The Importance of Form
by Paula Hugens, eZED Ltd | 8 June 2014
Whenever I discuss energy efficiency it comes as a complete surprise to most people that form is the most important consideration for an efficient design. People want to talk about their insulation or their amazing windows and ignore this simple rule. Later on, they struggle to comprehend why their project is so far over budget. You see, interestingly, good form will usually lead to a more cost effective building. Yes that’s right, less capital cost and increased thermal performance.
Good form is not necessarily about size, it’s related to the volume of the interior space and the overall surface area that envelopes that volume, that is, floor, walls and roof. We sometimes use the term ‘compactness’ but some people mistake this as meaning tiny or cramped. For those of you who did well in geometry it will make immediate sense when I say the most perfect form is a sphere, impractical yes, but it does have the greatest possible volume for the least possible surface area. This is why we do see the odd geodesic dome still popping up. Looking at the Wikipedia article on dome houses I was disappointed that this fact was not highlighted, nevertheless, I did like the quote “smart but not wise”. I concede in this example the perfect form doesn’t lead to a reduction in build cost, so lets think in more rectangular terms. Let’s use a Lego model to explain:
This first form is the outline of a very energy efficient two level detached home, it’s built to be at around 1:100 scale and has a generous treated floor area of 308m². The important figures to note are the overall surface area at 743m² and the volume at 986m³. So here’s a quick rule of thumb, try to have the surface area to volume ratio less than 1, in this example we have 0.75. So long as the walls are not paper thin and you keep your window coverage within reason, this home has the potential to perform very well.
Now consider this form is just a representation of the thermal envelope. Most homes have elements that are outside of the thermal envelope, i.e. the attached garage. It really doesn’t matter where you attach the garage and it can lead to ways of making the form far more interesting and appealing. Other features such as pergolas, eaves, wing walls, screens and decks, can dramatically change this basic form into something striking.
Check out the image of the Passive House that uses this technique. The Architect started with the same original form and pitched the roof into a wedge shape. This did reduce some of the internal treated floor area and volume but the surface area also reduced so it was not really compromised.
Detached homes do tend to end up being two level if they are to be affordable and energy efficient as it optimises the surface area to volume ratio. It gets even easier when considering multi unit buildings. Each apartment has a common wall and possibility a common floor and/or ceiling where heat loss is minimal.
We evaluate apartments individually for thermal performance so the common elements are discounted from the surface area. You dramatically reduce the surface area and the resulting heat losses through those elements, thus the surface area to volume ratio will also plummet. So the most energy efficient buildings end up being multi-unit residential buildings and they do it with relative ease. Metaphorically you could say they are sharing their body heat by huddling close together and if you are on the inside you will be the warmest.
This image charts how form can be incrementally improved with the multi-level highrise tending to perform the best. The single level standalone buildings in the foreground on the red mat do not make good candidates for highly energy efficient buildings.
These ideas could be incorporated to make for a more energy efficient and sustainable urban strategy in many regions around the world.
Copyright eZED Limited 2014