What Makes a Passivhaus?

Yes, of course you need great u-values and low air permeability to make a Passivhaus. But AECB member Alan Clarke believes that Passivhaus has something far more fundamental to teach us.

Reading the Zero Carbon Hub reports on zero carbon strategies, and on the performance gap, I realise that the idea of what I now think of as Passivhaus is not what other people think.

As an “extreme fabric” approach to low carbon emissions, the ZCH characterises Passivhaus in its zero carbon strategies report as having U-values 0.1-0.15, air permeability <1, low thermal bridges, plus heat recovery ventilation. Not wrong, but is that all there is to it?

When I first heard about Passivhaus I too understood it as a simple recipe of U-values, air-tightness and heat recovery. Then I opened up PHPP, the Passivhaus design software, and realised there was more: a model that dealt in actual energy use, so favoured efficient built form; careful analysis of glazing, trading solar gain for heat loss; and a proper understanding of heat recovery ventilation.

But now, with a good few finished projects under my belt I realise that isn’t what’s special about Passivhaus either.  We aren’t short of energy rating systems and models, but where they fall down is that the tighter the standard, the further the shortfall when actual energy performance is measured. What is interesting is that Passivhaus doesn’t suffer from the same “performance gap” – the one the ZCH is now wondering how to close.

One crucial item of evidence comes from co-heating – this is the direct physical measurement of the heat loss of a whole building.  Co-heating tests carried out by Leeds Met University in 2010 on a range of low energy houses showed that actual fabric heat loss was on average 50% more than design. Further tests on Passivhaus dwellings put these consistently at the top of the pile, all with a smaller performance gap than all the other buildings.

Co-heating test results: the three Passivhaus buildings (far right) showed a smaller performance gap than all the other buildings  credit:  Centre for the Built Environment, Leeds Metropolitan University
Co-heating test results: the three Passivhaus buildings (far right) showed a smaller performance gap than all the other buildings
credit: Centre for the Built Environment, Leeds Metropolitan University

This doesn’t just apply to a first few carefully monitored examples. Large scale studies of European Passivhaus developments have shown that Passivhaus heating energy consumption doesn’t show a performance gap either. There is significant variation between houses, thanks to the temperature choices of individual occupants, but the average comes out remarkably close to the predicted figure.

So why don’t Passivhaus buildings show the same problem with the performance gap as other low energy buildings? Perhaps this quote from Wolfgang Feist explains it:

I was working as a physicist. I read that the construction industry had experimented with adding insulation to new buildings and that energy consumption had failed to reduce. This offended me – it was counter to the basic laws of physics. I knew that they must be doing something wrong. So I made it my mission to find out what, and to establish what was needed to do it right.

And what was needed to do it right? First, a building physics model that quantified all the energy flows through the fabric to an acceptable level of accuracy. This doesn’t need to be a fancy dynamic simulation, but the input data does need to quantify anything significant – be it exact window frame dimensions, ventilation duct lengths, or the heat loss of a soil vent pipe.

Then you just have to build the building so that it matches the model.

For this you need to design practical details for insulation without thermal bridges, and an airtightness layer. Then with training in new skills, and experienced supervision, ensure they are built correctly.

Contrary to popular belief this isn’t impossible for UK builders, or even all that difficult. Passivhaus buildings are being built now by ordinary brickies, plasterers and carpenters.  Of course they had to learn to build some new details, use new airtightness products, and change some sequencing to get the airtight layer in the right place at the right time, but the basic skills are the same. Although the dreaded airtest is a bit daunting there is clear pride in delivering the workmanship needed to achieve a good result.

So although Passivhaus takes over from early US pioneers who built the first super-insulated airtight houses, the key to actually delivering this level of performance on a widespread scale is the PHPP model, and the realisation that when it comes to low energy building, details are important. To ensure that the necessary quality control is in place we have a certification system – but other than that it’s just physics, and a job well done.

That, to me, is what really makes a Passivhaus – a way to build a super-insulated building that performs how it’s supposed to.

alan mugshotAlan Clarke is a Certified Passivhaus designer and building services engineer. 

© 2013: Alan Clarke and AECB (for Terms and Conditions click here)


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