Notching Jamb, Jack and common studs for cables. Insulation position

[tvrulesme]

I'm building a small timber frame extension and planning the insulation and cable position currently. The studs are 140mm x 47mm C24 timber. 

 

First question is am I ok to notch cables (4mm T+E and 1.5mm 3 core and earth) into the studs which form the outer walls of the extention? I can see loads of info on notching joists but very little on studs.

 

I have two options in terms of placement of the insulation (100mm PIR + 62.5mm Insulated plasterboard) which have an effect on the u-values according to the wonderful Ubakus.

 

1. Service void below the level of the insulated plasterboard (blue line is position of the cable notched into the studwork, inside to left) giving a u-value of 0.163

2. Service void behind both sheets of insulation (blue line is position of the cable notched into the studwork, inside to left) giving a u-value of 0.158

 

Which is better or does it not really matter and I go for best u-value?

 

 

 

 

[ProDave]

Look at the normal option, all insulation towards the outside, vapour / airtight layer, service void then plain plasterboard.

 

None of your cables are in insulation, no penetration of vapour / air tight layer, and only the battens that form the service void need drilling or notching for cables.

 

A service void formed with 25mm battens and 12.5mm plasterboard is perfect for 35mm back boxes.

[JohnMo]

To me your wall build up makes no sense. Your current plans have both options punching holes in vapour tight membrane to get services through, be it electric or plumbing.

 

I would working from inside, plasterboard, service void, then your vapour and airtight layer. Run an OSB strip along the top of the wall to attach cables to.

[JohnMo]

 

Imagine the grey wall is your vapour membrane. Top horizontal batten to hold plasterboard then strip of OSB for wires etc. I also ran OSB everywhere other services would go to control penetrations.

 

I found 50mm battens made life easier than 25mm.

[tvrulesme]

Thanks @ProDave and @JohnMo. I blame Kingspan who I got the buildup from. I guess they care not of cables. 

[tvrulesme]

I was thinking of using a foil tape and silicone where the cables breach the vapour control layer based on:

 

As per the literature supplied with the product - an effective air and vapour barrier can be achieved by the taping of board joints using suitable insulation tape, the inclusion of a separate proprietary vapour control layer or the use of a foil backed plasterboard all of these methods are acceptable forms of air/vapour control

[JohnMo]

Not sure I would do kingspan between studs too much faff and getting it done well not easy. Mineral wool between studs something like frametherm 32, you should be able to fully fill, as you have external battens. Then PIR in full sheets over with a decent tape joints, service battens and plasterboard.

[JohnMo]

1 minute ago, tvrulesme said:

I was thinking of using a foil tape and silicone where the cables breach the vapour control layer based on:

 

As per the literature supplied with the product - an effective air and vapour barrier can be achieved by the taping of board joints using suitable insulation tape, the inclusion of a separate proprietary vapour control layer or the use of a foil backed plasterboard all of these methods are acceptable forms of air/vapour control

Would avoid if you can, it's just more time faffing about, try to keep jobs simple and idiot proof and unlikely to be missed and then hidden never to be seen again.