LASTWORD
The last WORD
A help or hindrance?
Dr Shaun Fitzgerald questions the use of PassivHaus design in the UK
I amoften asked aboutmy opinion on the PassivHaus design principles for the UK. Where do I stand, havewe got it right? It is natural to think that a new, improved building standard is helpful, butmany professionals have concerns about applying it blindly. Does the PassivHaus standard really move us forward in our desire for improved energy efficiency or does it actually hurt us? Dowe in factmake thingsworse?
energy (if used at all) required for cooling in summer. Both of these are laudable objectives. The PassivHaus solution firstly involves building a
T
Dr Shaun Fitzgerald is managing director and co-founder of Breathing Buildings. Breathing Buildings aim is to optimise energy usage in heating and ventilating. The company’s unique e- stack ventilation system provides low energy ventilation, using the principles of natural mixing ventilation in winter and natural upward displacement ventilation in the summer.
fabricwith great insulation properties – this reduces the amount of internal heatwhich escapes fromthe building inwinter and the amount of heat from outsidewhich gets into the building in summer. Whilst this is really sensible, the second piece of the PassivHaus standard involves the inclusion of a mechanical ventilation system. I don’t understand what is passive about this. It is the antithesis of passive or natural ventilation. The reason for the inclusion of amechanical ventilation systemis that you can pre-heat incoming air using heat extracted fromoutflowing air. However, this heat recovery process doesn’t come for free – fan power is needed to drive the air through themechanical ventilation systemand heat exchanger. The trade-offworks if you savemore heating energy than youwould have used by driving the fans. So, the key questionwe need to ask is: ‘When do the savings in heating bills exceed the fan power costs?’ Let’s take a domestic building in Germany, for
which the PassivHaus concept was originally devised. The internal heat gains are low and the winter conditions are such that there are many
he objective of the PassivHaus standard is to reduce the amount of heating energy used in winter and to reduce the amount of
days where the temperature struggles to get above freezing. Even if just minimum ventilation rates are used, heating may be needed as soon as the external temperature falls below 10°C with a traditional natural ventilation scheme (such as opening windows). Given that there are conditions much colder than that for hundreds of hours a year, the heating bills will be very high and a mechanical ventilation system with heat recovery will be preferred. Hence, the inclusion of this system in the PassivHaus concept. How about the case of a building with higher
heat gains per person in a more temperate climate? If one considers, for example, the heat gains in environments such as classrooms in the UK, then a simple energy balance for a building which has high levels of insulation shows that there is enough heat to maintain the interior at 21°C with the requisite amount of minimal fresh air delivered, even if the external temperature falls down to close to 0°C. Heat recovery is therefore simply not needed for most of the winter – in fact, if a mechanical ventilation system is included, energy is being wasted in the form of fan power being used unnecessarily. Clearly the actual design of the natural ventilation system is important in order to ensure that the incoming fresh air is mixed sufficiently with the hot interior air to avoid cold draughts, but the building itself does not need any additional energy input until the exterior temperature is below 0°C. What does this mean for the UK?Well, in most
non-domestic buildings we should be using the principles of PassivHaus for the building fabric, but not for the ventilation system. In domestic buildings, there may well still be a case for a whole house mechanical ventilation system in winter as the heat gains per person are lower, but any designer will need to carefully assess the internal gains before leaping to that decision.
ArchitectNews.co.uk | Architects Choice | 47
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