New Build Timber Frame Wall Build Up

[Gballam]

Has anyone built a wall build up like this? Any thoughts?

 

Walls – U-value 0.12 W/m²K

• Internal Finish: 12.5 mm Plasterboard
• Service Cavity: 50 mm battened void (filled with extra mineral wool?)
• Airtight & VCL Layer: Pro Clima Intello with joints taped using Pro Clima Tescon Vana.
• Structural Frame: 140 mm timber studs.
• Insulation: 140 mm full-fill mineral wool cavity insulation (e.g., Knauf RS45 or Rockwool RWA45).
• External Sheathing: 9 mm OSB3.
• Breather Membrane (Frame): Protect TF200.
• EWI: 160 mm high-density mineral wool (e.g., Rockwool Frontrock or density-equivalent RWA45 slabs).
• Weather Seal: High-performance W1-rated breather membrane (e.g., Tyvek Housewrap) installed over EWI.
• Cavity Fixings: Thermally broken structural insulation screws (e.g., HECO Topix-Plus Therm) installed at a 60–67° angle. Screws must achieve a minimum 50 mm embedment into structural timber studs.
• Ventilation: Horizontal structural battens (min. 38x89 mm) + vertical counter-battens 
• Fire Safety: Euroclass A1 non-combustible cavity barriers.
• External Finish: Cedral Click vertical cladding + starter/top profiles + vermin mesh.

[JohnMo]

Are you seeing any issues yourself?

[Russell griffiths]

Why. 
just build a wider wall and put the insulation in that, your Ewi sounds a nightmare to install and detail. 
 

just sounds all over complicated and not needed. 

[Gballam]

The why is trying to avoid thermal bridging without a twin frame. The issue is the complexity!

[saveasteading]

A service cavity filled with mineral wool isn't accessible any longer.  Just partially fill?

Your thermal bridging is through the studs but this will be calculated... it is normal.

Have you a target for insulation or throwing all you can at it?

[JohnMo]

Do you need the internal breather membrane? Cannot see what that does.

Between studs knauf Frametherm 32, easier to install and better U value.

 

I would do a simple twin stud, fill with frametherm 32, or blown in insulation. OSB board outside with membrane. Or 140mm structural, OSB outside with membrane, internally PIR skin over studs to achieve u value 

[Redbeard]

I have done something vaguely similar, but with wood fibre. Just stagger the outside and inside verticals and make it as 'fat' as you need to get your desired U value net of the timber fraction. In my case the external members were structural and the internals not so, but it can be either way round, or indeed the load can be shared.

Edited 2 hours ago by Redbeard

[Gballam]

Thanks all, very helpful thoughts. Yes I have been wondering whether a hybrid wall of 60mm woodfibre outside studs plus the mineral wool and then pir inside studs is more achievable 

[JohnMo]

27 minutes ago, Gballam said:

Thanks all, very helpful thoughts. Yes I have been wondering whether a hybrid wall of 60mm woodfibre outside studs plus the mineral wool and then pir inside studs is more achievable 

Think that 33% more complex than you need. Stud plus internal OR external, NOT both 

[Gballam]

Aiming for near-passive (and hoping to have walls around 0.13) so will need both internal and external the hybrid design

[Nickfromwales]

1 hour ago, Gballam said:

Thanks all, very helpful thoughts. Yes I have been wondering whether a hybrid wall of 60mm woodfibre outside studs plus the mineral wool and then pir inside studs is more achievable 

Go for a twin wall Larsen truss or I-beam, and lose all this unnecessary complication.   

 

PIR is a complete and total PITA to cut and fit (properly) so just go for blowing cellulose and gain the benefits of a one man insulation process (roof and walls) and also the hugely improved sound-deadening that you lose with PIR.

 

Your initial proposal is hugely over complicated, sorry. KISS wins every time, and will get you to PH targets quickly and simply, with far less to go wrong. The biggest issue will be getting all these layers, materials, installers to do their work meticulously, which most either can’t or don’t.