Natural materials, zero carbon and resilience
Natural and low-embodied-energy building materials, zero carbon and the Code for Sustainable Homes (CSH), on-site low- or zero-carbon energy, post-peak energy, energy returned on energy invested (EROEI)
While Passivhaus does not address the question of the type of materials used and the energy embodied in those materials, this is an area of interest for many who might also be interested in building a Passivhaus. Concern about using materials with low embodied energy, and more locally sourced materials, relates to the wider issue of the resilience of our preferred solutions to interruptions in resource supplies. The UK zero carbon standard suggests a very low-impact housing solution, and the question of how this relates to Passivhaus is therefore pertinent.
Zero carbon is the chosen UK government’s strategy for achieving low-energy housing in the future, and is planned to become a statutory requirement from 2016. For anyone interested in building a Passivhaus in the UK, understanding how the zero carbon and Passivhaus standards compare and might relate to one another will therefore soon be a necessity. A step change towards zero carbon is planned for 2013, so this need is already imminent.
Passivhaus does not stipulate the building materials you should use in your construction. The standard is focused on ‘energy in use’ because, in standard builds, this is by far the largest proportion of the energy used by our houses over their lifetimes. It is also an area of energy consumption we have historically failed to reduce effectively – in the UK, even eco-housing has tended to consume far more energy for space heating than was the original design intent. Also, as mentioned in Chapter 1, one of the goals of the Passivhaus Institut is to get the Passivhaus standard adopted as widely as possible, which means engaging with the mainstream construction sector. If the Passivhaus standard also stipulated that only low-embodied-energy and low-impact building materials were to be used, it would strongly discourage mainstream builders and risk making Passivhaus an econiche. However, there is no reason why a Passivhaus must be built with high-tech, high-impact, ‘non-natural’ materials.
The embodied energy of a material is the sum of all the energy inputs required for its production – from extraction to fabrication and transport, etc. The more highly processed a product, the higher its embodied energy is likely to be. A material such as steel, for example, will have a very high embodied energy (extraction, heated to high temperatures, heavy to transport), while a locally sourced straw bale will have a very low embodied energy (minimal processing, lightweight). Appropriate use of high-embodiedenergy products and materials should be a consideration for anyone involved in building, and it seems only responsible to use low-embodied-energy materials and products wherever possible.
There are of course ‘added’ values that a high-embodied-energy product might bring, including improved performance, structural strength, space saving or aesthetic benefits (the latter being less easy to quantify). Natural materials can also bring added value, especially in the way they can modulate air quality and handle moisture levels (see Chapter 10). Your choice of materials and products should, ideally, involve a sensible consideration of all these factors. In practice, financial and planning constraints will also often influence decisions.
Measuring embodied energy can be quite complex and the methodologies differ – how far do you go in including every input involved? There are ‘cradle-to-grave’ calculations (full life-cycle analysis), ‘cradle-to-site’ and ‘cradle-to-gate’ (up to the point when it leaves the factory). But whatever the methodology, the principle is clear and the data useful – within the UK, the CSH, BREEAM EcoHomes and the US LEED assessment systems all rate materials according to their embodied energy.
Clearly, in an ultra-low-energy build there is particular value in reducing or minimising the energy used in construction, provided it does not compromise the building’s energy-in-use performance, as to do so would quickly become counterproductive. In addition to using low-embodied-energy materials, there are also other ways to reduce embodied energy, if this is of concern to you.
• Avoid over-ordering materials.
• Reuse or recycle materials wherever possible.
• Retrofit and extend rather than demolish (if the existing structure warrants it).
• Use locally sourced materials, where possible.
• Use less highly processed materials, where possible.
• Avoid over-engineered solutions – especially steelwork and concrete foundations.
• Use assemblies that can be dismantled rather than destroyed (bricks with lime mortar is a good example of this, as the lime allows the bricks to be reused).
• Use ‘Lifetime Homes’ (a standard that aims to ensure that homes are suitable for the whole lifespan of occupants)3 design principles for creating flexible and adaptable spaces.
• Consider how big your house really needs to be.
• Build once – build well! Quality is often a good long-term energy-saving strategy.
