OSB3 as VLC in warm wall construction over oak frame

[David Cooke]

I'm planning an in to out 'warm' wall build up as follows - all outboard of a structural oak frame.

Plasterboard and skim
125 x 50mm stud at 400 centres (vented air gap)
15mm OSB3 taped joints (functioning as a VCL amongst other jobs). Could apply another VCL over this OSB if needed/use smartply if we have to!
120mm PIR
50mm Counter batten (vented air gap)
Breather membrane
Wood cladding

The aim here is to avoid the inevitable breaks and gaps that occur when an internally applied VCL is installed around a structural oak frame. I also like the idea of a VCL that retards but doesn't 'trap'. The design idea also allows for an unbroken insulation layer that fully wraps the structure without thermal breaks (other than counter batten screws)
Ubakus tells me the condensation risk is zero. Does anyone have any thoughts? I can apply a stand alone VCL over the OSB if needed.

Thanks for any thoughts!

 

Also posted in the Green Building Forum FYI!

[JohnMo]

14 minutes ago, David Cooke said:

120mm PIR

Why PIR, it could shrink, giving you breaks in the insulation. I take it's going between the 125mm studs?

 

I would increase depth of stud and do mineral wool, wood fibre or even cellulose pumped in.

 

If it's not structural why not use 600mm centres this will lead to fewer thermal bridges.

 

[Gone West]

59 minutes ago, David Cooke said:

Thanks for any thoughts!

 

My last self build was a timber I-beam portal frame, structure as follows:-

 

Plasterboard and skim

350mm Steico timber I-beam, fully filled with Icynene

15mm OSB3 square edge racking

Protect TF200 membrane

50mm timber battens fully filled with Rockwool RW6

Protect A1 membrane

50mm counter battens vented air gap

15mm cedar cladding

 

The walls and roof were both constructed in a similar way. The construction was successfully tested using the Glaser method for condensation risk. A WUFI analysis was not carried out. That may be of some use.

[David Cooke]

Hi JohnMo,

It's a warm construction - PIR outside of stud and OSB3. 

So unbroken layer of PIR on top of OSB3 (carried by the stud wall)

PIR held in place by counter battens.

Stud voids to be used as utility voids and probably not insulated unless client wants additional.

My question is whether it needs a dedicated VCL over the OSB3 or not. My guess is no - dew point being somewhere toward outer surface of a completely unbroken layer of PIR - where no moisture laden air can reach.

[David Cooke]

Hi GoneWest,

Was that a Passivhaus?