You’ll already know that the more surface area you have the greater your heat losses, that’s just basic physics. It’s why polar bears are big and round – good surface to volume ratio. Basic yes, but shape and form will have a profound effect on project. Get these right and you’ll have stunning design, while achieving your energy aspirations. Getting it wrong will be costly both to budget, energy consumption and long term comfort.
The step to achieving optimal shape and form is “form factor”, maximising usable floor area to surface heat loss area. Form factor is illustrated in the diagrams below, where we have taken a mid-floor flat and made it a bungalow with no changes to the inside or the amount of insulation in the walls, just increased the heat loss area. This simple change has the staggering effect of increasing fuel usage for heating by 136% even when the roof and floor are insulated well.
Interestingly, improving form factor by reducing external walls will not only save on your heating bills, but construction costs too, as you have less walls to build and they are the most difficult and expensive bits to build. You’ll also save on embodied energy as you’ll be using less materials. If the shape is simple there are less corners, reducing cutting and waste, and less junctions for air and water leakage. So, this is a real win, win with no extra costs!
So, external walls are expensive and keeping these to a minimum, results in savings of both the budget and energy consumption. But this isn’t just important to developers when deciding whether to build terraced or detached homes. It’s an issue that self-builders need to address too. Bay windows can significantly increase external surface areas with no or minimal impact on usable floor space, however you can achieve similar effect on breaking up an elevation with change of materials, using canopies and other tricks.
Adding usable floor space will improve your form factor, it’s a trick that can be used to improve calculations but this should be used with care. The bigger the home the costlier to build, heat and keep clean. Only build as big as you need. When a house becomes larger it usually becomes deeper with natural light and ventilation needing to penetrate further. These can be compensated for with larger windows and a bigger MVHR, but would need to be balanced against the risk of overheating and increased energy costs.
So, shape is everything, if you want to find out more give us a call, drop us an e-mail and watch out for our next blog.
If you would like more details the NHBC foundation have published “The challenge of shape and form.” A useful document which goes into more details about the effects of shape and form on the energy performance of a building.
You won’t be surprised to hear we collaborated on the document. Download your free copy here.
