Why Passivhaus?


When we started work designing a new house for ourselves the issue of energy efficiency was high on the agenda. It was 2010 and UK energy prices were rising, and continue to rise much faster than inflation. From 2005 to 2010 UK retail electricity prices doubled and gas prices rose by more than 80%. North sea oil and gas production is in decline and the prospect of being beholden to less-than-stable parts of the world like Georgia and the middle east for our supplies does not inspire me with confidence. The labelling of nuclear power as ‘low carbon’ can be of little comfort to the residents of Fukushima and the only people the government can persuade to build new nuclear power stations in the UK are a French company with Chinese investors, and then only at a 70% higher cost than the current wholesale cost of electricity. Investment in renewable energy is finally making a significant impression but still only made up 11.3% of the total energy generated in the UK in 2012. Being as self-sufficient as possible for the energy we use seems the only way to avoid constant price rises.

The other way of course is to use less of it. The energy used in housing made up 26% of the total energy used in the UK in 2011 and more than 60% of this energy is used to heat our homes. So the obvious thing to do is to build a highly insulated house with a very low space heating demand with the initial capital outlay being offset by lower energy bills in the future. If it can also include some energy generating measures such as solar panels that might be beneficial but would be secondary to the principle of avoiding the demand in the first place. In this ‘fabric first’ approach the idea is to avoid the need for the energy in the first place. This article on Eco-minimalism by Nick Grant sets out some of the reasoning behind it.

As well as a long-term financial incentive we also feel a social responsibility to at least not leave the world in a worse state than we found it. The climate change debate has been hijacked by zealots on both sides, either claiming that we’re all going to hell in a hand cart or that climate change is a left-wing conspiracy theory propagated to implement a socialist agenda and stop people doing what they want. The actual science gets ignored, or twisted by journalists more interested in a story that contradicts the last one than presenting the facts. We believe that climate change is occurring, that human activity is contributing to it and that we need to do something about it if extreme weather events, water shortages and political instability in parts of the world most affected are to be limited. Building our own house provides a great opportunity to learn more about low-energy construction and experiment in territory which might be too risky for a client. Our office has gained considerable knowledge in retrofitting existing dwellings through a number of projects in London, notably the 80% House and the Retrofit for the Future House, but we hadn’t yet had the opportunity to build a new house to such an high performance level. We decided to push things as far as we could.

Eco doesn’t have to mean mediocre architecture

There’s another factor that I must confess was at least equally important as the energy performance – it would have to stand on its own as a strong piece of architecture whatever its environmental credentials. One of our obsessions in the office is that architecture doesn’t have to be sacrificed to achieve excellent energy performance. It is an unfortunate fact that many ‘eco-houses’ have a pretty low architectural ambition and it could even be said in some cases that an environmental logic is used to avoid making design decisions on architectural grounds – it has to be like this because that’s the most efficient energy solution. With a ‘fabric first’ approach the great thing is the house doesn’t rely on clip-on gadgets for its performance. The walls and roof end up being thick to accommodate the necessary insulation but thick walls particularly can be a bonus aesthetically. We did make some choices for our house that go slightly against received Passivhaus logic and increased the insulation levels elsewhere to compensate. I’ll talk about these issues in another post one day. What we ended up with is not therefor the cheapest solution but as I say, there were other criteria.


So how do you know how much is enough insulation? How air-tight should it be? How much space heating will be required and how should it be provided? Even if you don’t need much heating how do you heat hot water? All these questions are very difficult to answer unless you have some way of calculating the energy performance of the building accurately, and that is what the Passivhaus Planning Package (PHPP) does. For some background on the principles of Passivhaus have a look at thePassivhaus Trust and the BRE’s Passivhaus websites. Terms such as sustainable, eco and green are vague, ambiguous and often used in a misleading way in the building industry. Passivhaus offers a clear and comparable standard founded on principles of building physics.

Building Regulations

Don’t the Building Regulations require an adequate level of energy performance? No. The government realise this and proposed revisions to Part L are due in 2014 and 2016 with the intention of making all new homes ‘zero carbon’ by 2016. This target was set by the Labour administration in 2006 but has been watered down following successful lobbying by the house building industry and the rate of implementation has been set back due to the recession. This article from Inside Housing gives a good synopsis of the situation. Whatever standards the Building Regulations demand there still needs to be a means of designing the building fabric to achieve it and the SAP calculation in Part L is just not up to the job. SAP is useful for raising the general standard of energy performance of average houses but it is not accurate enough for analysing a very low energy house. This AECB publication by Alan Clarkediscusses the differences between SAP and PHPP.

