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.

Thermographic images of the Tevesstrasse project, Germany: before (left) and after (right) its Passivhaus retrofit. Images: Passivhaus Institut

Table 1.1 Passivhaus mandatory technical requirements

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 burden needed to heat (or, in hot climates, to cool) the building, the less hard the bolt-on technologies have to work, and the smaller and potentially simpler to manufacture, maintain and use they can be.

Optimising the design from day one

The architect’s initial designs are modelled for energy performance using the Passivhaus Planning Package (PHPP), specialist software developed by the PHI (see Chapter 7). The PHPP makes it possible to test whether the design will achieve the Passivhaus energy and comfort standard. Once the design has been entered into the PHPP, it is possible to vary elements of it to measure the effect on energy performance. This is an iterative process where client preference, aesthetics, planning considerations, costs and any other practical constraints can be balanced against the design’s energy performance. Such an approach brings multiple benefits. Rather than over-engineering the design to make sure it reaches the Passivhaus standard, the design can be optimised. It also means that the client, architect and builder can make informed decisions about the design, that money is not wasted, and that everyone understands the significance of the design and why certain choices were made. Depending on the project, this preliminary detailed work helps to reduce the risk of unexpected cost overruns later in the project.

There is no doubt that this approach is good for the project and makes it easier to reach the Passivhaus standard. However, it is difficult for many to accept the risk of paying for more detailed design work than is typical in a standard build before planning permission has been given. The planning system is discussed in Chapters 4 and 14.

Solar gain and shading

Wintertime solar gain (heat gain from the sun through glazing) is an important part of how a Passivhaus stays warm. However, when a building is designed to minimise heat loss in winter, it is especially important to ensure that summertime solar gain is avoided as far as practicable. This can be achieved by the use of shading devices, which are discussed in Chapter 11.

Insulation and avoidance of thermal bridges

A Passivhaus needs more insulation than other buildings, and that insulation must wrap continuously around the building so that thermal bridges are eliminated (or, in a retrofit, minimised). A thermal bridge, commonly known as a cold bridge, is a gap in insulation that allows heat to ‘short-circuit’ it. This happens when a material with relatively high conductivity interrupts the insulation layer.

Form factor

The term ‘form factor’ essentially refers to the shape of the building: it is the ratio of the external surface area to the internal usable floor area, known as the treated floor area (TFA), and is a measure of how compact or spread-out the design is. A more compact design makes it easier and cheaper to achieve the Passivhaus standard because the walls, roof and floor can be a little thinner (have a higher U-value) than would be needed in a more spread-out design. Table 1.2 below demonstrates this.

Thermal comfort

Most discussion of Passivhaus tends to focus on saving energy, but just as central to Passivhaus methodology is the concept of thermal comfort, defined as the “condition of mind which expresses satisfaction with the thermal environment”. More simply, this means not feeling too hot or too cold. In designing a building, the effort to improve thermal comfort needs to be directed at reducing all cold surfaces and draughts. In Passivhaus, there are comfort design criteria (see Table 1.3 overleaf) intended to eliminate all draughts and cold surfaces, and to provide sufficient fresh air. Improving thermal comfort has an added benefit. In a conventional building, we often turn up the heating in cold weather to compensate for draughts and the chilly feeling (known as ‘cold radiant’) we experience when near cold surfaces. The occupants’ need to overcome the effects of draughts and cold radiant often adds to a conventional build’s real-world energy consumption. In a Passivhaus, this does not happen, allowing thermostats to be set lower for the same level of thermal comfort.

Table 1.2 Impact of form factor on U-values needed for a Passivhaus

Table 1.3 Passivhaus comfort design criteria

Airtightness and indoor air quality (IAQ)

Making a building less ‘leaky’ is important because air escaping in an uncontrolled way through the building fabric wastes energy, risks reducing the building’s lifespan (as a result of air carrying moisture into the fabric) and also makes it feel less comfortable during cold or windy weather. As statutory energy standards improve, there is a trend towards greater airtightness. The problem is that as buildings are made more airtight, indoor air quality (IAQ) almost always deteriorates. In some countries, the practice of opening windows daily to ventilate can help a little, but some research (see Chapter 12, page 192) has shown that opening windows provides only a brief improvement of IAQ. In the UK, where buildings were traditionally very draughty, there is no similar custom of regularly purging stale air. New windows often have trickle vents in an attempt to address this problem, but these are arguably ineffective because people are quite often unaware of their existence or significance and they simply remain closed. Even if they are used correctly, the rate at which air is changed is dependent on how windy the conditions are.

