Energy efficiency of a building is important for several reasons. It determines the building’s value on the market. People prefer energy efficient buildings because they have lower running costs. Last, but not least, building energy efficient buildings contributes to better and more sustainable environment – for example, buildings account for 39% of CO2 emissions in the United States!
Why shape affects building’s energy efficiency
Heat will gradually make its way to the outside of the building through building’s external faces (e.g. walls, roofs, terraces). The larger the area of external faces, the more place heat has to escape to the outside. In principle, to minimise heat transfer through the building’s external faces, the building shape should be as compact as possible (similar to a cube).
Energy efficient buildings don’t need to look boring. Inefficient features can often be avoided or replaced by alternatives which are still architecturally interesting. For example, recessed doors could be replaced by canopies over the entrance, and a similar ‘feel’ to bay windows could be achieved with window seats and alternative window reveal detailing (source).
Figure 1: Compact buildings do not need to be ugly (photo by Vecislavas Popa).
Comparing the energy efficiency of different design solutions should ideally be done in the early design stage as making changes later can be very time consuming and expensive. One option to improve building’s potential to save energy in early design stage is by calculating Form Factor of the buildings.
How to calculate building’s Form Factor
We can calculate Form Factor in different ways. Here we present two ways that are most commonly used nowadays. But first let us define the term building’s envelope area. Building’s envelope area is the sum of areas of external faces of the building (e.g. walls, roofs, terraces).
Note: If we needed to make detailed calculations, we would have to know the building’s thermal envelope area – the sum of the building’s insulated faces. The insulated faces form the main barrier for the heat. A building with an insulated ceiling has a lower Form Factor than the same building with insulated roof pitch (source). That is simply due to a pitched roof having a larger surface than the flat ceiling.
Heat Loss Form Factor
Heat Loss Form Factor (HLFF) is the ratio between the building’s envelope area (EA) and it’s net floor area (NFA). It is calculated as HLFF = EA / NFA.
This definition of Form Factor was used in a report The challenge of shape and form, published by British National House-Building Council in 2016.
HLFF is a number generally between 0.5 and 5, with a lower number indicating a more compact building. Passive houses aim to achieve HLFF 3 or less.
It is quite difficult to get a form factor below 2.5 on a stand-alone residential building (source). For example, a compact square 2-storey building with a 10 x 10 m² floor plan, still has a form factor of 2.97.
Larger buildings, however, can achieve an even better form factor. For example, a compact 4-storey block with 16 x 32 m² floor plan has a HLFF of 1.44. A 20-storey skyscraper with 20 x 20 m² floor plan has a HLFF of 0.76.
Surface to Volume Ratio
Another common way to calculate Building’s Form Factor is Surface to Volume Ratio (SVR). SVR is the ratio between the building’s envelope area (EA) and it’s volume (V), it is calculated as SVR = EA / V.
Note that the numeric value of Surface to Volume Ratio depends on chosen dimension units (eg. meters, feet).
This definition was used for example in German Energy Saving Ordinance, which sets the requirements for the total primary energy demand of the buildings by the Form Factor.
The Surface to Volume Ratio for a typical single family house is 0.8 – 1.0 1/m (0.24 – 0.3 1/ft). According to some sources, the recommended Surface to Volume Ratio for a passive house is ≤ 0.8 1/m; according to another source 0.5 1/m (0.15 1/ft) (source).
Like with the Heat Loss Form Factor, larger buildings can achieve better Surface to Volume Ratio than smaller buildings. The main reason for this is purely geometrical. Larger geometric bodies have a lower surface area to volume ratio than smaller geometric bodies.
Let us use the same examples as for Heat Loss Form Factor: a compact square 2-storey building with a 10 x 10 m² floor plan has a Surface to Volume Ratio of 0.771 1/m (0.23 1/ft). A compact 4-storey block with 16 x 32 m² floor plan has a SVR of 0.37 1/m (0.11 1/ft). A 20-storey (h = 60 m) skyscraper with 25 x 25 m² floor plan has a SVR of 0.2 1/m (0.06 1/ft).
The two ways to calculate Form Factor were compared in the paper Shape Factor as an Indicator of Heating Energy Demand by professor Kimmo Lylykangas from TalTech University. The results show that Heat Loss Form Factor is more accurate when comparing buildings with different heights. When the height of the space increases, the volume and the heating energy demand grow. Surface to Volume ratio falsely indicates that the building got more compact and therefore needs less energy than lower building. Heat Loss Form Factor on the other hand shows relatively good correlation with the simulated heating energy demand.
Figure 2: Example of different Heat Loss Form Factors and Surface to Volume Ratios based on different building shapes. You can download PDF image here.
Compare the Heat Loss Form Factor and Surface to Volume Ratio of the first two buildings. Although they share same floor plan, creating building with 3 storeys instead of 1 results in almost 50% better Form Factor and Surface to Volume Ratio. In the last two examples, you can see that both buildings that share the same Heated Floor Area differ by about 20% in Form Factor and Surface to Volume Factor.
How to incorporate Form Factor into your design process?
You can always calculate Form Factor by extracting heated floor area, envelope area and building volume from your favorite CAD application and do the required calculations manually. But this can be very time consuming, especially when it comes to validating different design alternatives.
It is thus better to use an appropriate software that does the work for us. One such example is Modelur software, which can calculate this automatically. This makes it a lot easier to compare different design alternatives in terms of Form Factor and Surface to Volume Factor. You can request free trial license here.
Benefits of considering Form Factor
Form Factor alone is not completely accurate energy consumption indicator, especially for buildings with complex plans. Other factors, such as the direction and speed of winds and amount of solar radiation, affect energy consumption, too.
But Form Factor can give a good estimate of building energy demand in the earliest stages of design process. Knowing Form Factors of different design solutions, allows us choose the one that is the most efficient. This way we can reduce heating (or cooling) demand of new buildings significantly – in some cases even up to 50% (source) – at practically no extra cost.
There are also other benefits of considering Form Factor. Buildings with better Form Factor often require less insulation and construction materials than buildings with poor Form Factor. This leads to win-win situation, where we can save money and reduce carbon footprint of the building by just choosing the design solution with better Form Factor.