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The Passive House – an energy-saving eco home

In the 1970s, I made several business trips to the north eastern United States in wintertime. Energy was cheap, and US houses were amazingly wasteful of heating energy. Clapboard walls with very little insulation were common, and heating systems often pumped large volumes of outside air starting at around -20°C, heating it to +25°C by passing it through a roaring gas flame. This made motel rooms so dry that you drew a painful spark from the door handle, as well as making your nose dry and sore.

During the latter half of the 20th century, people began to understand the main principles behind creating buildings that used energy less wastefully. New regulations and the huge increase in the price of energy have led to early 21st century homes that are much better insulated, with heating systems that are more efficient – but we still have a long way to go.

There are houses being built Germany, Austria and Scandinavia (with a few examples in the UK and Ireland) which are far in advance of the current building regulations. Called Passive Houses, their goal is to use as little supplementary heating as possible, relying on solar heating, the heat given off by cookers, hair dryers, computers and other appliances, and even that generated by its occupants. They achieve this by being extremely well insulated, by being built to maximise absorption of heat from the sun, and by being very airtight so that cold air doesn’t filter in and warm air doesn’t leak out.

To make the most of the heat provided by the sun, a Passive House in northern temperate latitudes must be built to face south, with large windows on the south side and only a few small ones on the north side – because north-facing windows only serve to let the heat out. Since it could otherwise get too hot in summer, the south side of a Passive House usually has a way of shading the windows. An attractive way of doing this is to grow vines over a trellis structure so that the leaves provide shade that disappears in winter.

Making a house airtight is all very well, but stale air must be exchanged continuously for fresh in order to make the house healthy and pleasant for human occupants. UK building regulations require that the air in a room occupied by people must be completely replaced every two hours. Traditional ventilation systems (extractor fans, open windows and vents) mean that the heating system has to heat up a houseful of outside air 12 times a day. Passive Houses use Heat Recovery Ventilation, in which the outgoing air is used to preheat the incoming air. It can’t recover all the heat, but it makes a very big difference.

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