The Passive House: thoughts and reflections

There were a couple of moments when the PassivHaus study tour seemed to lose all contact with normality and enter into a surrealist daydream. Picture around 20 people, crammed into a cellar, with a few more hanging out in the passageway outside unable to get in for lack of space. We were listening, or trying to listen, to the architect, Carsten Grobe, who was explaining with the aid of a translator about the mechanical systems employed to heat the Passive House, which he had designed and built back in 1998 and in whose basement we were now standing.

One of our party asked what the carbon emissions were like for this house. Carsten gave a long and detailed explanation as to why the indoor air quality was very good and insisted that they had no problem at all with carbon emissions. It was an eloquent answer to a very different question.

You might think that this was just a translation problem and that he hadn’t understood the question. But similar misunderstandings happened throughout the tour. Whilst the Germans had no problem explaining how they have engineered these houses to reduce space-heating demand to an absolute minimum, they looked flummoxed when we started going on about carbon emissions or used the dreaded phrase “zero carbon.” I began to realise that these are concepts that the Germans actually felt uncomfortable about and it highlighted a surprising difference in emphasis between Germany’s PassivHaus builders and the UK’s “carbon busters.”

You see, it’s no accident that one of the key benchmarks for Passive Houses is a maximum allowance for space heating requirements for each house and it is expressed in kilowatt-hours per square metre per annum — 15 is the magic number they are looking for in this context. In contrast, the UK regs now work in terms of carbon dioxide emissions per square metre per annum. Whilst the two units are related, they are not the same, not by a long chalk.

The kilowatt-hour (kWh) has the benefit of being simple to understand. It’s the unit we buy our gas and electricity in, and the related kilowatt is the preferred unit of power for our boilers, fires and electrical equipment. Even light bulbs are routinely rated in watts. Most people are familiar and easy with the watt, the kilowatt and the kilowatt-hour.

Carbon dioxide emissions, in contrast, are much trickier. They vary a lot depending on the type of fuel being burned. A kWh of gas releases less than half the CO2 of a kWh of electricity. Every fuel has a conversion factor, related back to mains gas, and some fuels have several. So the fuel you choose to burn in your house has a critical effect on the CO2 emissions but no effect on the kWh you use. Consequently, Carsten Grobe saw no contradiction in using electricity exclusively to power his ventilation/warm air heating system and to heat his domestic hot water when the solar panels weren’t producing.

The three schemes we visited around Hanover were all different, designed to show the different aspects of PassivHaus living. Carsten Grobe’s selfbuild represented one strand, the upmarket detached family home: we also saw a block of flats in Hanover itself and a terraced house set out on the Expo site to the south of the city. The weather was wet — very wet in fact — but not particularly cold for February, around 7°C, and I was certainly a little surprised to find radiators on in both the detached house and the flat. In fact, I was surprised to even find radiators at all, though they were small and unobtrusive.

Most Passive Houses are built without radiators, except for towel radiators in bathrooms. They aim to maintain an even temperature throughout the structure and Carsten explained that the only reason they had a radiator in their house at all was for odd occasions when their elderly parents came to stay and they appreciated a bit of extra warmth.

But just because Passive Houses rarely use radiators, it doesn’t follow that Passive Houses are “Homes without Heating” as we sometimes think of them as being. The actual space heating arrangements vary from site to site but the overriding principle seems to be to
a) having insulated the fabric down to a very low U value and having designed out all thermal bridges
b) you then fit energy-efficient triple-glazing into the openings
c) and then ensure that the resulting structure is virtually airtight.
d) Now you build in a mechanical ventilation system to supply fresh air
e) and you use a heat exchanger to take every available bit of heat from the extracted air and add it to the incoming air.
f) You then pass this pre-heated air through something called a post heater (a term I had not come across before) which brings it up to a comfortable temperature.

How much work this post heater has to do depends on how warm you like your house. The one in the terraced Passive House was capable of delivering 1.5kW and could produce an air temperature of 50°C if required. I was frankly surprised at this: it seemed to me that this was essentially a warm air heating system.

The standard method of heating new homes in Germany is to use underfloor heating combined with a gas boiler. PassivHaus eschews this approach on the grounds that you don’t need a full heating system in a Passive House. But it seems to me to offer an alternative that is no cheaper to install and possibly burns more carbon because it relies on electricity to power the fans and the air heater. It would certainly be no cheaper to run because electricity is habitually three times the price of gas.