The University of Bath (UK) has produced an ‘Inventory of Carbon and Energy’ database for hundreds of materials, based on energy inputs into their manufacture, together with the energy costs for transport.1 The figures are often given as ranges because so many inputs are variable that single assessments are not feasible.
As already noted, the embodied energy of the materials used to construct a house is far less than the energy that will be used in the house over its lifetime. Based on a three-bedroom standard house, BRE estimated in 1991 that energy in use would exceed embodied energy by 12 to 30 times, assuming a 60-year life.2 Based on this assessment, energy in use will overtake embodied energy in a maximum period of 5 years. Other studies may differ on the exact figures, but all clearly demonstrate that minimising energy consumption in use is much more critical than reducing embodied energy. It follows that there is room for some pragmatism in the choice of products, where higher-embodied-energy products can help to reduce energy in use.
Once a house is built to ultra-low-energy standards (a space-heating target of less than 40kWh/m².a), the proportion of energy used represented by embodied energy becomes more significant. For a ‘typical’ house the embodied energy is up to 10 per cent of total energy over its lifetime; for a low-energy house this may rise to 30 or 40 per cent.
When assessing embodied energy it can be helpful to consider whole building assemblies – combinations of building materials that make up a component such as a wall or roof construction – rather than just individual materials. Table 5.1 shows how some common building assemblies compare in terms of the embodied energy they represent. While these are not low-energy constructions, the equivalent low-energy assemblies would reflect similar differences. It is useful to note that heavyweight constructions tend to have higher embodied energy than lightweight constructions.
Use of high-embodied-energy components within buildings
The embodied energy of a material sometimes needs to be assessed in terms of the lifetime energy benefit it can bring. This might be in terms of energy it will generate (e.g. solar hot water panels) or energy it will save (e.g. insulation). Manufacturers of high-embodied-energy insulations will argue strongly that the enhanced performance (lower thermal conductivities than natural insulations) and the energy saved through the lifetime of the building make quibbling over the embodied energy of the product irrelevant. Some argue (cynically or realistically, according to your taste) that the fossil fuel energy will be consumed anyway, so it’s better to use it to save energy in the future.
In retrofits, spatial and structural constraints imposed by the existing building structure can make the use of high-embodied-energy but high-performance insulation a pragmatic compromise, since this may allow the existing structure to be saved while still achieving an acceptable energy performance level.
Table 5.1 Embodied energy in some common building assemblies
* m2 refers to the area of the building assembly.
Source: Lawson, B. (1996) Building Materials, Energy and the Environment
kWh/m² – kilowatt hours per square metre.
MJ/m² – megajoules per square metre. A megajoule (MJ) is a unit of energy. 3.6MJ are equal to 1kWh.
Windows have high embodied energy compared with many other building components. Clearly, though, a simple, single-glazed, wooden-framed window has a considerably lower embodied energy than a triple-glazed, argon-filled window with insulated wooden-aluminium frames. If embodied energy were the overriding criterion in our choice of components, we would be installing single-glazed or perhaps air-filled, double-glazed, wooden-framed windows, despite their poor energy-in-use performance.
What, in fact, do we mean when we talk about natural materials? Normally this refers to materials and products that have undergone minimal processing or are in their raw state; in addition, there may be greater opportunity to source such materials locally. Both these factors correspond to a significantly lower embodied energy.
Another attraction of using more natural materials is that they can contribute to better indoor air quality (IAQ) . Natural materials and products will usually not ‘off-gas’ in the way some products do (examples include various carpets, paints, stains, insulations, glues, etc.). Off-gassing is the evaporation of volatile chemicals at normal atmospheric pressure: a process that introduces contaminants into the indoor air. The air in a typical home contains a large range of these volatile organic compounds (VOCs). The more airtight the building, the greater the impact of off-gassing on air quality. Some natural materials can even improve air quality by absorbing VOCs and also by absorbing and de-absorbing (releasing) moisture, thereby helping to regulate internal humidity levels. We discuss these potential health and air-quality benefits in more detail in Chapters 9, 10 and 12. The health impacts of poor IAQ are widely acknowledged, and this is therefore a very important consideration when building a home.
