What shape of building works best

At this year's AECB conference two weeks ago, Peter Warm gave an informative talk on the Passivhaus certification process. One of his PowerPoint slides showed the impact of a building's form on the wall U-values needed to get to the 15kWh/m2.a target.

By dividing the total "heat loss" area by the "treated floor area", you get a ratio which describes how compact the building form is. The "heat loss" in most cases is, essentially, the sum of the areas of the building footprint (ground floor), roof and external walls; all measurements taken externally. The "treated floor area" or TFA is the usable internal floor area; calculated according to the convention used in Germany (as you would expect, given that the Passivhaus standard originates there). The higher the form factor ratio, the lower the U-values need to be to reach the target.

Non-compact detached houses, particularly bungalows, score the worst, sometimes with a ratio as high as 5. They need walls with a very low U-value of around 0.05 to get to the 15kWh target. Blocks of flats typically have a ratio of 2 and they only need wall U-values of around 0.15 to reach the Passivhaus target.

Semi-detached and terraces are somewhere in-between.

Form factor ratio	Typical wall U-value
4 < 5	0.05
3 < 4	0.10
2 < 3	0.15

This means that, if you want to build a Passivhaus with reasonably sensibly sized walls (i.e. less than 500mm thick) or using natural materials or at a sensible cost, you really are going to have to pay attention to the shape of your thermal envelope; the building itself can be any shape you want but the "warm" space (contained by the thermal envelope) needs to be compact.

In our project, our form factor ratio is 2.53 - so I'm feeling quite pleased with myself, even though this was partly by luck.

In Passivhaus refurbs I think that building form is as least as important as orientation and solar gain, if not more so.