And yet Passive Houses sell themselves in part by saying that you can offset the extra costs of building to the higher standard by saving money on not having to install a full heating system. Hmm. I was beginning to have my doubts.

Carsten gave us another little trick used by PassivHaus designers. He said it was extremely difficult to get a really low theoretical heat demand from a detached two-storey structure as there is just too much external envelope in proportion to floor area. An easy way around this conundrum was to add a third storey, in his case a basement, and to include this in the heated envelope. The floor area goes up 50% whilst the space heating demand rises much less. As you are looking to meet a target expressed in kWh/m2/annum, the job of reaching PassivHaus standard becomes that much easier.

You could argue that this is daft and that the total energy load is actually increased in order to meet some notional standard. In fact, you’d be right but it’s a criticism that can be levied at most of the other energy rating schemes as well, certainly all the ones that work on a floor area basis.

Maybe, I am carping. I suppose I do carp rather a lot. But I know others were also uneasy. We had Julia Hailes, author of the Green Consumers Guide, with us in our party and she was flabbergasted to see that Carsten’s house was full of halogen and tungsten lights with not a CF bulb anywhere to be seen. There is in fact no particular requirement in the PassivHaus code to use energy efficient lighting or appliances: this seems bizarre. Julia also asked at one point just how you would incorporate a cat flap into a Passive House, a question that was met by complete bafflement from our hosts. Maybe the word cat flap was lost in translation, or maybe it was another case of different agendas.

The embodied energy question was raised several times by various delegates. Was the extra investment in, say, triple-glazing filled with krypton really worth it either financially or in terms of carbon emissions? Again, it seemed to be something that the Germans hadn’t addressed. They seemed to be solely interested in designing homes which required less than 15kWh/m2/annum for space heating: there didn’t seem to be any room for critical reflection about this.

The first day of the tour was spent in the hotel listening to various presentations. One of the most interesting presentations was delivered by John Willoughby, the energy consultant and sage, who gave a 20-minute slide show on the History of Low Energy Building in the UK. He ran through many projects starting with the Wallasey School (1961), the Wates House at CAT (1976 and still the best insulated building in Britain), various solar houses in the 1970s, the Pennyland and Linford projects in Milton Keynes in the 1980s, right through to Hockerton and Bedzed, both realised within the past few years. He also drew our attention to the increasing number of energy codes:
• Code for Sustainable Homes
• EST Good-Best-Advanced Standards
• Eco Homes
• Bill Dunster’s Zed Standard
• PassivHaus
• AECB Gold and Silver Standard

He suggested we were becoming good at standards but not very good at implementing them. He also referred to another energy sage, David Pickles, who I remember seeing give a talk once where he said that low energy living is 10% technology and 90% lifestyle. This little aphorism kept plopping into my head as we made our way around the three Passive House sites the following day. The German PassivHaus builders really didn’t seem to care about this aspect of energy efficiency at all: they just wanted to get their buildings up to PassivHaus standard and move onto the next one. PassivHaus seems to be very much an engineering concept, designed to provide comfortable space heating with the minimum energy possible. But that, unfortunately, is it.

So I came away feeling PassivHaus is a flawed standard. It’s very good on space heating demand (in advance of anything we have built in the UK, except for a tiny number of exemplar projects like Bedzed), but questionable on heat delivery systems, so-so on hot water heating and almost non-existent on lighting and electrical goods used within the home. What is more, there don’t appear to be any plans to update or improve the PassivHaus standard to take into account the rising concerns about carbon emissions. As such, I really don’t see PassivHaus translating wholesale across to the UK, but we can certainly learn a lot from what they have achieved to date.

The principle lesson perhaps being that, whilst it is all too easy for me to carp on about what is wrong with the PassivHaus standard, or why it is not quite the dog’s bollocks (try translating that!), they have built loads of them. And the more they build, the less theoretical it becomes and the more practical it appears. Whereas we may be full of good intentions, the Germans (and the Austrians, who seem to be even more enamoured of PassivHaus than the Germans) have now built around 6,000.

They are also very keen to promote the PassivHaus concept to the non-German speaking world. To this end, the next PassivHaus annual conference will for the first time be bi-lingual in German and English. So if you want to go and learn more about it, you need to head for Bregenz in Austria on April 13/14.