compactness ratio of a building example

The compactness ratio or Form Factor of a building

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).

3D model of one of the two passive houses of Cavriago.
3D model of one of the two passive houses of Cavriago.

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.

Fuel efficiency is not one of the core goals pursued in the design of this kind of car. Even if you were to implement the most efficient engine, this vehicle is never going to get as efficient as an aerodynamic, compact, and lightweight car. The same concept applies to the architectural design of a building. It is just as senseless to develop the geometry of the building without keeping an eye on the compactness of its thermal envelope, and to try and make it “efficient” at a later stage.
The more complex the geometry of the thermal envelope, the more expensive it is to make it efficient. The shape of some buildings is so complex, that it becomes physically impossible for them to reach the Passive House standard, regardless of the amount of insulation installed and the money spent.

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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