A Passivhaus is many times more airtight than typical new builds, so a reliable and consistent method must be used to keep the indoor air fresh and healthy when the windows are closed. Passivhaus uses a carefully designed, highly efficient and quiet heat recovery ventilation system, known as mechanical ventilation with heat recovery (MVHR). In an MVHR system, no air is recirculated, only the heat is recovered and recirculated; and, if correctly designed and installed, there should be no noise or perceptible draught from the movement of air. The result is that IAQ in winter is maintained at a very good level. In the summertime, ventilation can be achieved by leaving windows tilted open and/or using the ventilation system. Chapter 12 explores in more detail how heat recovery ventilation works in a Passivhaus.

Designing correctly for airtightness, or minimal air leakage, is part of the Passivhaus architect’s job (see Chapter 9). However, the Passivhaus airtightness standard is a particular challenge for contractors because once the airtightness layer has been created, it is very easy to damage it accidentally later in the build. To avoid this, everyone in the construction team, including subcontractors, needs to understand exactly what the airtightness layer is, why it is important, and what changes in working practice are needed to reliably create and protect it. Chapter 9 explores the challenges contractors face in delivering the airtightness standard onsite.

Annual space heat demand / heat load

These terms refer to the energy and the maximum power needed to heat your home. Achieving either the 15kWh/m².aannual [specific] space heat (or cooling) demand – which represents around 90 per cent less space heating energy than in a typical UK building – or the 10W/m² [specific] heat load is one of the key requirements for a Passivhaus (see Table 1.1, page 18). (Note that, unlike the heat load, the ‘cooling load’ requirement for a Passivhaus in a hot climate is not yet fully specified as part of the Passivhaus standard.) Some people ask why these requirements aren’t made even lower. Why not completely design out the need for any space heating or cooling? While this is technically possible, it significantly increases the build costs for relatively little additional energy saving. The choice of 15kWh/m².a represents an optimum point where the need for conventional central heating (or, in hot climates, air conditioning) is eliminated. Space heating options in a Passivhaus are discussed overleaf.

Annual primary energy demand

The 120kWh/m².a annual [specific] primary energy demand requirement (see Table 1.1) is designed to ensure that consumption of energy for hot water, cooking, appliances, lighting and all other uses is efficient. It effectively makes it impossible to use internal heat gains from inefficient appliances or poorly designed hot-water pipework as a tactic to get around the 15kWh/m².a space heating requirement. It also discourages the use of electricity for direct water heating (for instance, an immersion heater) and encourages the use of solar hot water where practicable. Eliminating excess heat gain from inefficient domestic hot water systems and appliances also has the benefit of reducing the risk of your house overheating in summer.

Working as a team – building trust between architect, builder and client

Working as a team may well sound like a clichéd aspiration, but it is an ideal we fall short of in many real-world builds. Being able to achieve an effective, trusting and cooperative working relationship between architect, builder and client is critical to the success of a Passivhaus build. In the same way that a Passivhaus build demands much attention to technical details, this ‘human’ detail is one that must also be addressed.

Typically, architects work on their designs with minimal input from those who will be tasked with building them. While this approach does not preclude a close, trusting relationship between the key parties, it does make it harder to achieve in practice. If the builder is involved in the initial stages of the project and has a meaningful input into the construction details, he or she should be able to help reduce the complexity of the build without affecting the aesthetic or the function. More importantly, early involvement of the builder will help to build trust and mutual respect with the architect. The builder is more likely to accept the design and to have a deeper understanding of its rationale.

Common misconceptions about Passivhaus buildings

While it is fairly easy to grasp what a Passivhaus is about, it remains an abstract matter for those of us who have never experienced one, and so most people find it hard to predict how they would feel about living in such a place. We have long experience of buildings that are draughty and hard to heat, or stuffy and tricky or expensive to keep cool. Many of the misconceptions about Passivhaus reflect this. Here are a few of the most common.

