Changing from masonry to TF

[SuperPav]

So after even more deliberation, we're now considering going for a TF build instead of masonry (existing Cotswold stone bungalow, going to two storeys), given the costs and duration the local builders are coming out with to do the walls in blockwork. 

 

We're now considering knocking down and building up in timber frame from the existing footings (with a levelling course of marmox thermoblock to create a thermal break and also to get it to the tolerancing required for a sole plate). Hopefully some TF companies will be willing to work with that.

 

My question is, most of the house will be clad in Cotswold stone, and my footings are about 320 wide (1.5 brick). The outer leaf will need to therefore be about 110mm of stone, and I'm presuming TF always needs a residual cavity of 50mm. I'm therefore left with ~165mm on the footings, is that enough/realistic for a fully built up 140mm studwork wall? Given what we're starting with and cost constraints, I'd be happy with anything around the 0.2 mark for wall U-values... The outer leaf could probably be corbelled out by 20mm or so, but beyond that I think BC would start getting a bit twitchy.

 

I'm mainly hoping that this would result in a watertight shell much quicker with a supply & erect compared to the 12 weeks or so the builders are quoting for the walls and roof to go up. 

Slightly concerned about the required structure to support the three structural ridge beams, but I guess the TF suppliers will sort that out. Calculations show a 440x180mm Glulam beams will take the loads, it's just transferring them down to the foundations!