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What shape is particularly advantageous for a passive house?

The more compact a building is, the easier it is to meet an energy-efficiency standard. The surface-area-to-volume ratio (S/V ratio for short) has a considerable influence on a building's energy requirement. The S/V ratio indicates how large the surface area S (such as wall, ceiling, roof and window surface areas) is in relation to the building volume V, and thus to the living space provided. The larger this ratio, i.e. the higher the S/V value, the greater the thermal energy requirement per m² living space/usable space is, for a given set of energy-efficiency measures. The more compact a building is made, the more cost-efficiently it can be constructed, partly because the requirements applying to insulation thickness are then less strict.

Larger buildings have a lower and therefore more favourable S/V ratio than smaller buildings.

Buildings with a simple geometrical shape, such as cuboids or cubes, have less surface area in relation to their volume and thus a better S/V ratio than buildings with numerous protrusions, oriels or dormers.

The following sketches show different (building) shapes and their degree of compactness in terms of surface-area-to-volume ratio (S/V ratio).

On left: spherical form (< 0.3); in centre: cube (approx. 0.5), on right: large surface area (> 0.8) (source: Stefan Prokupek, GrAT)

Detached houses typically have S/V values between 0.7 and 1.0, large buildings achieve lower values, down to 0.2. Detached passive houses should have values below 0.8, if possible. A higher S/V ratio must be made good by rather thicker insulation, in order to comply with the required thermal energy rating of 15 kWh/m²a.