Most architects are familiar with the maxim “Form follows function”, coined by architect Louis Sullivan in the late 19th century, but the impact of building form on energy efficiency is less widely understood.
Determining a building’s form is one of the most fundamental decisions that we make in the design process – a decision that is made long before we’re ready to create a detailed energy model of the project. Luckily there are some very simple calculations that can help us assess a building form’s potential, even when we’re still sketching on tracing paper and building massing models.
How does building form impact energy efficiency?
It’s simple: the more surface area the building has, the more heat loss will occur. By surface area, I mean the total area of the thermal envelope: the walls, floor and roof that separate heated space from unheated space. A compact thermal envelope will lose less heat (during the winter in a cold climate) than a sprawling one.
The example below shows three different floorplans that all have the same floor area. The shape of the floorplan changes its perimeter, which in turn changes the total surface area of the walls. Increased wall area means increased heat loss. If we are aiming for the same level of energy efficiency for all three options, we need to add insulation to the less compact ones to compensate for the additional heat loss resulting from their increased wall area. A compact building form delivers cost-effective energy efficiency.
Complex building forms also have more junction details, which means that there is a greater likelihood of thermal bridging or air leaks if those details are not carefully designed and constructed.
How do I assess building form?
Here are a couple of simple calculations that can help you determine the compactness of your building’s thermal envelope:
These target values are not absolute. If your building has a surface area to floor area ratio of 3.2, you aren’t doomed, but it will be more difficult for you to achieve good overall energy efficiency. These ratios are very useful when comparing different design options, to determine which one performs best from a thermal energy standpoint.
The take-away
A compact thermal envelope will make it much easier for your building to achieve excellent energy efficiency. It will reduce heat loss through the envelope, and limit the number of tricky junction details that you will need to resolve.
A building with a compact thermal envelope doesn't need to be boring. There are many ways to add architectural interest that don't impact the thermal envelope's compactness:
Arrange unheated spaces (garages, porches...) around the building to create an interesting spatial composition;
Explore cladding material, colour, pattern and texture variations;
Use passive shading devices such as brise-soleils to animate the facade (remember to minimize thermal bridges in their attachment detail).
Good design stems from creative solutions to constraints!