Form Factor and the Effect it has on Costs

Form factor is the ratio of the thermal envelope area to the treated floor area (TFA). The thermal envelope is the area of walls/roof and floor measured to the outside face of each thermal element e.g. if you are insulating under the ground floor slab it will be measured to the under-side of the insulation. TFA is the usable floor area as calculated in the Passivhaus PHPP software.

Calculate the form factor early on, as this will give you a good guide on whether your project costs are at the cheaper or more expensive ends of the spectrum. Aim to design a detached house with a form factor of below 3.0, if costs are critical. Our guide below gives an indication of how form factor effects costs. As a rule of thumb the more spread out a building is, (common with single storey houses) the worse the form factor.

Form factor also impacts on the thermal performance in PHPP as importance is given to the amount of usable floor space (TFA) contained within the outer envelope. Having full height spaces on the ground floor for instance reduces the amount of TFA and will make meeting the passivhaus standard more difficult – this may be of great importance to a client.

Please checkout the Design Guide for more information about PH15.

PH15 costs for guidance only. Costs were correct as of