDave]

What exactly is this?  It sounds like the external stud walls of a timber framed house.

At the moment I see no mention of a gap between d and e.

I do quite a lot of houses like this, and you need to educate the architect / builder.  What you need is the wall structure with all insulation completed. THEN install 25mm by 50mm timber battens following the line of the timber studs.  This creates a service void for all cables.  Plasterboard goes over the battens and fixes to them.  Very conveniently, 35mm back boxes fit nicely in the 25mm void plus the 12.5mm thickness of the plasterboard.

As it happens I am off to one exactly like this tomorrow where the spec is almost identical and I have already warned him what he wants is virtually impossible.

When will architects and builders learn?

I could also ask where is the airtighness membrane, but that is an even harder one to persuade builders to fit.

 

[Sharpend]

What about fire barriers around the sockets and switches? I’ve done my fair share of timber framed properties and have had to use the fire pads or putty either inside the back box or moulded externally to it. That’s not to mention fire  stop mastic between board and box if the boarders have been over zealous cutting the board. 

 

[Evans Electric]

I do quite a lot of houses like this, and you need to educate the architect / builder.  What you need is the wall structure with all insulation completed. THEN install 25mm by 50mm timber battens following the line of the timber studs.  This creates a service void for all cables.  Plasterboard goes over the battens and fixes to them.  Very conveniently, 35mm back boxes fit nicely in the 25mm void plus the 12.5mm thickness of the plasterboard.

Are you doing it like this Dave  ?    Interesting .

 

[ProDave]

No like this

The principle is you can fill the entire timber frame space with insulation.  Then the battens create a service void before the plasterboard goes on.

Usually the frame is filled with insulation, then over sheeted with more celotex type insulation to mitigate cold bridging of the frame.  This picture (the first I could find anywhere) shows OSB instead.  Imagine that being 25mm thick celotex.

If you want to go th whole hog and make it an airtight building as well as a well insulated building, you would cover the whole lot with an airtight membrane with all joints taped.

 

[Hefferl]

What exactly is this?  It sounds like the external stud walls of a timber framed house.

At the moment I see no mention of a gap between d and e.

I do quite a lot of houses like this, and you need to educate the architect / builder.  What you need is the wall structure with all insulation completed. THEN install 25mm by 50mm timber battens following the line of the timber studs.  This creates a service void for all cables.  Plasterboard goes over the battens and fixes to them.  Very conveniently, 35mm back boxes fit nicely in the 25mm void plus the 12.5mm thickness of the plasterboard.

As it happens I am off to one exactly like this tomorrow where the spec is almost identical and I have already warned him what he wants is virtually impossible.

When will architects and builders learn?

I could also ask where is the airtighness membrane, but that is an even harder one to persuade builders to fit.

Dave yes the first install scenario I'm talking about is the first floor of an extension. Ground floor is masonry first floor is timber frame.

I was trying to choose between two ways to do the job:

You're right there is no gap between d & e - so I see this as 102.

The other way I was considering was between c & d where there will be a void. The stud is 140mm and the celotex is 100mm so that creates a gap of 40mm between the 100m celotex (inside the stud) and the 12mm celotex (on the inner face of the stud)

So its like Deke's drawing but without the Pro Dave Battens!

 

[ProDave]

No it's not like Deke's drawing, as there is a layer of insulation, so the cables end up in between layers of insulation.

I could lecture for hours how this is going to be a carp extension in terms of air tighness and energy eficciency but I won't bore you.

So are you going to to derate the cables?  I dod not even know you could get 12mm celotex, the thinnest I have seen is 20mm.

Why can't the builder fill the whole 140mm thickness of the frame with insulation then the service void battens?  Yes the room gets very slightly smaller, but you have no cable derating issues and the customer gets a better job.

As I say, I dispair at some architects and builders.

 

[Sharpend]

The ones we did had a breathable membrane on the exterior of the OSB then the studs at approx 140mm this was filled with celotex which was 120mm ish thick which left a small void, we would then either put a timber nog across between studs to mount the back boxes which were 35mm or use a metal strip to fix the back box (the name is which escapes me) the fronts were then double boarded with 15mm boards. Externally the walls were brick faced with I believe insulation between bricks and osb. When each house was checked for air tightness not one failed. Mind every joint floor to wall was sealed with mastic also. 

