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What is a passivhaus?

If you build a house that is super-insulated and super-draughtproof, it will need very little energy to heat – in effect, the occupants could heat the house just by being alive.

This is principle behind the Passivhaus standard, which – as you might guess from the spelling – originated in Germany.  Strictly speaking, only a building which has been certified by the Passivhaus Institute in Darmstadt can be called a Passivhaus.

The key criterion is that the energy used to heat the building amounts to no more than 15kWh per square metre (m2) of floor area per year. There is wide variation of course, but most UK houses use about ten times as much as this.  For example, according to calculations by the Scottish Passive House Centre a typical Scottish new-build 4-bedroom semi with 116m2 of floor area would use 150kWh/m2/yr.

Holding on to the heat

The idea of superinsulation is obvious, but what about super-draughtproofing?  A great deal of heat is lost from a typical house when warm air leaks through cracks and gaps and is replaced by colder outside air.  The word ‘airtight’ is often used to mean the elimination of this interchange, but it is a somewhat misleading term because it doesn’t mean you can’t open the windows!

A high standard of airtightness means that there is almost no unplanned ventilation – draughts (leakage you can actually feel) or infiltration (micro-leakage which you won’t be aware of without testing for it).  To achieve Passivhaus standards of airtightness, a great deal of care must be taken at the construction stage – for example, extremely adhesive tape is applied carefully to every join.

What about fresh air?

But if you eliminate the unplanned ventilation – the leaks and drafts – what about necessary ventilation? There must be a supply of fresh air to maintain indoor air quality. An important aspect of Passivhaus design is the use of HVR – heat recovery ventilation. Fresh (cool) air is pumped in and circulated at the same time as the stale (warm) air leaves.  The two air streams pass over a heat-exchanger, so the incoming air is warmed by the outgoing air. Pumping air does use a small amount of energy, hence the alternative name of ‘mechanical ventilation and heat recovery’ (MHVR).

How much is this ‘small amount of energy’? Well, 25 watts would be a reasonable power demand. Let’s spell this out a bit: a ventilation system operating all year at 25W (though in practice it might be a bit less than a full year – remember, you can open the windows) amounts to an annual energy consumption of 220kWh. If the Scottish 116m2 semi was operating to the Passivhaus standard of 15W/m2/yr, annual heat demand would amount to 1,740kWh. Adding the MHVR figure of 220kWh brings this figure up to 1,960kWh. Compare that with the typical energy use of the semi, which at 150kWh/m2/yr adds up to 17,400kWh – ten times as much over the year.

There are very few passive houses in the UK as yet, and very little awareness of the skills necessary for their construction, but there is a growing interest in this area of eco-building (though not everyone is convinced that it is the best way forward). The following are good starting points for further reading:

Read more on sustainable building techniques: http://info.cat.org.uk/ecobuild

This article was written by Martin Parkinson whilst volunteering for the CAT Information Service.

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