Some natural building materials will be better suited to a Passivhaus build than others. We have not yet come across a straw-bale Passivhaus in the UK, for instance, although there are now examples of these with low airtightness levels. We understand that there are now Passivhaus-level straw-bale buildings in Austria (individual bale humidity and density measurements needed to be taken to ensure performance for certification). Part of the difficulty lies in managing the levels of movement common with certain natural materials, and ensuring that this does not compromise the airtightness strategy. Natural materials also tend to have lower thermal performance levels (the lambda value – a measure of thermal conductivity – of phenolic foam, say, is 0.021-0.024W/mK (watts per metre per degree kelvin) versus sheep’s wool at 0.035-0.04W/mK – see Appendix B). Natural materials also have less consistent lambda values, which means that more conservative figures have to be used in the PHPP. All this makes a significant difference to the depth of insulation required to meet ultra-low-energy building targets.
Essentially, there is no reason why a Passivhaus cannot be built using natural materials for the majority of the construction elements. Timber-frame construction is commonly used in both Europe and the United States, and frames can be insulated with natural materials such as hemp, sheep’s wool, woodfibre board or even recycled waste paper (cellulose). For Passivhaus, some elements of your building will necessarily need to be more highly processed, i.e. less ‘natural’; in particular the triple-glazed windows (although these often have timber frames; it is also likely that they will be produced more locally in the future – see Chapter 4, page 55). Also, airtightness tapes use very specific glues that cannot be classed as ‘natural’. Of course, even a ‘natural’ building is never 100-per-cent natural – there are always light fittings, kitchen appliances and heating systems, etc. to consider, all of which are manufactured using more processed materials.
Carbon sequestration by natural materials
Some natural materials sequester carbon, i.e. they store carbon within their structures as they grow (timber being an obvious example). If then used in a building, they effectively ‘lock up’ this carbon over the building’s lifetime. Some materials are particularly good at sequestering carbon – hemp and straw being two notable examples, since they can be grown as crops on farms and have useful insulating qualities as well. Using such materials means that we can begin to see buildings as potential carbon stores. This is not always a straightforward equation, as the environmental benefit depends on whether the overall volume of the resource material is increasing as a result of your use or not – for example, increased demand for tropical hardwood is likely to have the opposite effect, i.e. of diminishing the area of rainforest. On the other hand, European-sourced timber, where the forest area is being managed and increased to match demand, will be effective. Building with timber is also a better use for our limited timber resource than burning it for fuel4 – burning, of course, releases the carbon back into the atmosphere. If farmers could grow break crops (secondary crops, such as hemp, grown as part of a crop rotation system and often used as soil improvers) for use in buildings, this could help to increase the amount of carbon sequestered in our buildings.
Passivhaus and zero carbon
‘Zero carbon’ and ‘Passivhaus’ have both become popular marketing buzzwords in recent years, and many will be interested to know how the two relate to each other, which we explore here.
UK policy – the Code for Sustainable Homes (CSH)
Most relevant public policy and discourse has focused on reducing carbon dioxide emissions, in an attempt to respond to the global climate predicament. The UK government has legislated to make mandatory reductions in the UK’s CO2 emissions: to at least 80 per cent below 1990 levels by 2050, with an intermediate target of a 34-per-cent reduction by 2020. Reduction in CO2 emissions is measured relative to 1990 levels.
Housing contributes approximately 30 per cent of the UK’s total CO2 emissions, of which about half is from space heating. The Code for Sustainable Homes (CSH) is the UK government’s chosen vehicle to set reductions in CO2 emissions from the housing sector, although this also incorporates broader sustainability measures. The CSH uses a weighted rating system, consisting of nine categories, which address anything from providing bike storage to installing a solar hot water system. Category One, ‘Energy and Carbon Dioxide Emissions’, is the most important of the nine.
Dwellings can be rated at one of six levels within the CSH system, Level 6 (L6) being the most stringent and termed ‘zero carbon’. Zero carbon is currently set to come into force as a standard for new residential homes in 2016, with a similar requirement for non-residential new buildings to be in place by 2019.
The ‘zero carbon’ definition
The definition of zero carbon has been under review and discussion since the initial proposal of the standard. The intention has always been that it would mean 100-per-cent reduction in net emissions relative to a dwelling compliant with the 2006 Building Regulations (England and Wales) – according to the initial definition, this was where any emissions created were offset by those ‘saved’ using on-site renewable capacity. Under what exact terms this will eventually be enforced and what the uplift in cost might be (and to whom) remains unresolved at this time.
