Since 1991, Part L of the Building Regulations has set out the minimum energy standards that must be achieved by all new non-domestic buildings. It has also directed refurbishments towards a minimum standard of energy performance. While subsequent editions of Part L have increased the minimum amount of insulation required in buildings, the effect on the actual energy performance of non-domestic buildings has not been dramatic. The limited effectiveness of the recent regulations in reducing energy usage in non-domestic buildings was largely the result of an emphasis on insulation. By 2008, the levels of insulation required were such that typically less than 15% of a non-domestic building’s energy use was associated with heat loss through insulated elements. The larger elements of energy usage such as air leakage, ventilation and electrical energy usage have not been aggressively addressed by the regulations to date. In 2002, the EU proposed that all member states should not only harmonise the method used to define the minimum energy performance of buildings, but also that a more holistic calculation should be used to ensure all forms of building energy usage were considered. This proposal was ratified in the form of the Energy Performance of Buildings Directive (EPBD) in 2008. The 2008 Building Regulations responded by the introduction of the Non-domestic Energy Assessment Procedure (NEAP), which was usually implemented by the use of Simplified Building Energy Model (SBEM) software. SBEM contains several known deficiencies and loopholes but is still considered a reasonable compromise between complexity and accuracy for the purpose of setting holistic minimum energy standards for most buildings. While the 2008 regulations did address a large range of sources of energy consumption in buildings, the targets set were so low that the effect on building performance was a relatively small improvement on the previous methods that relied on insulation performance alone. The EPBD Recast was then introduced in 2010 with the intention of generating dramatic improvements in building performance that would produce Near Zero Energy Buildings (NZEB) by 2020 and significantly increase the amount of on-site renewable energy used in buildings. The 2017 version of Part L (currently in draft form) is part of Ireland’s response to this legislation and is set to notably improve the minimum performance of new non-domestic buildings in Ireland.


The calculation method used to determine compliance with Part L involves the generation of a reference building that is similar to the actual building under assessment but with a number of fixed parameters such as a 40% glazed area, set U-values, infiltration levels and system efficiencies. The reference building is also assumed to obtain 20% of its energy from renewable sources. The performance of the actual building (as calculated by the tool) is compared with the reference building and, if the energy usage is lower than that of the reference building, then the requirements are met. The 2017 version of Part L increases the performance of the reference building by 40-60%, depending on the exact building under consideration. The energy referred to in all cases is the building’s primary energy and excludes all energy used by the end users such as computer and equipment usage. End-user energy is intended to be addressed by other elements of the EPBD, such as energy labelling. Primary energy refers to energy used at power stations in the case of electrical energy and a small (10%) production factor is added to most fossil fuels that are used directly on site. While a 40-60% improvement in energy usage sounds dramatic, it should be kept in mind that most new buildings constructed over the last ten years have often exceeded the minimum requirements by the order of 40-50% so the increase in energy performance required is not overly restrictive. The requirements for renewable energy, however, have drawn much more debate. In addition to the reduced energy usage, all new buildings must generate 20% of their energy from renewable energy sources, although this may be reduced to 10% where the energy performance of the building is more than 10% better than the reference building. This option of further reducing energy use is likely to be selected for most buildings. The 20% (or 10%) requirement can be provided by photovoltaics (PV), wind, solar thermal, biomass, combined heat and power (CHP) or heat pumps. For most buildings, it is likely that the most practical option will be a combination of PV and heat pumps. Achieving the 10% renewable energy requirement is relatively straightforward for most buildings, but can be problematic for some building types – particularly where a building has not been designed from the early stages with consideration of the requirement. Ironically, buildings that have almost no heating load can find it particularly difficult to achieve the renewable energy requirement, as the provision of heat with a heat pump is not on option open for such buildings. In addition, if they have a relatively small roof area and a high cooling load, then it may not be practical to resort to the use of PV to meet the renewable energy requirement. Buildings that have a high cooling load and almost no heating load may, however, not be designed optimally and consideration should be given to reducing cooling loads – even at the expense of increasing heating loads slightly. In addition to the headline energy requirements, the proposed changes to Part L also include a number of revised requirements relating to overheating, plant efficiencies, air leakage, ductwork leakage, lighting efficiency, controls and other aspects of energy performance. In most cases, these additional requirements will be picked up by default when complying with the overall energy targets, but designers must still be aware of the detailed requirements.