“You are not allowed to open the windows / you are locked away from nature”

In summer, a Passivhaus relies on windows that can be opened. In warm weather, the most effective way of purging any heat built up during the day is to open the windows at nighttime. During the winter heating season, there is no reason why a window can’t be opened, but most people will feel less need to do so than in a standard build because the MVHR system ensures good air quality. A few people who previously always slept with an open window in summer and winter continue to do so in their Passivhaus home. This reduces the temperature (in winter) slightly and adds a little to the building’s energy use, but the Passivhaus will still function as intended.

“The air is too dry in a Passivhaus”

This can be an issue in Passivhaus designs, but it is also true of conventional centrally heated buildings. However, the problem can be minimised, provided that the MVHR system is specified, installed and commissioned correctly. Dry indoor air is caused in the wintertime because cold winter air holds a lot less water vapour (moisture) than air at typical room temperature. Even if the winter air is completely saturated with water vapour (i.e. it has a 100-per-cent relative humidity (RH)), as happens on a cold, rainy winter day, it still holds a lot less moisture than indoor air at 20°C, even if that indoor air is only at 50 per cent RH. This means that dry winter air entering the building, whether controlled via an MVHR system or uncontrolled through leakage gaps in the building’s structure, will feel very dry (i.e. have very low RH) as the air reaches room temperature.

Dry wintertime indoor air is more of a problem in the colder and drier winter climate of central Europe. In warmer, damper winter climates, such as those of the UK and Ireland, the moisture content of dry indoor air can be increased by, for example, keeping indoor plants and drying clothes on a washing stand – as well as by normal cooking, washing and bathing activities, and by the occupants’ breathing. Use of particularly hygroscopic materials (those that act as a water vapour buffer by absorbing and releasing a lot of atmospheric moisture), such as clay or hemp, can also help to moderate fluctuations in internal RH. There are MVHR units that recover humidity as well as heat (‘ERV’ units, which are used in the USA), but this tends to be at the expense of efficiency.

“A Passivhaus overheats in summer”

With appropriate design strategies, this should not be a problem. As mentioned, it is essential to be able to open windows at night during warm spells. In the daytime, windows exposed to summer sunlight should be adequately shaded externally (see Chapter 11). The Passivhaus standard requires that the internal temperature does not exceed 25°C for more than 10 per cent of days annually, although we suggest aiming for a more stringent target of 3 to 5 per cent – this should not require much additional shading and will make a big difference to comfort levels in summer. In hotter, tropical and subtropical climates (see box overleaf), the effort to keep within the 15kWh/m².a requirement focuses on the annual cooling rather than heating demand.

“There is no heat source such as a cosy fire”

In a Passivhaus there is no physical need to light a fire in order to feel cosy, for the same reason that we don’t feel the need to light a fire in a conventional house during temperate summertime weather (where external temperatures are similar to room temperatures). It is possible to install a very small wood burner in a Passivhaus, but it needs to have a very low output for space heating – typically less than 2kW. Using it on all but the coldest winter days could overheat the house, unless you were very careful about how much wood was added (although this could easily be remedied by opening windows). In a Passivhaus, a wood burner needs its own combustion air supply from outside and exhaust to outside. Also, the air supply and exhaust must be detailed correctly where they enter and exit the building, to avoid thermal bridging (see Chapter 8) and optimise airtightness (see Chapter 9).

This Passivhaus in Louisiana illustrates design strategies for a hot climate. Image: Catherine Guidry

Suitable wood burners that deliver very little space heating are coming on to the market, so this could be an option if having a log fire is a priority (and if there are no constraints on wood supplies). Otherwise, some log burners have a marble or granite surround, which acts as a thermal mass – absorbing and storing heat, and so limiting the rate at which the fire’s heat is released. Alternatively, there is no reason why a home cannot be designed with some areas that are outside the Passivhaus thermal envelope. A standard log burner or even an open fire could be used to heat those spaces.