Carbon emissions have been separated into what are now termed regulated and unregulated carbon emissions. Regulated emissions are those from fixed building services, i.e. heating, ventilation and lighting; unregulated emissions are those relating to energy used by the building occupants, e.g. from cooking or electrical appliances. The initial plan was that 100-percent reduction in both would be the target – the L6 standard. Concerns about whether this was a realistic target in practice led the government in 2011 to alter the definition of zero carbon to include only the regulated carbon emissions. This means that zero carbon can now refer to either Level 5 or Level 6 of the CSH.
The Zero Carbon Hub (see Resources), a public– private partnership, has been working since 2008 to support the delivery of zero-carbon homes, and this includes the development of a final definition for zero carbon. The Hub’s extensive work has included publishing various advisory papers and carrying out useful consultations.
Achieving 100-per-cent reduction in carbon emissions, even if from regulated emissions only, involves the significant use of on-site renewable energy sources. The practicalities of having enough roof space, not to mention the cost burden, has led to a further strategy being introduced – allowing a certain proportion of the emissions reductions to derive from off-site sources or what are termed ‘allowable solutions’. What exactly these will be or how they will be delivered is again unknown, but will most likely involve a payment towards the cost of introducing carbon-saving projects. The proportion in carbon reductions that will have to be achieved by the house itself is termed ‘carbon compliance’. The split between these two is a matter of further debate. Initially it was thought that 70 per cent of the emissions reduction would be met by carbon compliance and 30 per cent by allowable solutions. An excellent paper by the Zero Carbon Hub5 highlights that this still remains very demanding and unrealistic for some house types. The recommendations in that report suggest the following carbon compliance levels:
Zero carbon and stamp duty
The original 2006 zero carbon definition – relating to both regulated and unregulated emissions – is the one used by the UK’s tax authorities (HMRC) to validate exemption from property sales tax (stamp duty) on homes below £500,000 in value.
• 60 per cent for detached houses
• 56 per cent for attached houses
• 44 per cent for low-rise apartment blocks.
The carbon compliance percentage achieved on any project will be determined by the efficiencies achieved by the building fabric and building services plus any on-site low- or zero-carbon energy source. In the original proposal for the zero carbon standard there were no specific building fabric efficiency targets. This was addressed in the (2010) Code for Sustainable Homes: Technical Guide, which set a new criterion, the Fabric Energy Efficiency Standard (FEES), for the dwelling’s space heating.6 Since the zero carbon standard for new homes is not to be enforced until 2016, FEES is likely to be developed further before then. At the time of writing, it helpfully introduces a Passivhaus-style space heating energy target – for the first time moving away from a carbon emissions measurement. We feel this is a positive development. The lower the space heating target you can achieve, the less you need to make up to meet the overall carbon compliance percentage. Reducing the on-site carbon emissions through excellent fabric performance is also key to reducing the need for additional and expensive on-site energy production. Energy generating systems will also have a shorter expected life (a solar hot water system, for example, will last 10 to 25 years) compared with the general building fabric (average 60 years), so prioritising investment in fabric makes good sense.
For the zero carbon level, the FEES target is (currently) 39-46kWh/m².a. The range reflects different house types – for example, it is easier to improve performance on a mid-terrace than on a detached house. The UK’s Standard Assessment Procedure (SAP) method for measuring floor area is more generous than that of Passivhaus (see Chapter 7, page 94), which means that the zero carbon target is actually equivalent to over 50kWh/m².a in Passivhaus terms. The consultation7 for the next updated Building Regulations, Part L (2013) is also proposing an interim Target Fabric Energy Efficiency (TFEE) requirement for new dwellings, of 43-52kWh/m².a, adopting the same measuring standard as the zero carbon FEES levels. As yet there is no final decision on the standard they propose to adopt – the TFEE or full zero carbon proposed FEES levels. Whichever is adopted will then also link into Level 4 of the CSH. The interim TFEE would apply from 2013 until 2016.
