The arithmetic is not complicated. A 140 square metre plate with a reasonably compact perimeter has about 48 linear metres of external wall. Fifty millimetres across that length is 2.4 square metres, and once you account for the internal returns and the reveals it lands closer to four.
What is complicated is whether those four square metres are worth having. Distributed evenly around a perimeter they are invisible: every room is 60mm wider and nobody notices. Concentrated in one place, by thinning the wall only along the north face where the thermal demand is lowest, they are a usable room.
On four of the six plans we tested, the honest answer was that the extra area was not worth the thermal penalty. On the other two it was, and in both cases it was because the plan had a long single-aspect edge where the wall thickness was doing nothing but standing up.
The reason to run the numbers at all is that the decision is usually made by default, at the point where someone types a wall type into a model, and it is never revisited.
We now do this calculation in the first fortnight, alongside the section. It takes about two hours and it has changed the plan twice.