Comfort and Air Quality

A major attraction of Passivhaus to me is the air quality and comfort of the internal environment. To ensure enough fresh air in a very air-tight building a mechanical ventilation system (MVHR) is required. A heat exchanger ensures that most of the heat in the exhaust air is transferred to the the incoming air (ours is 90% efficient). This means that in the winter we are getting constant fresh air at around 18 degrees C compared to a normal house where very cold air leaks in or is allowed in through trickle vents from outside. In the summer we just open the windows like any other house, and at night or when we’re out the house is still being ventilated if the windows are shut. The constant air flow provides a very even temperature and humidity. In a colder or less well ventilated house extremes of humidity can cause condensation and mold growth which have been linked to health problems such as asthma.

The need for accurate energy modelling becomes more important the more air-tight the building envelope becomes. Building Regulations currently demand an air permeability of 10 m3/(hr.m2). At this level MVHR is not required. Passivhaus requires an air-tightness of 0.6 air changes/(hr.m2). Somewhere in between these figures it becomes necessary to provide some mechanical ventilation. It is generally agreed that it would be advisable to provide some mechanical extract if the air permeability is less then 5 m3/(hr.m2). I rented a house a couple of years ago that was very well built with an air permeability of 2 m3/(hr.m2) but no MVHR and the windows were running with condensation every morning.

Limitations of PassivHaus

I am not religious about Passivhaus – it doesn’t give perfect solutions to every aspect of environmental performance, but it is the most useful tool we have found to produce genuinely low-energy buildings. It concerns me that when you look Passivhaus on the internet you get a list of very similar articles evangelising its benefits without much discussion of its limitations. We are founder members of the Passivhaus Trust, Bob is a certified Passivhaus designer and I have built my own soon-to-be certified Passivhaus. Suffice to say we think it is a good thing, but I don’t feel the need to give you a hard-sell. Our priority as designers is to do as much as we can to reduce the energy consumption of the building industry and Passivhaus is a very effective way of doing it. I have set out some of the issues to be aware of below:

– PHPP is only a tool for modelling the energy performance of a building in use. It does not address wider issues of sustainability such as:
– embodied energy of building materials / equipment
– It does not reward low water use or rainwater recycling
– ecology
– waste management
That doesn’t mean these issues can’t be addressed in a Passivhaus – of course they can. The PHPP software just doesn’t take it into account.

– PHPP only looks at energy use, not carbon emissions. The standard was developed in Austria where electricity is a low-carbon energy source as it comes mainly from hydro-electric power stations. In the UK our electricity comes mainly from gas and oil which releases a lot of carbon into the atmosphere. It is then up to the building user where they get their electricity from. We buy ours from Good Energy, a UK supplier from 100% renewable sources (mainly wind).

– There is a shortage in the UK construction industry of the attitude and skills required to build to the exacting standards of Passivhaus. There are some excellent contractors out there who are fully engaged but for Passivhaus to gain wider adoption in the mass housing market there needs to be a fundamental shift in attitude.

– The word Passivhaus is not protected – anyone can call their house a Passivhaus. In my experience getting ours certified by a Passivhaus assessor, which is an additional cost, made a difference in the standard we achieved because we had to keep pushing with the air-tightness, cold bridges etc to get it over the line rather than just stopping when we’d had enough. For a potential purchaser certification gives an assurance that they are getting what they think they are buying.

– The build cost will be higher than for a building regulations level house but that gap will narrow as the building regulations standard is raised towards ‘zero carbon’. Cost comparisons carried out to date have been based on one-off houses, often built by designers and contractors who were learning new skills on the job using specialist products. As consultants and site workers become more familiar with the construction methods and products become more widely available costs will reduce. Published data suggests an additional building cost of around 10 – 15% and there would be some additional design time. On larger projects the cost uplift is less because the standard is easier to achieve with a lower surface area-to-volume ratio. I will write another post specifically about costs when I have analysed the costs of my own house.

– Some people would say the need for mechanical ventilation is a disadvantage and I was against it when we first heard about Passivhaus. Having understood better now how it works I am converted. The cliche about not being able to open the windows is a myth. Having lived with ours for almost a year the air quality is great, and you really appreciate it in the winter.


Our decision to build a Passivhaus was something of an experiment and we’re very pleased we did it. It isn’t the only way of building a low-energy building but it has been devised as a sensible standard that actually means something in terms of energy performance.