Does anyone know what the current regs are for using a stud wall as a second skin on an external wall, ie thickness of insulation, vapour barriers, does it need to be ply backed etc etc

Cheers

Being really unhelpful – but depends, what is the space?
Building Regs Approved Documents

You need to achieve a U-Value of 0.30.

Assuming the outer skin is brickwork, it should be as follows, from inside to outside:

12.5mm Plaster Board
Vapour control layer – could combine this with plasterboard if you use a duplex grade plasterboard.
Timber studs (depth depends on structural requirements)
70mm Celotex FR4070 between studs.
9mm sheathing board
Breather membrane
25mm Cavity
Brickwork

It does depend on what your external skin is. There are also wall lining systems that Gyproc do that will also work.

A U Value of anything upto 0.35 w/m2k

basically:

12.5mm plasterboard
VCL (500 gauge polythene)
Timber frame/studs – 140 x 38mm or 100 x 50mm (structral dependant)
90mm Kingspan TW55
10mm OSB
breather membrane (could use a Tyvek Reflex for a better U Value)
50mm Cavity (Minimum)
Facing brickwork

The above should achieve a U Value of 0.26 W/m2k

If your getting the B.regs done by NHBC make sure you get a BBA certificate for the insulation as only the Kingspan is approved (ridgid insulation only), if your going to use a mineral insulation (breathable) such as Knauf Frametherm you will be alright.

A U Value of anything upto 0.35 w/m2k

Only if it's a new-build

50mm Cavity (Minimum)

Sorry, my bad – should have said 50mm

Thanks guys, just what I wanted to know. Its non structural so guess just need stud wide enough to take the insulation, ansd all this to sit on dpc I take it. (Its a garage to living space conversion with single brick skin at preent)

The thickness of your insulation will depend on the stud centre for thermal bridging.
Rigid pu insulation is expensive tough, could be cheaper to use a 140mm stud & mineral wool. Try & stick to 600mmc/c for the studs although as in any timber frame you will be looking at a min. 12% thermal bridging.

Drop me an email if you want a quick U value calc doing for building control

Teggs, having an argument at the moment with the NHBC regarding the bridging element on a timber frame, they use 15% as a default although I always base it on 10-12% they wont budge, any thoughts??

Could always use a layer of 12mm Celotex TB3000 on the inside of the studs to eliminate any thermal bridging.

Also, as I said before Gyproc do products, such as the Thermaline Super, that will do the job too.

Most insulation manufacturers have U-Value calculators on their websites, that take into account the thermal bridging effects.

Hi Glenn

15% is the default value recommended if your using accredited details which I would presume would not necessarily be applicable here.

You can use 12% (actually 12.5%) using accredited details & meeting certain criteria set out in the U value conventions guide which is all to do with the head/sole plates & noggins & stuff.

I know it would be laborious, but if you have the frame design then you could always calculate it but its a crap job.