It is quite possible to use a heat source that burns woodchips or wood pellets. The challenge when designing a single domestic-scale Passivhaus building is that such boilers have tended to have higher heat outputs than are needed. But in multi-home Passivhaus projects a single wood-fuelled boiler could be used to serve a number of homes. Chapter 5 explores the options for making a Passivhaus zero carbon (no net greenhouse gas emissions), including using wood as a fuel.

“The ventilation system is noisy, overly complex technology that uses energy to run and transmits noise between rooms”

A Passivhaus-certified MVHR unit, correctly designed and installed in a building designed and constructed to Passivhaus standards, is not noisy and saves much more energy than it uses. An MVHR works entirely differently from air conditioning, with which it is sometimes confused. With heat recovery ventilation, fresh air from outside is slowly introduced to parts of the house (typically the sitting room and bedrooms), each room being supplied separately via its own duct. From these rooms the air moves through common areas such as halls and stairwells, and is extracted from the kitchen and bathrooms. The rate of air movement in an MVHR system is a lot slower than in an air conditioning system, making it quiet and energy-thrifty. Sound attenuators (soft cylindrical inserts) are placed if necessary at specific points in the ducting system to muffle any residual noise from the MVHR. Chapter 12 looks at ventilation in a Passivhaus in detail.

“A Passivhaus can’t be built with natural materials or ‘low-impact’ materials”

There is no reason why a Passivhaus cannot be constructed using low-embodied-energy and low-impact materials (building materials that do not require large energy inputs to make and deliver to site, and which can be reused, recycled or otherwise returned to the environment without significant impact or cost to the environment). Such materials are discussed in Chapter 5. The Passivhaus standard has a narrow focus on energy consumed in the use of the building, and the Passivhaus Institut has deliberately chosen not to stipulate any standards for embodied energy or other environmental impacts of the materials chosen. This is simply because the PHI does not want Passivhaus to be seen as an eco-niche solution. Passivhaus will make an impact on energy use and carbon emissions only if it is put into practice as widely as possible. This approach is showing dividends in Germany and Austria, where public authorities are moving towards making Passivhaus the norm for new building.

“Passivhaus architecture is boxy and unimaginative, not joyous or uplifting”

Passivhaus is not intended to be a single architectural style, although it may sometimes be seen as such because so many early examples were built in a relatively small part of central Europe. It is likely that the designers of those early Passivhaus buildings drew their inspiration from the region’s existing architectural legacy and practice.

More recent examples in North America and the UK are beginning to change this assumption, as can be seen from the buildings pictured overleaf. In 2010 the PHI started an awards scheme for good Passivhaus design, since it is keen to encourage design diversity: as the PHI sees it, Passivhaus needs to adapt and adopt the design traditions of the societies where the buildings are constructed, not least because those traditions were born of years of practical experience of building in very different climates, with different local materials and skill sets.

Different styles of Passivhaus buildings. Above: SurPlus-Home, USA. Winner of the 2009 Solar Decathlon. Image: ENERGATE^® Top left: The first home in the Channel Islands to be Passivhaus-certified. Image: David
Family home in Chemnitz, Germany. Image: Passivhaus Institut
Centre for Disability Studies, Essex. Image: Simmonds.Mills

Passivhaus design tends to encourage the creation of a fairly compact thermal envelope, as we noted earlier (page 21). This does not prevent you from building a Passivhaus that is very spread out – but doing so makes it more expensive to achieve the Passivhaus standard, as building elements (materials or objects that are part of the structure of a building) need to be significantly thicker to achieve the lower U-values needed in a Passivhaus. In practice, economics, local architectural tradition and the constraints of the local planning system are much more significant determiners of building shape.

It could be said that it has always been part of an architect’s remit to design buildings that are ‘joyous’ or ‘uplifting’. Architects work with many constraints: climate, project budget, planning laws and Building Regulations, space-and site-specific restrictions, the skill set of the builders, and so on. Clearly, it would be untrue to say that Passivhaus uniquely inhibits architects from designing joyous buildings. The requirements of Passivhaus do influence appearance, but this can be handled sensitively or clumsily, depending on the skill and imagination applied. New aesthetics often take time to filter into public appreciation (we tend to warm to familiar features), so this is not a definitive test for aesthetic value.

