Wall composition of timber extension.

[Suited72]

Hi, i'm building a wood constructed extension on to the existing house. Couple of questions on the wall composition - there isn't a lot of wall.

Got a couple of concerns on the outer side of the wall.....is the air gap needed or can the void be filled with 25mm celotex? If air gap, are top vents needed and how are they generally done?
I'm planning to have it rendered, but didnt want to run the render boards to the floor and getting stained - any suggestion of what to use to fill the gap to the floor? I know normally it would just be a stop bead, but here there will be a 50mm kick back to the brick face.

Also any other suggestions from anyone who has done something similar or knowledge of would be great.

 

[^woody^]

The timber frame should have a minimum 25mm air cavity and continuously ventilated top and bottom - mesh screened strip 10mm strip. Cladding should me kept a minimum of 150mm above external finished levels, so the foundation brickwork is taken up the floor level.