PassivHaus:

A help or hindrance?

Posted on 27/04/12

I am often asked about my opinion on the PassivHaus design principles for the UK.Where do I stand, have we got it right? It is natural to think that a new, improved building standard is helpful, but many 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?Do we in fact make things worse?

The objective of the PassivHaus standard is toreduce the amount of heating energy usedin winter and to reduce the amount of energy (if used at all) required for cooling insummer. Both of these are laudable objectives.The PassivHaus solution firstly involves building a fabric with great insulation properties - this reduces the amount of internal heat which escapes from the building in winter and the amount of heat from outside which 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 a mechanical ventilation system is that you can pre-heat incoming air using heat extracted from outflowing air. However, this heat recovery process doesn't come for free - fan power is needed to drive the air through the mechanical ventilation system and heat exchanger. The trade-off works if you save more heating energy than you would have used by driving the fans. So, the key question we 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 originallydevised. The internal heat gains are low and the winter conditions are such that there are many days where the temperature struggles to get above freezing. Even if just minimum ventilation rates areused, heating may be needed as soon as the external temperature falls below 10°C with a traditional natural ventilation scheme (such asopening 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 inthe PassivHaus concept.

How about the case of a building with higherheat 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 mostnon-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 wholehouse 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.

 

Dr Shaun Fitzgerald

Managing Director of Breathing Buildings



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