Fabric performance and ventilation strategies
The zero carbon space-heating target range is clearly less stringent than that of Passivhaus (at 15kWh/m².a), and this reflects a reluctance to limit ventilation strategies to MVHR. There is a body of opinion that advocates natural ventilation solutions such as passive stack ventilation (a whole-house ventilation system that uses naturally occurring pressure differences to draw air in through trickle vents in windows and then up and out through ducts in the kitchen and bathrooms). This necessitates higher air-leakage rates, which come with a significant energy penalty. The air-leakage benchmark for zero carbon is being currently mooted at an air permeability of 3m³/hr/m². This is approximately equivalent to 3ach (air changes per hour) for an average-sized house (see Chapter 9 for more details). There has also been discussion regarding the capability of the general construction industry to achieve very low air change rates, i.e. below 3ach. This compares with the Passivhaus standard of 0.6ach – again, a far more stringent standard.
Without mandatory fabric energy targets, there was an early tendency for those looking to meet the zero carbon standard to rely on relatively complex and hard-to-maintain low-carbon heating solutions to achieve the carbon emission reductions. This concentration on renewable technologies can easily lead to a less informed focus on building fabric, which will increase other risks. By beginning to increase levels of insulation in our homes and making them much less ‘leaky’, we are changing the way they physically behave, and an understanding of this is key. Making radical changes without understanding carries four major risks:
• reduced indoor air quality (IAQ)
• moisture within the fabric causing mould and deterioration of materials (and exacerbating the risk of reduced IAQ)
• unacceptable overheating in summer
• underperforming buildings (in energy terms).
Introducing FEES will – rightly – refocus on the building fabric, but this will need to be coupled with appropriate modelling methods and training so that these changes in building construction do not lead to the risks just noted. Linking the required measurements to as-built performance (measuring real performance after occupation), rather than design performance (using energy modelling software during design), as now recommended in the zero carbon consultations, will assist with this.
The debate about naturally ventilated and mechanically ventilated solutions for houses primarily pivots on which will deliver better indoor air quality (IAQ). There are, thankfully, a growing number of studies being undertaken to provide solid data on this important issue. In light of the data available to date, we are convinced of the very real benefits of a controlled mechanical ventilation system – as long as it is designed and installed appropriately. For those interested in this subject, we would recommend the 2012 report of the Ventilation and Indoor Air Quality Task Group set up by the Zero Carbon Hub, entitled ‘Mechanical ventilation with heat recovery in new homes’,8 which contains an extremely useful review of the studies on links between IAQ and health. In its introduction it notes that “at low permeability levels, reliance cannot be placed on the ability of the home to ventilate itself”. This is why MVHR is an essential component of Passivhaus, which sets the lowest air permeability standard (0.6ach), and why the zero carbon standard proposes a much higher air permeability standard of 3m³/hr/m² and then allows other ‘natural’ ventilation solutions – which in turn necessitates less stringent space heating targets.
A zero-carbon Passivhaus
There is a misconception that Passivhaus “will only get you to L4 (Level 4) of the CSH”. If you built a Passivhaus and chose not to address any of the other sustainability criteria assessed in the CSH, then this might be true. In particular, Passivhaus does not include the water usage criteria demanded by L5 and L6. However, if water usage is addressed in a Passivhaus design, it should comfortably meet L5.
In fact it should be easier and cheaper to meet a zero carbon standard in a Passivhaus than in a structure that is not designed as a Passivhaus. This is simply because the building fabric of a Passivhaus leads to the exceptionally low energy requirement for space heating, even relative to a zero-carbon house (see Table 1.1, page 18). The Passivhaus standard also includes a total primary energy demand of 120kWh/m².a, which will also help to achieve zero carbon because it encourages efficient energy use across all electrical appliances and uses within a building.
Together, this means that if regulated and unregulated carbon emissions are taken into account, a Passivhaus can reach a zero carbon level with minimal additional renewable devices. If regulated emissions only are considered (the current zero carbon definition), a Passivhaus has been shown to achieve a 65- to 70-per-cent reduction in regulated carbon emissions compared with a compliant Part L (2006) dwelling, when calculated using the PHPP, without the use of any on-site low- or zero-carbon energy provision.9 This meets all the current zero carbon ‘carbon compliance’ recommended emissions reductions (44, 56 or 60 per cent). In making a comparison between Passivhaus and zero carbon, it should not be forgotten that, by using the PHPP and applying Passivhaus methodology, a much more reliable prediction of real-life performance is achieved. And, as we discover later in this book, Passivhaus also addresses summer overheating risks (see Chapter 11) and IAQ (see Chapter 12) much more reliably than a non-Passivhaus low-energy building.
