What is a Passivhaus?

A Passivhaus building is designed to be very comfortable and healthy, and to use vastly less energy than conventional buildings, irrespective of the climate. This is achieved by careful design informed by building physics and, crucially, by thoughtful and careful construction by a properly skilled and motivated team.

Passivhaus originates in Germany; the German word Passivhaus literally translates as 'passive house or building', since Haus refers to a building as well as a house. The word is being incorporated into English, although the concept is still often referred to as 'passive house', particularly in the United States. When people hear the term 'passive', they sometimes assume that this means no heating system or that the design relies on 'passive solar design', i.e. utilising heat from the sun. There is some truth in these assumptions. A Passivhaus does require a trickle of heat to maintain 20°C, although not nearly enough to justify a central heating system. It relies on high levels of uninterrupted, all-round insulation, airtight design and heat gained from the winter sun through the windows (solar gain); however, solar gain is not in itself sufficient to heat a Passivhaus. A Passivhaus is also more comfortable and healthier than a standard build, as there are no draughts, no condensation or mould in cold spots, and the air is fresher. We will see later in the book how and why this is the case.

The thermographic images below, in which surface temperatures are represented with colours, show how high levels of insulation transform a building's energy performance. A well-insulated building in winter has cold external surface temperatures (shown in blue). Thermographic imaging also highlights any 'thermal bridges' – gaps in insulation that allow heat to bypass it.

The Passivhaus energy standard

A building is a Passivhaus if it meets a voluntary technical standard that, being international, has to be met regardless of the local climate. It was developed by the Passivhaus Institut (PHI), an independent research institute founded in Germany in 1996. The Passivhaus standard is defined by the core technical requirements listed in Table 1.1 opposite. However, understanding Passivhaus is about a lot more than these numbers. It is a process informed by some key principles, which we will briefly explore next (and in more detail in the rest of the book).

Any building that meets the standards in the second column of Table 1.1 is a Passivhaus. If you don't understand the entire table now, you should become familiar with the ideas as you read the rest of this book. Each requirement is also discussed in the sections that follow. Definitions of terms and units are given in the box below the table, and are also explained in the glossary.

To be sure that a building designed as a Passivhaus genuinely meets the Passivhaus standard and to have independent verification that the design will work as intended, it helps to be certified. Chapter 3 explains what Passivhaus Certification involves and what benefits it brings.

Passivhaus and other building standards

Different standards are not, for the most part, quantitatively comparable, either because they do not measure the same things or, often, because the conventions and assumptions on which they are based are different, making direct comparison impossible. Some are country-specific and stem from government initiatives. Others, such as Passivhaus, originate from non-governmental organisations and are therefore voluntary. Some standards have a narrow focus, while others, such as the UK's Code for Sustainable Homes (CSH), attempt to cover a broad range of sustainability goals. The CSH was originally conceived as an assessment system. By contrast, Passivhaus is not only an energy performance standard but, critically, is also intended to be a design process – which, if applied intelligently, will get you to your low-energy goal.

To some extent, it is possible to mix and match standards. For example, there is no reason why water usage targets from one code cannot be mixed with energy usage standards from another. Some projects are required to meet statutory assessment-based targets, but you can still choose to work to a tougher, voluntary standard. Similarly, there is no reason why other areas of sustainable building not covered by standards or codes cannot be addressed in the design. For example, Passivhaus does not address the properties of building materials – such as embodied energy (the energy used in the sourcing, manufacture and transport of a material), breathability and recycle-ability – but clearly there is nothing to stop you choosing to address these independently of any standard, while still achieving the Passivhaus energy-inuse standard. These issues are discussed further in Chapter 5.

The Passivhaus concept

Underlying the Passivhaus concept are several key ideas, discussed on the following pages. While some are 'common sense', others are less obvious and more technical in nature.

Getting the fabric right

The Passivhaus approach concentrates above all on 'getting the fabric right'; in other words, on designing, specifying and constructing the foundation/floor, walls, roof and windows correctly to achieve the Passivhaus standard. Money spent on the building fabric should be seen as an investment for the life of a building (normally a minimum of 60 years), as production of the building materials can consume considerable energy. Designing the fabric to last means that the invested money, energy and carbon provide enduring benefit. In contrast, money spent on 'bolt-on' technologies, such as those needed to provide hot water or space heating (e.g. a boiler or a hot water tank), is a shorter-term investment – 20 or perhaps 30 years at the very most. Those systems may well be replaced several times during a building's lifespan. The smaller the energy...