June 17, 2021 Reviewed
Form Factor refers to a building’s surface-to-volume ratio.
This is important for building performance because it directly affects heat loss.
Heat Loss & Form Factor
In Passivhaus and high-performance design, the “Heat Loss Form Factor” is measured as the ratio of the thermal envelope surface area to the treated floor area (TFA). In other words, it’s the ratio of the surface area that can lose heat (the thermal envelope) to the floor area that gets heated (TFA).
It’s worth noting that the thermal envelope may differ from the building envelope, depending on how the thermal envelope is defined (for example, uninsulated exterior walls in an attached garage).
The Heat Loss Form Factor is a measure of a building’s compactness. A more compact building is easier to make energy-efficient, while a less compact building requires more insulation to achieve the same level of energy efficiency.
Think of it like a person trying to stay warm: they curl up into a ball to retain heat, rather than lying down with their limbs spread out.
Image Source: International Passive House Institute
Heat Loss Mitigation
A building with higher surface to volume ratio has more surface to lose heat through. To reduce or remove heat movement through the thermal envelope the insulation must be adequate. A higher surface to volume ratio would require much higher insulation levels.
Image Source: NHBC Foundation
Correlation Between Form and Costs
So, when considering the cost of building, the Form Factor also provides insight into potential increased expenses.
If the Form Factor is high, thicker and more insulation will be required, which can be more expensive. Thicker insulation may also necessitate different design and structural solutions, potentially leading to increased thermal bridging, which then requires even more insulation to counteract. The opposite is true for a low Form Factor.
Additionally, more complex building forms generally have higher construction costs—think about intricate rooflines and larger surface areas of materials.
If high performance is a priority in your build, this is a simple rule to consider. A simple form can not only increase energy efficiency but also help reduce build costs for several reasons. In some cases, it can make the difference between being able to afford a high-performing home and not.
It is this very reason our affordable high performance plan range consists of simpler forms.
Find out more about our design build or high performance plan range.
