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 percent 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 Code for Sustainable Homes.

Building regulations compared to CSH
Building regulations compared to CSH

The Zero Carbon Hub, 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.

Allowable solutions

Achieving 100-percent 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 Hub highlights that this still remains very demanding and unrealistic for some house types. The recommendations in that report suggest the following carbon compliance levels:

  • 60 per cent for detached houses
  • 56 per cent for attached houses
  • 44 per cent for low-rise apartment blocks.

Describing Allowable Solutions

‘Allowable Solutions’ has been a useful catch-all term for any approved carbon-saving measures that would be available to developers from 2016 to allow for the carbon that they would not normally be required to mitigate on site through Carbon Compliance (see illustration).

The expectations that have become associated with Allowable Solutions are:

  1. That the developer would make a payment to secure emissions reductions through (largely) near-site or off-site, carbon-saving (Allowable Solutions) projects;
  2. That, independent of the developer, there would be an opportunity to aggregate a number of Allowable Solutions payments to deliver larger scale carbon-emission reduction projects;
  3. That Allowable Solutions would be affordable and (per unit of carbon) would cost, at least initially, less than Carbon Compliance;
  4. That wherever possible, Allowable Solutions would be linked with local projects that would bring local benefits.
Zero carbon comliance
Zero carbon comliance

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. 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 which means that the zero carbon target is actually equivalent to over 50kWh/m2.a in Passivhaus terms. The consultation 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 l5kWh/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. 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.

A zero-carbon Passivhaus

There is a misconception that Passivhaus "will only get you to L4 (Level 4) of the Code for Sustainable Homes". 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 Level 5 and Level 6. However, if water usage is addressed in a Passivhaus design, it should comfortably meet Level 5.

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

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. 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 the Passivhaus also addresses summer overheating risks and IAQ (Internal Air Quality) 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. (‘Zero-carbon energy’ normally refers to biomass fuel, while 'on-site' refers to energy generation.)

Carbon compliance
Carbon compliance