Timber framed cladding on top of cavity brick wall

[rssteve]

I'm in the process of building a lean to porch and up to wall plate level. The side infield are going to be timber framed then cladded. 1 side is actually being built ontop of a flat roof so I think that is sorted and will be as follows.

Plasterboard, VCL, 25mm celotex, 100mm studs full filled with celotex, osb, breathable membrane, batten, then cladded with an air gap top and bottom.

The other side which is built ontop of the cavity wall I am unsure of. At a guess how does the following sound?

Inside out.
Plasterboard, vcl, 25mm celotex, 100mm studs full filled with Celotex, breathable membrane. Then the gap for the cavity.

Then built on the outside course. 100mm stud laid on a dpc, breathable membrane, batten, then cladding with air gap top and bottom.

Is there a need to install osb or ply on this side?

 

[^woody^]

Two leafs of timber frame? Why, that's a complete waste of time.

Yes you need sheathing board on timber frames.

 

[rssteve]

The area of the timeber part will be approx 1800mm wide and 700mm high, at a 22 deg angle. It's not supporting the build it's simply an infill.

What would you suggest woody? The idea of the 2 sets of timber frames was only because the brick wall is approx 310mm wide and I would want the inside wall level and not set out. The cladding would protrude the outside enough for the vent and so drips don't sit on the brick wall.

The bit I need to infill and clad is shown in the second photo.

Edit
The porch will have a full height ceiling up to the rafters.

 

[Charlie George]

Why not just continue with block/brickwork, no reason to change to timber.

 

[rssteve]

The top parts are being cladded to match the rest of the house/estate as per my planning permission. I've already got most of the materials needed for this purpose so it's just my time which is free. I don't want to employ a brick layer for another day plus the cost of the bricks. I'm simply unsure how to tackle the job I asked in the first post.

I think I'll close the cavity at wall plate level with some cement board. Build the inner leaf as per the usual method talked about in my first post, then build the outer leaf stud, breathable membrane, batten then clad with vents top and bottom.