Even where a project aim is to achieve zero carbon, it is worth giving serious consideration to using the PHPP and applying Passivhaus methodology and principles to the building fabric design, i.e. to meet or, even better, to exceed, the FEES target. There are many efficiency gains and no conflicts.
Broader sustainability criteria
As we have seen from the issues relating to ventilation strategies discussed above, the Passivhaus focus on building fabric performance, and scientific research into and testing of this, is critically important. The fact that other sustainability criteria are not included as part of the Passivhaus standard has ensured that this focus has been maintained. This is not to say, however, that other sustainability criteria apart from carbon emissions (which zero carbon focuses on) and energy in use (which Passivhaus focuses on) are not important: the fact is that Passivhaus buildings can be made from many different materials and construction methods (both lightweight and heavyweight), and the Passivhaus standard is perfectly suited to combining with more diverse sustainable assessment systems. Unfortunately, official certification using two different assessment methods on one building will have cost implications, but in technical terms there is no inherent incompatibility between Passivhaus and other systems, such as the CSH or BREEAM, or the US systems LEED and HERS. Other standards can usefully widen the Passivhaus approach to consider some broader sustainability issues, including recycling, water management and ‘Lifetime Homes’ recommendations.
On-site low- or zero-carbon energy
While a zero-carbon Passivhaus may not require any or only minimal on-site or zero-carbon energy for carbon compliance, you may still want to consider such options. If the aim is only to meet the zero carbon FEES targets (not the full Passivhaus target), then some on-site or zero-carbon energy will be essential. (‘Zerocarbon energy’ normally refers to biomass fuel, while ‘on-site’ refers to energy generation.)
Offsetting carbon emissions from electricity use
As can be seen from Table 5.2 overleaf, UK grid electricity is carbon-intensive relative to other fuels used in the UK. While it is possible to construct a building that is not connected to the grid, this is not often a practical option and it would certainly make no sense, economically or environmentally, to deploy off-grid solutions at any significant scale, as long as there is a functioning electricity grid! Most homes are and will be grid-connected, so unless and until the grid itself is supplied entirely from non-carbonemitting sources, generally a zero-carbon building will need to offset ‘dirty’ grid electricity by photovoltaic (solar-generated electricity) panels. Take, for example, a building located in southern Britain with an unshaded roof of sufficient area, appropriately oriented, and an installed solar photovoltaic array of sufficient size – say, 4kWp (kilowatt peak). Over the course of a year, this would generate electricity equivalent to the annual consumption of a reasonably frugal average-sized household (say, 3,200kWh per year), assuming no electricity is used for hot water or heating.
Figures are as at 2011, on a gross calorific value basis. Source: Carbon Trust / Defra10
Space heating and hot water
In summertime, water can be heated using solar thermal panels. Installed on an appropriately oriented roof, they provide zero-carbon heat, assuming the electricity needed for the pumps is also zero carbon.
Wintertime hot water and space heating can be provided from biomass fuel, which can be zero carbon (generally, ‘biomass’ refers to any organic solid fuel, but in the context of space and water heating here, we use it to refer to wood in pellet, woodchip or log form). In a conventionally built zero-carbon house, biomass fuel would typically be burned in a physically substantial woodchip or wood-pellet boiler; preferably one with a lower output than that required for a standard house. In a Passivhaus, a similar biomass boiler could provide enough heat for a small apartment block or a terrace of Passivhaus homes via a heat main (a system of insulated pipes that run between buildings). Woodchips and wood pellets are not truly zero carbon, because the processing and transporting of chips or pellets almost always requires some fossil fuel input, but they are nevertheless very low-carbon fuel sources.
