Vienna - Temperatures may be falling and energy prices are still high, but an increasing number of Austrians living in low-energy houses have little to worry about this winter.

Martin Mueller, 40, his wife Barbara, 35, and his children have been living in a so-called "passive house in Perchtoldsdorf, just outside Vienna, since 2003.

Unlike conventional buildings, such houses need hardly any heating and cooling, but draw energy from sunlight, the earth and the people dwelling within.

"Having three children, I wanted to build a house that I can take responsibility for in terms of the energy balance sheet," said Mueller as he sat in his living room, surrounded by his two sons Daniel, 12, Christoph, 10, and his daughter Anna, 7.

While Mueller explained his reasons for this increasingly popular lifestyle in Austria, the house's air duct system drew fresh air from the outside and heated up a few degrees as it ran down a pipe to the ground and then on into the cellar.

There, it flows through a heat exchanger or "reverse refrigerator", as Mueller has dubbed it, and permeates every single room in the house through holes near the ceiling, ensuring a near-constant temperature of 21 degrees Celsius.

"It's very modern," said Anna said proudly.

The heat stays inside thanks to triple-pane glass windows and wooden walls that have been insulated with cellulose.

Extra warmth is provided by people giving off body heat, by household activities such as cooking and by sunlight coming through big windows on the south side of the building.

Although many passive houses have no additional heating, Mueller decided to install a small heater which burns wood pellets and turns itself on for short periods during the cold winter nights.

The family's heating and electricity bills have dropped to a mere 400 euros (519 dollars) annually over the 960 euros (1,209 dollars) they used to pay when they lived in a 80-square-metre flat.

"Now we have 128 square metres on two levels, plus a cellar, and we pay 400 euros a year," said Mueller, who works as a procurement manager at a rubber goods manufacturer.

The first passive houses were built in the early 1990s in Germany, where more than 10,000 such residential buildings have since been constructed, according to the Passivhaus Institut in Darmstadt, Germany.

But with 4,000 such buildings, Austria has the highest per-capita rate of low-energy houses in the world, "and the trend is really catching on," said Guenter Lang, who heads an Austrian consortium of companies offering components and services for eco-friendly buildings.

Regional governments subsidise low-energy houses thus making the switch increasingly popular.

Austria's westernmost province, Vorarlberg, now only subsidises the construction of apartment buildings if they are low-energy. In Vienna, 24 per cent of new housing constructed in 2009 will have no conventional heating, Lang said.

But although the financial incentives are important, passive house owners such as the Mueller family say they also enjoy other benefits.

Even if windows are closed at night, the air is fresh in the morning, and the constant ventilation prevents mould in the bathroom, Mueller explained.

As his eye caught the electrical towel radiator, he bemoaned it as "a bad investment."

"We never need it for heating," he said. (dpa)