When addressing the energy efficiency of a building, one of the most important players in the game is the compactness of its thermal envelope.
Is there a way to measure the compactness of a building? Yes: it’s the compactness ratio, sometimes referred to as the Form Factor.
This value is obtained by the sum of all surfaces of its envelope, divided by its gross heated volume.
Therefore, the Form Factor does not depend on whether or not a building is insulated, or on how much. Orientation does not play a role either: it only depends on the geometry of the thermal envelope.
Form Factor example
Let’s take one of the two passive houses of Cavriago as an example:
Sum of envelope surfaces: 554,56 m2 (5969,09 ft2);
Gross heated volume: 880,13 m3 (31057,63 ft3);
Form Factor: 0,63 1/m (0,19 1/ft).
When we were designing the two passive houses of Cavriago, we worked to keep the compactness ratio as low as possible, in order to maximize energy efficiency and minimize costs. Regardless of its insulation, orientation, or building systems, the lower the compactness ratio is, the more efficient the building is going to be, for both heating and cooling.
Energy efficiency starts with the very first preliminary architectural design: architects need to be aware of the consequences of their decisions in terms of energy efficiency.
The first energy designer is the project architect
If the architectural design does not keep the compactness ratio under control, making the building efficient may be very difficult and expensive. Because of poor architectural design, regardless of the insulation, some buildings cannot be upgraded to meet the passive house standard.
To draw a comparison to cars, we can say that compactness is to the thermal envelope, what an aerodynamic shape is to a fuel-efficient car.
To ease the work of architects during the preliminary design phase, the Passivhaus Institut released DesignPH, a plug-in for SketchUp that allows you to develop the architectural and energy design at the same time.
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