Form Factor
It’s true that many Passive House projects can look a little plain externally. This isn’t due to any lack of creativity but because of an underlying principle around heat loss. Simple building forms tend to be more energy efficient.
We’ve all seen a ‘Grand Designs Ecohouse’ [sic] with cantilevered first floors and energetic roofscapes darting into the landscape beyond. But whoah there Crazy Horse - each of those faces has a heat loss associated with it, and all that structural gymnastics creates thermal bridges. Complex 3D folded forms are harder to get airtight and bespoke details need specialist thermal bridge analysis. Wrapping heated spaces around unheated basements and garages creates further surface area for heat to escape. If energy saving is your goal, there’s a lot to be said for simplicity.
A sustainable building is (Premier League Footballers take note) only as big as it needs to be, minimises its surface area and simplifies its form. Heat Loss Form Factor is the metric we use to define this, describing the relationship between heated volume and surface area. Passivhaus targets <3 and LETI recommends 0.8-2.5 covering the different domestic typologies: detached, semi-d, mid-terrace and flats. Typically, the worst Form Factor is found in a bungalow, and the best in a mid-terrace house or mid-floor apartment.
Form is also linked to orientation, with the ideal plan being rectangular with its long edges facing due north and south. The orientation and pitch of a roof has a clear impact on the ability to harness solar energy using PV. Software like DesignPH can help designers find the sweet spot for form factor and orientation.
Retrofit Academy slide for the optimum PV orientation and angle
Decisions at concept stage will heavily influence the final heat balance of the building. Fixing on an idea too soon may cost you, and the planet, for the next 50 years. The Passive House Designer and architect (if different) have the tools to optimise the design whilst also meeting their client’s expectations for architectural delight.
