If your first idea of an ‘eco home’ is one loaded with solar panels, think again: low-energy building is much more back-to-basics. As Meredith Bowles of Mole Architects, explains here, it’s about designing and constructing places that, through a combination of measures such as harnessing the sun’s energy and maximising air-tightness, require very little energy to heat and cool them in the first place. The zenith of this ‘fabric first’ approach is Passivhaus, a stringent set of standards devised in Germany in the late-1980s. In the UK, architects such as Bowles are building low-energy homes that are beautiful as well as sustainable, as Mole’s work testifies. Here’s a practical guide to fabric-first.
Can you give a broad overview of what’s meant by ‘fabric-first’?
Fabric first basically prioritises passive design principles over technology. So reducing energy consumption by increasing insulation, reducing heat loss and air infiltration and using heat from the sun is considered before deciding to create energy using solar panels or wind energy.
What is passive solar design?
Passive solar design is a method of designing that makes use of the heat from the sun, so cuts down the amount of heat needed from other heat sources. To make a building work well using passive solar design principles, the orientation of the building is critical. We need to gain heat from the sun through south-facing windows, and conversely avoid losing too much heat by restricting windows on the north side of the building. It’s also necessary to store heat once it’s inside using heavyweight materials as a ‘heat sink’, and insulating the walls and roof well to avoid heat loss.
In practice, how often is it possible to construct a building in this way, when there might be other buildings casting shadows, or a perfect view that faces the ‘wrong’ direction?
All our designs consider basic passive solar design. It’s rarely impossible to add windows on the sunny side, which not only makes sense but of course makes us feel better too. We’ve had to deal with projects where the ‘view’ is to the north; skylights on the south side can still get sunlight into a north-facing room. A smaller window looking at a view can still frame it beautifully, which is better than a load of glass that opens onto a shaded area that no-one will sit out on.
What about insulation?
The more the better! It’s actually a law of diminishing returns; once you’ve put in about 300mm (or 200mm of high-grade insulation) the amount of improvement becomes far less per extra millimetre of insulation. At this point it’s better to look at other aspects such as window performance.
What about windows and doors? Is triple glazing always necessary?
It’s the overall performance of the frame and window that matters. A high-performance window with a thermally broken frame will outperform a poor frame with triple glazing. You need to get the manufacturer to give the U-value of the whole window with the frame too.
Great, so you’ve built a completely air-tight home. So how does fresh air circulate?
Air-tightness primarily refers to the amount of air leakage through the building fabric – in other words, when the building was put together, are there gaps in the construction that let warm air escape? Mechanical ventilation with a fresh air supply means that you can transfer heat from exhaust air that would otherwise be lost. The alternative is leaving basic ventilation holes in the windows/walls to let in fresh air, which somewhat defeats the object of all the efforts to make the building warm. There’s a system called passive stack ventilation that allows the stale warm air to escape through a chimney, while fresh air comes in through humidistat controlled vents.
Does fabric first encourage a certain system of construction – timber-frame versus masonry, for example?
No, fabric first can be applied to either. Whilst there are arguments for both masonry and timber construction systems, it’s been shown that either system can perform to produce well-performing buildings. At Mole we promote a mixture – timber-frame buildings with a masonry core inside.
Is a low-energy house nicer to live in?
A house without draughts is definitely more comfortable! A low-energy house doesn’t have hot and cold areas closer or further away from a heat source; everywhere is a good temperature. It does preclude gathering around a hot fireplace in a cold house, which is something people miss. There’s always a loss with a gain.
As an architect do you find it difficult to balance all of the above with aesthetic appeal?
No! Good performance doesn’t come at the price of poor aesthetics. The very best architects achieve both, just the same as in any other design field.
Are there any certified Passivhauses you admire for their ability to balance all the various rules with aesthetic beauty?
My favourite is the winner of the first international Passivhaus awards, the Liebefeld House in Switzerland. It shows just how much can be done using the strict parameters of Passivhaus design.
What other difficulties are there to overcome in building to Passivhaus standards? Is the additional cost a discouraging factor?
Fortunately the cost of Passivhaus products is continuously falling so soon it will be much closer to normal costs. The main difficulty is in site management and building standards; all the site operatives have to be very aware of the ways the airtightness needs to be achieved, and work together to make it happen. There’s no room for slipshod workmanship.
Do you expect Building Regulations to tighten further in the coming years, meaning that an approach more in line with Passivhaus standards will be more common?
I don’t doubt the regulations will continue to change. The incorporation of much more sophisticated methods of measurement in recent years is very different to how the regulations worked 10 years ago – there’s already been a change in perception. The simple change to incorporate air tests as mandatory and at a lower rate would be simple to implement and be a step closer. It’s the performance of our existing buildings that is the real challenge…