Passive houses

‘Passive house’ is a term which needs a bit of explaining. Roughly speaking, it is a house constructed facing the sun, using this as its primary heat source, particularly in winter, spring and autumn. But where did the idea originate?

In the early 90s, Dr Wolfgang Feist at the University of Darmstadt was focussing on potential energy savings in the construction of housing. For the first time ever, he began comparing the investment costs of an energy-saving measure with the annual savings one can expect to make from it. It would of course be ludicrous to spend more money (or energy) on installing insulation than it is able to save during its lifespan.

The results of these practical calculations and research was more than just surprising. Up until now, we have been wasting energy – because if you try to minimise total costs when building a house (investments plus running costs of heating over a 30–40 year period), you automatically arrive at the concept of a passive house. The fact that such a house could manage on just 10% of the heat consumption of a typical new build at the time simply knocked the experts of their feet. During the years of war and the oil boom, nobody had properly recalculated. It wasn’t until the 80s that energy costs began rising rapidly for the first time.

Essentially, the passive house idea consists of five improvements, two of which cost nothing: 

  • Constructing buildings with a large, south- facing façade, facilitating the use of passive solar energy.
  • The heat given off by the inhabitants supplies half of the energy required to heat the house, thanks to insulation.

It goes without saying that this is not without additional investment costs: 

  • Insulation in external walls needs to be increased by at least 30 cm! 
  • Triple-glazed, sealed windows are necessary. 
  • A mechanical ventilation system with heat recovery is necessary – and pays off!

All in all, the additional costs amount to around 5–10%. This is not just my personal estimate, but a figure which was determined 10 years ago in Austria from around 300–400 passive houses. The figure also matches our experience in Freiburg.

In previous years (since 2008 to be more exact), construction and property costs have got out of hand significantly. Even for normal, non-passive houses, people charge and pay €4,000 per square metre. This is absurd, or can be explained away with luxury. Since the first financial crisis, many wealthy people have been withdrawing their money from stock exchanges and insurance policies because they believe investments will protect their money from capital loss. This is certainly true to an extent. However, whether tenants, even now, will actually be able to afford the resulting exorbitant rent prices before the next crash, is questionable at best.

The meaning of a passive house becomes most obvious when considering heating costs. In a 90 m2 apartment, three people generally pay under €100 – the cost for hot water must be subtracted, however. In a passive house, more money and energy is generally spent on hot water than on heating.

But can you open the windows?

This is one of the most common questions

people who live in passive houses get asked.

The short answer is ‘yes, whenever you want’, but consider the following. How many hours a day do you leave the windows open in your current house when it’s -15°C outside? At such a low temperature, the answer is likely no more than a few seconds. This is exactly what is required in a passive house. The temperature will tell you when you want to close the window again. And anyway, unlike in a typical house, you have fresh (but not cold) air 24 hours a day thanks to the ventilation system.

What’s more, the ventilation pipes are no dustier than the ambient air, and problems regarding bacteria cannot occur, since none of the waste air (in which condensation could occur) is fed back into the house. 

Needless to say, a good engineer and a good installation company are required to ensure the ventilation system does not produce any unwanted noise. It’s all a question of engineering and experience.

And in the transition period?

When it’s between 12°C and 16°C, you can even leave the windows open slightly if you like – the heating system in a passive house usually doesn’t switch on until below an outdoor temperature of 12°C. Between 20°C and 12°C, internal heat gains (showering, cooking, waste heat from electrical devices) are sufficient to keep the house warm without a loss of comfort. We’re assuming 21°C.

When it’s very cold in winter, we are helped by a further effect. Moisture can no longer stay in the atmosphere, falling as rain or snow. This means clear skies, and although it can get very cold at night as a result, you can count on the sun shining the day after. Indoor temperatures of 24°C are then the rule rather than the exception. A portion of this energy remains into the evening and night.

At the moment (2013), even passive houses require a small amount of heat from external sources. But provided heating costs continue to rise (and everything points towards this being the case), the most economical house to heat will be a zero-energy house – by investing a little extra in insulation, an active heating system can be dispensed with entirely. This is the original definition of a passive house. As ‘emergency heating’, you can always light one of the rings on the gas hob, for example when returning to a cold house after a winter holiday.

Translation: Richard Marsh

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