 

[Hefferl]

No it's not like Deke's drawing, as there is a layer of insulation, so the cables end up in between layers of insulation.

I could lecture for hours how this is going to be a carp extension in terms of air tighness and energy eficciency but I won't bore you.

So are you going to to derate the cables?  I dod not even know you could get 12mm celotex, the thinnest I have seen is 20mm.

Why can't the builder fill the whole 140mm thickness of the frame with insulation then the service void battens?  Yes the room gets very slightly smaller, but you have no cable derating issues and the customer gets a better job.

As I say, I dispair at some architects and builders.

This is how I picture it - a void between the 100mm celotex and the 12mm celotex by virtue of the stud being 140mm and the celotex being tight up against the OSB.

So cables in that void is one option I considered - the other option is between the 12mm celotex and the plasterboard so the cable tight against the inner surface (102)

The ones we did had a breathable membrane on the exterior of the OSB then the studs at approx 140mm this was filled with celotex which was 120mm ish thick which left a small void, we would then either put a timber nog across between studs to mount the back boxes which were 35mm or use a metal strip to fix the back box (the name is which escapes me) the fronts were then double boarded with 15mm boards. Externally the walls were brick faced with I believe insulation between bricks and osb. When each house was checked for air tightness not one failed. Mind every joint floor to wall was sealed with mastic also. 

Yes this sounds very similar - breather membrane outside of OSB - vertical tiling on top of that.

 

[ProDave]

This is how I picture it - a void between the 100mm celotex and the 12mm celotex by virtue of the stud being 140mm and the celotex being tight up against the OSB.

So cables in that void is one option I considered - the other option is between the 12mm celotex and the plasterboard so the cable tight against the inner surface (102)

View attachment 9791

Yes this sounds very similar - breather membrane outside of OSB - vertical tiling on top of that.

So what are you proposing?  Cut channels in the 12mm celotex where the cables run?  I guess that would work and since you have cut the 12mm celotex the cable is no longer between insulation.

The reason for over boarding the frame is with the SAP assesment required, the frame no longer meets building regs without the extra layer of insulation.  I just wish builders / architects would think it through a bit better.

 

[Hefferl]

Thanks Dave.

Of the two options, I'm now leaning towards running cables in the void. So cable will be loose in 40mm void formed by the gap between the insulation to either side. What i can't decide is what installation method covers this the closest? I will probably opt to run 16a/20a radials using 4mm cable

 

[Evans Electric]

Why not the usual ring main ?   Domestic  cables don't usually carry their maximum loads   other than showers & cookers  .  

Up to you though , you're the  man doing the job .      Cables are not totally enclosed in insulation so is it not an Installation Method 100  ?   

 

[ProDave]

Well I have just first fixed the one I am doing today.

The guy would not budge and do the battened service void thing.

So the solution we agreed is the first fix is a very rough affair as I can't fix any cable drops as no insulation is in and nothing to fix it to.  The agreed "solution" is the owner will fit the insulation, full fill rockwool between the studs and 20mm celotex over the studs.  HE will then chase channels into the celotex for the cables before the builders come back and fit the plasterboard.

What could possibly go wrong?  All a bit unsatisfactory.

 

[Hefferl]

Well I have just first fixed the one I am doing today.

The guy would not budge and do the battened service void thing.

So the solution we agreed is the first fix is a very rough affair as I can't fix any cable drops as no insulation is in and nothing to fix it to.  The agreed "solution" is the owner will fit the insulation, full fill rockwool between the studs and 20mm celotex over the studs.  HE will then chase channels into the celotex for the cables before the builders come back and fit the plasterboard.

What could possibly go wrong?  All a bit unsatisfactory.

So do you see this as 102 Dave?

Are you derating the cables at all?

 

[Evans Electric]

What could possibly go wrong?  All a bit unsatisfactory.

As you said earlier ,  no one seems to be addressing the fact that there,s no service void .