Timber Frame Building

2 Jul 2013
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United Kingdom
Hi All,

I've got a 1960's bungalow which I'm planning to build a first-floor extension of timber frame construction in a couple of years' time. So, I've started doing some preliminary drawings and research before I involve a timber frame designer...

I'm wondering if anyone here can answer a question on the build-up of a timber frame. Most of the kits which I've seen are (out to in)

Cladding > 45mm Batten > Breather Membrane > 9mm OSB > 140mm stud with 120mm PIR between studs > VCL > 45mm battens to form service void > Plasterboard and skim

I want to know why nobody seems to build-up frames as (again out to in)
Cladding > 45mm Batten > Breather Membrane > 160mm stud with 120mm PIR between studs > 9mm OSB > VCL > Plasterboard and skim

This would give you a service void in the insulated cavity, whilst also providing a patress to all walls internally via the 9mm OSB (which is there to prevent racking). Only thing I can think is that the VCL would have to be penetrated to get services through - I don't know if this could go on before the OSB in order to avoid this.

Ultimately, I'd like all stud walls lined with OSB directly behind the plasterboard to form a patress - so I'm trying to avoid a second layer of OSB. The frame would be first fixed with electrics and plumbing on-site, so it wouldn't be an inconvenience to fit the OSB and VCL on site.
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Mine's 90mm CLS. Filled with celotex and lined with 50mm on the face, and the services are channeled in to the face of that and behind the plasterboard.

The frame thickness is generally based on the insulation that will go in it, rather then needing to be 140/160mm for strength

The OSB goes on the exterior of the frame not interior.
Perhaps a void would not work too well because any condensation from the inside of the house needs to be able to pass to the outside through the breather membrane. A void might not aid this process.

In your example I guess there is also the question of the placement of the timber onto your cavity wall in tems of the loading.
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