The form factor of a building characterises the compactness of its form. It is calculated as the ratio between the area of the thermal envelope and the treated floor area (TFA). The TFA measures the habitable floor area of the building and excludes internal partitions, flights of stairs, voids, unheated rooms and restricted height spaces.
The lower the form factor, the less energy will be required to heat the building, since the heat loss area is minimised in relation to the heated volume, hence its use in characterising the thermal efficiency of the building form. The form factor is also a good proxy indicator for heat loss due to thermal bridging: the lower the form factor, the lower the number and length of building junctions, and the lesser the likelihood that complex and expensive detailing will be required to mitigate it.
The image on the right illustrates the form factors of typical building geometries, ranging from high- and mid-rise apartment blocks with a low form factor to single detached bungalows with a high form factor.
The lower the form factor, the easier it is to achieve Passivhaus levels of space heating demand, though there is no specific form factor requirement for certification. This allows trade-offs to be made with other aspects of the fabric performance specification, for example insulation thickness. Such trade-offs are best evaluated on a project specific basis, but a simple rule of thumb states that halving the form factor would allow insulation thickness to be halved without a negative impact on the space heating demand.
Reducing the form factor can therefore have positive knock-on impacts on internal area, thermal bridging, simplicity of detailing and construction, greater freedom in material specification, efficiency of material use, lower embodied carbon and construction cost.
The form factor is a key performance and build-cost indicator that should be assessed and rationalised during the initial design stages. It is recommended that a Passivhaus consultant is involved as early as possible to review the design proposals.
Recommendations: Low Form Factor Design
- Aim for a form factor of less than 2.
- The thermal envelope should enclose all internal heated spaces within a continuous line of insulation, avoiding complex junctions and structural penetrations.
- Avoid building bungalows and single detached dwellings – build multi-storey blocks and terraces instead.
- Minimise non-useable internal floor area such as communal circulation, voids and restricted height spaces.
- Create separate ancillary buildings or lean-to structures for bike stores, bin stores and other unheated spaces, otherwise co-locate these spaces within the building and keep them outside the thermal envelope.
- Avoid stepped or cantilevered façades, shifted floor plates and inset balconies, stepped party walls, recessed entrances, bay windows and dormer windows – keep the thermal envelope simple and floor plates stacked.
- An efficient form factor need not imply a boring building architectural interest and articulation can be created outside of the thermal envelope, for example through materials and finishes, shading elements or external self-supporting balconies and porches.
- The cost savings achieved through embracing a low form factor design approach may enable greater freedom in the specification of external finishes.