We have lived through a period when energy was cheap and its use (apparently) consequence-free; when it was feasible to build very energy-hungry structures. The world is changing and energy consumption is becoming a real constraint for growing numbers of people. The accepted aesthetic will therefore be challenged, but alternative aesthetic solutions do evolve.

What is the significance of refurbishment or retrofit?

Most of the housing stock that will exist in 2050 (the year widely referred to as the target for 80-per-cent reductions in carbon emissions) already exists today. In the UK, new build replaces only a tiny proportion (around 1 per cent) of the housing stock annually. This replacement rate is constrained by economics and by land ownership patterns, planning law and culture. Because of this, most energy/carbon reduction in the housing sector will come from retrofitting existing building stock.

No agreed standard definition of refurbishment or retrofit exists, but in the UK it generally refers to fairly superficial changes, such as new kitchen, bathroom or decorative changes, rather than addressing any backlog in maintenance of the existing fabric or services. However, a refurbishment/retrofit nearly always provides an opportunity to improve the building’s energy performance at much lower additional (marginal) cost. It pays to plan strategically, so that no work completed in earlier phases has to be undone later. That way, it is possible to make substantial and cost-effective improvements over time, in stages, as resources allow. The terms ‘refurbishment’ and ‘retrofit’ are often used interchangeably, but since ‘refurbish’ implies superficial changes as opposed to changes to a building’s fabric, in this book we will use the term ‘retrofit’ to describe the adaptation of existing buildings to ultra-low-energy standards.

Passivhaus and retrofitting

A minority of Certified Passivhaus projects have been retrofits of existing buildings. However, in practice, reaching the full Passivhaus standard – in particular the 15kWh/m².a space heat demand and 0.6ach airtightness limits – is not often feasible. This issue is explored in Chapter 6.

During 2010 and early 2011, the PHI piloted a new energy performance standard for residential retrofits, known as EnerPHit.³ This was launched at the Passivhaus Conference in May 2011. EnerPHit allows a maximum annual heat demand of 25kWh/m².a and an upper airtightness limit of 1.0ach, if the 0.6ach target can be shown to be impracticable. EnerPHit also sets requirements for individual elements of a retrofit, should the 25kWh/m².a requirement not be met. The EnerPHit standard is therefore more complex than the Passivhaus standard. As of November 2011, only retrofits in certain climates, including central Europe and the UK, can be certified to the EnerPHit standard. The key points of the EnerPHit standard are:

•  annual specific space heat demand – below 25kWh/m².a

•  airtightness – normally 0.6ach; if this is demonstrated to be unachievable, then up to 1.0ach with additional evidence provided

•  evidence and calculations to demonstrate that moisture management issues have been adequately addressed

•  only existing buildings that are unable to meet the full Passivhaus standard are eligible for EnerPHit certification.

If the 25kWh/m².a limit is exceeded, EnerPHit certification is also possible by meeting criteria for individual components and building elements:

•  walls – externally insulated wall (>75% of total wall area), U-values below 0.15W/m²K; internally insulated wall (<25% of total wall area), U-values below 0.35W/m²K

•  roofs / top-floor ceilings – U-values at or below 0.12W/m²K

•  floors – U-values at or below 0.15W/m²K

•  windows – installed whole window, U-values at or below 0.85W/m²K

•  external doors – installed whole door, U-values at or below 0.80W/m²K

•  linear thermal bridges – at or below +0.01W/mK; point thermal bridges +0.04W/K

•  ventilation – heat recovery efficiency at or above 75%; electrical efficiency at or below 0.45Wh/m³.

While retrofit is more challenging than new build, the EnerPHit standard provides an achievable benchmark that encourages the same high standards, rigorous methodology and attention to detail that Passivhaus demands for new builds. It is worth remembering that, at 25kWh/m².a, the EnerPHit energy performance standard is still about twice as demanding as that of the Fabric Energy Efficiency Standard (FEES) defined in the UK 2010 Code for Sustainable Homes: Technical Guide.⁴ Similarly, the EnerPHit minimum airtightness standard is about three times as demanding as the zero carbon (CSH Level 5/Level 6) standard. As such, achieving the EnerPHit standard remains a big challenge for architects and builders. However, it is easier to achieve in the mild climate of, say, south-west England than in central Europe, where it was piloted.