I'm not sure what you mean by 'twin wall construction' but have you considered a single skin block work wall with external insulation and render over to give you your 'U'-Value? Should be quite easy to achieve with an overall wall thickness of 350 -400mm depending on the insulation used and the thickness of the block.

http://www.scanhome.se/downloads/drawings/s10_super_passive_converted_attic.pdf

Sid the above link shows a cross section of the twin wall frame I am building. The outer wall comes as a closed panel with windows and doors fitted from the factory. Then 145mm of rockwool (120mm shown on this drawing) is installed and covered with the air tightness membrane and the final 70mm stud is built on site which acts as the service cavity and it is also insulated with rockwool before the plasterboard is installed. This gives a calculated u value of 0.095. You can of course adjust the thickness of the middle layer of rockwool to suit the U value you require.

If you want to go with a block construction I would suggest you go for a 250-300mm wide cavity with pumped bead. I have a very strong aversion to EWI, it looks good on paper but in is prone to a lot of problems as you are relying on the render system to maintain it's integrity over a very long lifespan.

The modern render systems are I understand very effective and are more than capable of withstanding the usual knocks and wear and tear and can be patched if anything more major occurs. They have been used effectively on the continent for some time now and are are a widely accepted form of upgrade to old building stock as well as new build. The added advantage is that they can be built using common trades and according to your own design. Timber frame can of course be similarly specced but I have always found it to be much more expensive. I suppose it depends upon your budget. I haven't used the render system yet but I will be as soon as I can afford to. I prefer the internal solidity of block to timber frame. Personal preference I know but valid all the same.

Why would you go for pumped bead Chris? Even the ones with the polymer additive can be problematic I understand due to settlement and leakage over time if any work is subsequently undertaken. Additionally using a cavity system adds cost and creates a larger over all cross section.

Hi Sid

Scanhomes don't deliver to the UK in any case, the point was just to show you a typical cross section of a twin wall build up. I would imagine that it should be within the capability of any timberframe manufacturer to do you a twin wall if they so choose, it's not particularly complicated.

@Matt I don't wish to derail Sid's thread with discussion of EWI as he says he wants to go TF in any case, but if you search the forum you will find a report I posted from the Swedish test institute SP into failures in EWI render systems. What you decide to use is your choice, I'll be going TF in any case.

Posted By: mattwpriceTimber frame can of course be similarly specced but I have always found it to be much more expensive.

Do you mean timber frame is more expensive than block?