Rendering over timber section in block wall

[Mr Blobby]

For reasons that make me use bad language, our block build house has a couple of window openings that need to be made smaller, but only on the outer leaf of a block cavity build.

Window height must be reduced by about 300mm.  The obvious thing to do is to insert a intel and a course of blocks, but that may be a little too destructive. 

Builder and joiners are suggesting a timber stud, to be built with ply across the face and then steel mesh across the front to render the timber stud to the blockwork.  My concern here is the timber getting wet and rotting/warping and then render cracking.  Breather membrane and bitumin paint on the ply has been suggested. 

 

What's the best approach here?  Does the timber need to be ventilated?  Would render board at the front be better than ply?  Can mesh be nailed to render board?

 

[nod]

Mesh directly over the timber then render if sand and vent 

Breathable  renders Cement board fibre mesh and base coat