In a conventionally built zero-carbon house, a log burner with a back boiler would deliver zero carbon heat, if the wood was located nearby and had been cut up with an axe! In a zero-carbon Passivhaus, the problem to date has been that there have been few wood log burners or other types of biomass boiler with a low enough space heating output to make them suitable for a Passivhaus. However, this is beginning to change as the first Passivhaus-suitable models come on to the market. For people living in rural areas, it can make sense to use a low-output wood burner as a heat source.
However, large-scale wood burning for home heating is not practicable because, even with much-improved woodland management, the UK’s wood supplies are insufficient to meet the large demands that our poorly insulated housing stock would place on them. If all UK homes had been built to Passivhaus standards, it would be much easier to provide the small residual amount of heat they would need with a well-managed wood resource. This theoretical scenario is not very realistic in the UK, with its large, densely populated cities, because it would require the transport of large quantities of solid fuel from the countryside, and the concentration of wood burning would be detrimental to local air quality.
The final point to remember in relation to all these space- and water-heating options is that, unlike the fabric of the building itself, they do not for the most part represent investments that will survive for the lifetime of the building. New builds are typically designed for a 60-year lifespan. Passivhaus buildings should last longer owing to the low air leakage rates and the Passivhaus focus on build quality. Over the lifetime of a Passivhaus, space heating and hot water provision will probably need to be replaced several times (just as in a conventional house). So there is no guarantee that a zero-carbon Passivhaus (or any other zero-carbon house) will remain so. For example, a notionally zero-carbon wood-pellet boiler can be replaced by a boiler run on fossil fuel, such as natural gas. In contrast, it is much harder to change the fabric of the building and ‘break’ the building’s original design intent.
Passivhaus and post-peak energy supplies
While public policy in many European countries, including the UK, has focused on responding to climate change, the UK government has singularly failed to accept, let alone attempt to respond to, the separate and equally intractable predicament we face with energy. As we noted in Chapter 2, energy supplies are very likely to be expensive and ultimately less reliable in the coming decades, and this will impact on the economics of Passivhaus as well as on other forms of building. Two concepts that are key to this issue are ‘net energy’ (or ‘net energy gain/ balance’) – the remaining energy available to society after the energy needed to obtain it from an energy source has been subtracted – and ‘peak net energy’ – the maximum rate at which net energy can be extracted from a source. Peak net energy is a more accurate term than the more commonly used ‘peak oil’. It refers to the rate at which we are able to extract net energy from our environment, and it is fundamental to our success as a species. Net energy can be expressed as a ratio: energy returned on energy invested (EROEI)11 (sometimes referred to as ‘energy returned on investment’; EROI) – the number of units of energy produced for each unit of energy consumed in order to produce it. Figure 5.1 below shows a simplified representation of EROEI.
The fossil-fuel era started with the large-scale mining of coal in England in the eighteenth century and grew even faster as oil production began in earnest, commencing in the United States. Globally it heralded a period of high EROEI or high net energy, and this ‘easy’ energy has given the rich world a material standard of living unprecedented in all human history. The flood of energy has also allowed us to create a society of extraordinary complexity and specialisation, again unprecedented in all human history. We have had access to so much energy that society has had incredible freedom to decide how that energy is used. Oil is the most important energy source of all, as it is an enabling or master energy source that allows us to unlock others – take oil out of the system and virtually all other forms of energy production are severely compromised. Oil is critical to our transport systems; indeed transport is the hardest sector to decarbonise, because our transport infrastructure is designed to operate with liquid fuels.
Figure 5.1 Energy returned on energy invested. Source: Hall et al. (2002)12
While there has been periodic concern about oil ‘running out’ – notably in the aftermath of the 1973 oil crisis – it has become clear that natural resources do not generally suddenly run out; instead, the rate of extraction reaches a maximum or peak before going into decline. Technical advances can extend the tail of the decline, and other human interventions, such as geopolitical factors, can reduce the total amount ultimately extracted. The EROEI of the energy extracted before the peak is a lot greater than that of the energy extracted post-peak. Globally, we are approaching, are at or have passed a peak in net energy from oil extraction. There has been much debate about when this will occur, or whether it has already occurred, but few question the fact of a peak in production. Unfortunately, changing infrastructure is itself very energy-intensive, so there is no ‘silver bullet’ that will suddenly transform our energy future. As EROEI drops below 8:1, society becomes far more constrained by its energy choices.
