Insulating Timber Frame Extension

[Build71]

Hi

First timer on the forum. I'm sure this question has probably been asked before - but I couldn't find it anywhere.
Im building a standard single storey extension and all of the specs / drawings I can find online - show a typical construction of

12.5mm Plasterboard
40mm Celotex TB4000 over studs
Vapour Control Layer
100 x 50 mm stud wall with 90mm Celotex GA4000 between studs
12mm Plywood Sheathing
Breathable Membrane
50mm Cavity
Brick Cladding

Is there any reason why I can't move the extra layer of insulation outside of the studs (between the ply sheathing and the studs) - as it would give me a much more useful 10mm gap for dropping down wires etc behind the plasterboard...

12.5mm Plasterboard
Vapour Control Layer
100 x 50 mm stud wall with 90mm Celotex GA4000 between studs
40mm Celotex TB4000 over studs
12mm Plywood Sheathing over Insulation
Breathable Membrane
50mm Cavity
Brick Cladding

Thanks

 

[freddiemercurystwin]

The sheathing is often a structural element that provides rigidity to the timber frame, so needs to be fixed directly to the timber studs, so you would need longer wall ties and have the added hassle of building the insulation into the cavity wall. Is this timber frame finger in the air stuff from you or designed by an SE?

 

[Build71]

Ok.
Thought there must be some reason for it.
I’ll just stick with the original plans…
Thanks

 

[noseall]

Ok.
Thought there must be some reason for it.
I’ll just stick with the original plans…
Thanks

Use 120mm timbers for the framing, thus creating yourself a small void behind the 90mm insulation.

EDIT: Use 100mm insulation. More readily available and more useable.

 

[robinbanks]

Or use battens between plasterboard and VCL to form a service void. If you choose an aluminised VCL this will also provide an air gap which will enable the reflective surface to work as intended

Agree with Freddy re the sheathing providing structure; if you want insulation outboard of it you should look to bond it onto the sheathing - expanding foam would work; lay insulation, spray liberally (you can get nozzles that convert foam guns to spray op), leave to expand and go tacky, drop wall panel on and wait half an hour, stand panel up

 

[freddiemercurystwin]

If you fit insulation in the cavity you wouldn't bond the insulation onto the sheathing board but specify wall ties that should be the type that hold the insulation against the inner skin.