Part L’s role in delivering sustainable dwellings

Phil Kelly, Head of Sustainability and Building Physics, Ramboll

The definition of a sustainable home is subjective. Low energy, net-zero carbon, healthy, comfortable; all these metrics play a part, but market incentives and legislative drivers typically fail to consider them in a holistic manner. Part L of UK Building Regulations currently sets legally-binding limits for overall carbon emissions and space heating energy demand only. A mixture of enhanced fabric performance, additional low-carbon technologies, district energy and renewables have all played a part in meeting these moving targets for over a decade, and as fabric performance continues to improve, reducing space heating demand, the focus has shifted towards the carbon emissions associated with hot water and ventilation.

The carbon emissions targets of Part L, together with industry’s modern-day good practice standards for air quality, acoustics and overheating, drive designers to consider more complex design solutions with bespoke user operational assumptions. None of these metrics, however, are captured adequately within the Standard Assessment Procedure (SAP), the calculation methodology within residential Part L, which is already too generic to act as a true indication of performance.

Should Part L continue to present energy and carbon emissions data to residents if it does not allow the envisaged operational procedures to be adequately represented? This has already contributed to the ‘Performance Gap’ within the industry, and we have reached a point whereby carbon emissions can no longer be considered in isolation when making design decisions. Legislation should reflect this reality by better linking Part L (Conservation of Fuel & Power) and Part F (Ventilation).

The consultation for the proposed amendments to Part F & Part L in 2020 (due to close 10 th January) outlines some substantial changes aimed at furthering new residential design towards a net-zero carbon future. The new carbon factors for fuel alone will encourage a shift towards electrically-fuelled energy strategies in the hope that the National Grid will achieve its low-carbon aspirations.

The Future Homes Standard (FHS), presented as part of the same consultation, includes very little detail beyond the aspiration to achieve a 75-80% reduction in CO2 emissions beyond current standards. The proposal to adopt the FHS in 2025 coincides with the Committee on Climate Change (CCC) recommendation that no new homes are connected to the gas grid by the same year. Both proposals have, however, missed the opportunity to address three key challenges that are pivotal in achieving the UK’s 2050 net-zero carbon commitment.

Embodied carbon, namely, the emissions associated with a building’s entire lifecycle from cradle to grave, but excluding the operational energy consumption, could be up to 70%* of the overall carbon emissions of a dwelling. Failing to adopt a lifecycle carbon analysis and design approach could result in design solutions that incur greater embodied emissions during the construction stage in order to reduce energy-related emissions during the operational phase- thus merely moving the problem, rather than directly addressing it.

Furthermore, the performance of both the dwelling and its respective systems will fail to meet expectations if aggregated over an entire year. The efficiency of Air Source Heat Pumps (ASHP) and the carbon intensity of the electrical grid changes both hourly and seasonally. Electric ASHPs have been shown to pass the carbon emissions targets of the new draft London Plan (which incorporates a comparably decarbonised national grid) with flying colours, but is this system truly operating efficiently and with a low-carbon fuel source on a cold, dark winter afternoon?

Currently, Part L does not do enough to ensure a dwelling’s performance in operation matches the design intent. Furthermore, with Building Control departments significantly under-resourced and non-compliance penalties woefully insignificant, enforcement is already lacking. Successful enforcement of Part L is therefore also linked to local planning policy enforcement in councils, who set their own energy and carbon-related targets, as these are typically also based upon the SAP methodology, and must be submitted upon construction. The consideration in the Part L consultation that there may be ‘no further need’ to allow local authorities to set higher standards beyond building regulations from either 2020 or 2025 would therefore be a disaster.

Part L of UK Building Regulations has an important part to play in reducing the energy consumption and carbon emissions within the construction industry. However, its failure to consider the lifecycle outside the operational phase, the limited granularity and accuracy of the calculation process, and the lack of enforcement, will result in the true performance of dwellings continuing to vary from their initial design expectations.

*RICS, Whole life carbon assessment for the built environment – 1st edition, November, 2017