Reception and debate

As the legislation introduces significant changes to the ways that buildings are designed, it has inevitably sparked significant interest and debate from all those involved in the design and construction of buildings. • Is the legislation too onerous? A number of individuals and construction bodies have expressed concern that the legislation may be too onerous and will impose excessive costs on building procurement – or, in some cases, may make it impossible to procure buildings of some types or in some locations. This view is, however, not supported by the large number of test calculations that have been performed for various building types that have concluded that most buildings will be able to achieve the new requirements will little additional cost. In many cases, the additional capital cost is estimated (by the Department of Housing, Planning, Community and Local Government Regulatory Impact Analysis) to be in the order of 2-5% for compliance. • Does the legislation go far enough? While the term ‘Near Zero Energy’ is completely subjective, it requires a healthy imagination to accept that a 40-60% improvement over the worst-case requirements of current construction qualifies as ‘near zero’. Defining a 10% renewable energy requirement to be a ‘significant proportion’ of the remaining energy is also an optimistic interpretation of the term. It can be argued that the requirements do not comply with the directive on that basis. The directive, however, implies that member states should define ‘Near Zero Energy’ as the point where the measures required to implement the standard are ‘cost optimal’ and the current proposal aims to achieve a balance between costs and energy performance in the current construction environment. It must also be kept in mind that the legislation must be reviewed every five years and adjusted to reflect changing technology. This is essential, particularly in the context of the break of almost ten years since the last revision of Part L and the current pace of change in construction. • Why is natural gas-fired CHP considered to be a renewable form of energy? The inclusion of natural gas-fired CHP as a form of renewable energy may be surprising, as it is by definition not a form of renewable energy. However, in practical terms, its inclusion is not a concern – the ‘renewable’ content in the case of CHP is defined as the net primary energy saving, which is less than 1% in most cases, and other forms of renewable energy will still be required. • Is the primary energy factor for electricity potentially unrealistic? Reducing the carbon impact of electricity is a key part of Ireland’s low-carbon strategy and a consideration of the impact of electrical energy should potentially acknowledge the average impact over the building system’s lifetime, rather than the current carbon impact of the national grid. The proposed legislation uses a primary energy factor for electricity that is based on the predicted carbon impact of electricity in 2018, even though the majority of the buildings that the legislation affects will not begin operation until well after 2020, making the carbon factor out of date even before the legislation is applied. The argument against a more progressive, future-oriented electrical carbon factor is that there are no definitive guarantees of future reductions in electrical carbon impact. Designers should, however, take a view on the probability of further reductions in grid carbon when selecting building systems. • Is the legislation aggressive enough for refurbishments? While the legislation does tighten up on the requirements for building refurbishments, the standards that apply are much lighter than those for new buildings. There also remains a significant degree of flexibility and interpretation in the wording of the legislation. The significant improvement of existing building energy performance remains the responsibility of designers and their clients rather than being dictated by legislation. This may be addressed at a later date, as Ireland’s national retrofit strategy is expected to be updated shortly.

Application and conclusion

The legislation applies to any new public buildings that have started design after January 2017 and to all new buildings where planning is applied for after December 2018 or where the external walls are not complete by December 2020. It is, however, prudent for designers and clients to comply with the legislation in all buildings currently in design, as the costs will easily be recovered within the lifetime of the building’s system in most cases. The proposed Part L requires a significant improvement in how buildings are designed and constructed while avoiding significant additional capital costs in most cases. A small number of buildings may find the legislation challenging to comply with, but buildings that are carefully designed from inception with the legislation in mind are likely to comply in most cases. The legislation does not bring buildings close to a ‘zero energy’ performance, but should be viewed as a minimum requirement. Most buildings will hopefully continue the current trend of exceeding the minimum requirements of legislation, viewing the minimum standards as a base case to compare proposed improvements against. Author: Chartered engineer Chris Croly is environmental engineering director with BDP.  He has led the design and delivery of a range of innovative low-energy new build and retrofit/renovation projects in the commercial and public buildings sectors.