Dare I suggest that your article on SAP has the world upside-down (23 July, page 22)?
The science world has only the vaguest quantitative measure of global warming and the contribution to it of carbon dioxide; the possible contribution of the UK to making a change in this is swamped by developing countries elsewhere; and the possible contribution of our new housing is largely swamped by the existing stock. Meanwhile a growing portfolio of social studies demonstrates that calculated savings are swamped by householder behaviour. This is called “multiplying a guess by an estimate”, and out of this comes a series of largely political arbitrary targets.
Perhaps it is good to have targets, but energy conservation is quite different from the structural Building Regulations. If a beam is weak, it falls down and someone may be killed. If a roof has the incorrect thermal resistance, then the householder has a slightly larger (or smaller!) bill, but the effect on the world is undetectable. What is certain is any elaboration of the calculation procedure will add a new corps of approved designers, calculators and quality controllers - in other words, increase the overheads on each house. Does 2% sound about right?
So perhaps our objective should be a system that is dead easy to use, and, by and large and on average, delivers the arbitrary political target. Unfortunately, the scientists have got hold of the system-building, and seem to have persuaded you that the objective is accuracy to seven decimal places.