What Is Passive House?

In times past, buildings were extremely efficient. They were compact in form, with thick walls of cob, earth, stone or brick and small windows. Centrally located wood fireplaces provided relatively sustainable heating while driving fresh air ventilation through wall vents, and chimneys with high thermal mass held their heat when the fire died down. Then came mechanical heating and cooling, cheap centralised energy, cheap glass and thin, lightweight timber frames. We now live in bigger houses than ever before, with bigger windows, more complex forms and higher expectations of thermal comfort. All of these things have their benefits, but they also contribute to very high household energy use.

Passive House is a rigorous, voluntary building standard developed in Europe in the late 1980s/early 1990s, which aims to reduce energy use by up to 90% compared to typical building stock. It is a performance-based standard applicable to any building size or type in any climate, and mandates the following requirements:

  • Minimal heating (total annual heating demand of less than 15kWh/m2 OR maximum heating load of less than 10W/m2)
  • Minimal cooling (less than 10% frequency of overheating (over 25°C) OR total annual cooling demand of less than 15kWh/m2)
  • Minimal overall energy use (total annual primary energy demand of less than 120kWh/m2)
  • Minimal air leakage (less than 0.6 air changes per hour at 50 pascals)

These requirements can be met by any means available, but generally they mean a building will have:

  • A compact, efficient shape with a low surface-area-to-volume ratio
  • Passive solar heating through carefully located high performance glazing
  • Excellent insulation, weather seals, and an air tight envelope without thermal bridges
  • Constant, 100% fresh air mechanical ventilation with heat recovery
  • Efficient lighting, appliances and hot water supply

What I love about the Passive House standard, and the methodology used to design Passive House buildings, is that it’s based on a detailed understanding and calculation of the thermal physics of a building – something sadly not covered in detail in a typical architectural education. It’s great to be able to quantify the effect of design and specification decisions with confidence.
Passive House is quite new to Australia, and as yet there are no certified buildings (although there are several under construction). Hopefully in the near future we will see more and more buildings that are efficient and comfortable all year round.

Passive House was originally developed in Germany – a cool temperate climate not too dissimilar to ours for much of the year, but with colder extremes in winter and cooler extremes in summer. The standard has been popular across central Europe, as well as in countries such as Ireland and Canada, where high heating bills provide a big incentive.

More recently it has also been applied with great success in warmer climates, such as Spain, Portugal, Italy and across the pond in New Zealand. Even the Austrian Embassy in tropical Jakarta has achieved the Passive House standard, saving many thousands of dollars in cooling costs every year. Different approaches are required around the world to deal with different temperature and humidity conditions, but the Passive House standard itself is climate-independent. A mild climate such as Melbourne may not have the huge heating bills of Ireland or the huge cooling bills of Indonesia, but the cost impost of building to Passive House standard will ultimately be less here too. (We have no cost data in Australia yet, and early adopters will pay more, but in Spain – with a similar climate – the upfront cost impost is 3-5% over conventional building.)

Passive house explained in 90 seconds

So Passive House is relevant to any climate, but the other question I’ve been asked is: “Is it relevant to our lifestyle?” Australians love the outdoors, and on beautiful sunny days all we want to do is throw open the doors and windows and leave them that way. Contrary to popular belief, there’s absolutely nothing stopping this in a Passive House. But when it gets bitterly cold or stinking hot, a Passive House will let you close everything up, keep the heat in (or out), and still have a constant supply of fresh outdoor air. A well-designed Passive House gives the occupant total control over internal comfort, with minimal energy use.

How hard is it to achieve Passive House in Australia?
There is nothing easy about the Passive House standard. It requires:

  • Careful planning to get the right building volume, orientation and glazing;
  • Careful detailing to avoid thermal bridges, gaps in insulation or breaks in the air-tight layer; and
  • Careful construction to ensure tight-fitting insulation and appropriate sealing of any required penetrations

To achieve the Passive House standard in extreme northern European climates can require vast areas of south-facing triple-glazing, minimal windows elsewhere and very thick walls (up to half a metre!). All of this adds cost, reduces internal space and limits how spaces can be arranged. But in Australia we are lucky to have much greater access to sunlight, and comparatively mild temperature differentials between inside and outside. In many areas we are also blessed with comparatively large blocks and open spaces, which provide more opportunities for ideal siting and orientation. With the right site, planning, detailing and specification, it is quite possible to design a passive house building in Australia without too much more insulation than is required by the National Construction Code, and often with double-glazing only. Add solar panels and a water tank and you can achieve a very high level of efficiency.

Perhaps the most difficult element of the Passive House standard to achieve – and one that is equally difficult in any climate – is the air-tight sealing. It has certainly proven the most challenging component of a Passive House-inspired retrofit project we’re currently working on in West Melbourne. 0.6 air changes per hour at 50 pascals is an extremely high standard. It exceeds every building code requirement around the world (at least until Brussels mandates Passive House construction from January 2015), and requires careful cooperation between the designer, contractor, and many key subcontractors – such as plumbers and electricians.

In Australia we are used to houses with open wall vents, absent or ineffective door and window seals, rough holes for services and exposed, uninsulated floor framing. Ventilation is definitely important, and provision for natural cross-ventilation in the appropriate weather conditions should be incorporated in any design, but first and foremost the point of any building is to shield us from outside conditions. Leaky buildings do little to help this. Whenever we heat or cool a poorly sealed building we lose vast amounts of heated or cooled air to the outside – literally throwing energy (and money) out the window.

This was brilliantly summed up in a recent post on the Australian Passive House Association’s Facebook page – by a self-proclaimed whinging pom: “Would you pay full price for a fridge if the doors didn’t seal? It’d keep your food cool but the pump would be running all the time costing you loads in electricity. Why do Australians put up with leaky houses, the most expensive things they will likely ever buy? Esky without a lid anyone?”

It is indeed a sad fact that many houses in this country – even new ones – are very poorly sealed by international standards. Whilst there isn’t any widely accessible data on the average air changes per hour of Australian housing, a study of 68 Canberra houses found an average of 17.96 air changes per hour at 50 pascals! Certainly – at 30 times this level of performance – the strict Passive House standard is not for everyone. But the principles at least could help us drastically reduce energy consumption while improving comfort levels. Personally I hope that increasing local interest in Passive House – and high performance building fabric in general – filters through to better building standards across the board. There is a great opportunity to have a huge impact on our national energy consumption by taking a physics-first approach to building design.

Written by Jim Stewart