Joseph Tainter, in his seminal work The Collapse of Complex Societies,13 and others have examined how societies have historically adapted to a shrinking EROEI, i.e. a decline in net energy. In simple terms, societies past and present have used their ingenuity to increase the efficiency with which they employ their resources to counter the impact of reduced net energy flow rates. This is very much the path our global society has taken over the past few decades. For example, growth in computer and Web-based systems has allowed increased economic activity for a given energy input. The downside of this trend is that our productive systems have become much less able to cope with quite short interruptions in the energy supply. Supermarkets, banks and supply chains almost everywhere rely on ‘brittle’, just-in-time inventories:
Transitioning to a lower-EROEI future
EROEI figures are sensitive to many assumptions and variables. Historically, the production of oil, coal and natural gas had EROEIs of between 100:1 and 20:1; the EROEI of renewables ranges from around 35:1 to little more than 1:1! (Biofuels produced in temperate climate zones, seen as a panacea in some circles a few years ago, have an EROEI close to 1:1.) But it is evident that energy generation based on renewables provides far less net energy than fossil fuels have done in the past. And the EROEI for fossil fuels is also declining, as extraction becomes more difficult and the ‘lowhanging fruit’ is mostly gone. It is clear, then, that we need to reduce our overall energy demand in order to adapt to the lower-EROEI energy sources that will be available to us in the future. To help achieve this, high-performance buildings in terms of energy invested for energy conserved make the most sense.14
if any part of the supply chain is unable to deliver, even for a few hours or days, the impact is disproportionate. In these cases, efficiency has been achieved at the expense of system resilience.15
How resilient is Passivhaus?
A Passivhaus build, like any conventional building project, is dependent on reliable energy flows – energy is not just needed on-site; obviously, it is also needed to manufacture and shift building materials and to transport the build team. The specialist components used in the first UK Passivhaus builds typically travelled around 1,000km from factories in central Europe to the UK. These distances should shrink over the coming years. Already more companies are producing suitable products, and even UK manufacturers have started responding to the demand for much higher-performance windows and Passivhaus-certified MVHR units. However, fast-forward a couple of decades and it may prove impossible to get timely delivery of bespoke specialist components such as windows. The rationing of liquid fuel – either by price or via a rationing scheme such as tradable energy quotas (TEQs)16 – will make local and regional products comparatively more attractive and feasible. Whether MVHR units will be produced on a local or regional basis is hard to say. However, these present less of a problem as they could be pre-ordered by regional stockists.
As we suggest in Chapter 14, part of the policy change needed to facilitate growth of Passivhaus in the UK is to encourage domestic manufacture of Passivhaus building components and to make changes to the structure of the construction sector, including training and skills (see also Chapter 4). These changes would also help to make Passivhaus a more resilient approach, particularly if using more low-impact building materials. In other words, there is nothing about Passivhaus that makes it intrinsically less resilient than other forms of building in a lowernet-energy future. Nevertheless, clearly if the net energy available to society drops below the levels required to maintain a functioning industrial society, we will be living in a world where more immediate needs – food and security – take precedence. It is not possible to predict exactly how our energy predicament will play out. It depends, in large part, on the wisdom and far-sightedness of the policy decisions we have taken, are taking now and will take in the future.
There is no innate conflict between Passivhaus principles and a preference for building with natural and low-embodied-energy materials. Some elements of a Passivhaus will necessarily need to be less ‘natural’, in particular where specific performance criteria must be met, e.g. for windows or ventilation systems, but this will be true for most buildings. There is also no innate conflict between ‘zero carbon’ and Passivhaus; in fact, we would recommend that the Passivhaus methodology should be adopted to efficiently and accurately meet or exceed the zero carbon FEES targets. Passivhaus can help to meet carbon compliance with little or no reliance on on-site low- or zero-carbon energy. Furthermore, by opting for a mechanical ventilation strategy, good levels of indoor air quality will be more readily achieved. In turn, other assessment standards can usefully widen the Passivhaus approach to consider some broader sustainability issues.
In a more energy-constrained future, Passivhaus has the potential to offer a resilient housing solution, not least because of its efficiency and because it does not rely so heavily on short-life complex technology, although there is a need for more localised supply chains for some of the Passivhaus components.