Ready for zero: Focus on Parts L and O 

With tougher targets for both homes and non-domestic buildings, the government is continuing to push construction towards zero-carbon, with further cuts to follow in 2025. Tom De Saulles explains

The introduction of the Future Homes and Future Buildings standards, planned for 2025, is a key element of the government’s net zero-carbon agenda. The aim is to deliver new buildings that are zero-carbon ready: in other words, able to become zerocarbon with no future retrofitting required as the electricity grid continues to decarbonise. The first steps have already been taken with recent updates to Part L of the Building Regulations, which came into force in June. The revised Part L includes tougher limits on carbon emissions for new-build, with a 27% cut for non-dwellings and a 31% cut for dwellings, compared to the previous 2013 edition.

The intention is for the 2025 Future Homes and Buildings Standards to cut this much further, made possible by even higher levels of energy efficiency and lower-carbon heating. While this is still a few years away, the recent Part L revisions are part of a phased approach to cutting emissions on the road to net zero-carbon.

Part L volume 1: dwellings

For new homes, a key change is that, while gas boilers are still permitted, the use of heat pumps is strongly encouraged and makes it easier to achieve the emissions targets. Even when not used, there is a requirement for dwellings to be heatpump- ready for future installation. This essentially means central heating systems must, where possible, be capable of operating with a maximum flow temperature of 55°C, which will entail larger radiators, with implications for room layout and aesthetics.

This could make underfloor heating a more attractive and practical option, particularly when hot water pipes are embedded in concrete or screed, which transfer the heat relatively freely to the room. This ability to conduct heat, combined with the floor’s large surface area, makes even lower flow temperatures possible down to about 35°C, significantly increasing heat pump efficiency. Underfloor systems of this type also open up the potential for smart control with time-of-use tariffs, lowering operating costs and carbon emissions by using the floor as a thermal store to retain and release heat as required.

In contrast to previous Part L changes, the latest cut in emissions has more to do with services, reflecting the progress that has already been made to improve fabric performance. This makes a switch to low or zerocarbon heating systems the next logical step for lowering emissions. Photovoltaics (PV) also play an important role and are likely to be needed to meet Part L requirements, which have been set on the basis that PV is installed.

There are now four separate Part L targets for new dwellings:

  1. Fabric Energy Efficiency Standard (kWh/m2/y), determined by the building fabric
  2. Primary energy (kWhPE/m2/y), determined by the building fabric and the fuel used
  3. CO2 emissions (kgCO2e/m2/y), determined by the building fabric and the fuel used
  4. Minimum fabric and building services standard.

The Fabric Energy Efficiency Standard (FEES) continues the government’s “fabric first” policy, the rationale being that fabric performance is built-in, maintenance-free and lasts the lifetime of the building, unlike technological solutions. Before the Part L consultation process, the government was minded to drop FEES, but relented when most of the consultees thought otherwise. Another notable change relating to the building fabric is a requirement for detailed design drawings to be submitted for all thermal bridging junctions, together with on-site inspections and photographic evidence. The use of what is now termed “recognised details”, from sources such as the Local Authority Building Control, continues to be strongly encouraged.

This is a much more efficient and cost-effective option than applying the default Y-value for thermal bridging which, although still technically possible, is no longer feasible from a practical perspective. Primary energy is a new Part L performance metric for dwellings and non-dwellings. It takes account of factors including the energy used to produce/deliver fuel and power to the building, and the efficiency of the heating system. Nearly two-thirds of respondents to the Part L consultation disagreed with using primary energy as a key performance metric and, in response, the government has said this approach will be reviewed prior to the introduction of the Future Homes and Future Buildings Standards.

As before, the Standard Assessment Procedure (SAP) tool for assessing Part L compliance sets a Target Emissions Rate (TER) based on a notional building of the same size and shape. The notional building uses a set of reference values for the fabric and services performance, which includes U-values that remain largely unchanged, with only a small uplift for roofs and windows. While the wholesale adoption of the notional dwelling U-values can be applied to the actual home, some trading between values still gives a degree of design flexibility.

However, the tougher Part L targets mean the scope for this is now more limited. For example, this could well result in the notional dwelling U-value of 0.18W/m2K for external walls becoming widely adopted, even though a higher backstop value of 0.26W/m2K is permissible. This is no bad thing, as a value of around 0.18 is more readily achievable these days and should arguably be the baseline for all new-build. It is worth noting that for the 2025 Future Homes Standard, the draft notional dwelling has a wall U-value of 0.15.

Part L volume 2: buildings other than dwellings

The metrics for buildings other than dwellings are the same as for homes, although there is still no Fabric Energy Efficiency Standard. The emissions target (TER) continues to be set using a notional building in the same way as for dwellings (still using the SBEM compliance tool). There is the same new requirement for heating systems to be designed for a maximum flow temperature of 55°C or as close to this as possible, aimed at making heat pumps a more practical option from the outset or at some future point. However, in contrast to new dwellings, heat pumps are not necessarily as well suited to all of the building types that volume 2 covers, particularly those with a significant hot water demand such as hospitals, or those with large open spaces to heat such as distribution warehouses or industrial process buildings.

For this reason, the government is mindful that a one-size-fits-all approach to low/zero-carbon heating will not work as well in the Future Buildings Standard, which will need to account for the diversity in non-domestic buildings. This will be one of the areas addressed in a technical consultation planned for 2023, which will be carried out in parallel to a separate consultation for the Future Homes Standard. Fabric performance has seen more of an uplift for most non-dwellings, with U values for the notional side-lit building improving from 0.26 to 0.18W/m2K for exposed walls, and from 0.22 to 0.15W/m2K for exposed/ground floors. Roofs and windows have also been given an uplift. The treatment of thermal bridging is similar to dwellings, with a strong push to use suitably calculated construction details, or incur a significant performance penalty, which is likely to result in the need for enhancements to other aspects of the design.

Part O: Overheating mitigation

Part O is a completely new section of the Building Regulations. The combination of warmer summers and the ongoing tightening of U-values has steadily increased the risk of overheating, and Part O addresses this directly, focusing on new residential buildings. The aim is to ensure the application of practical measures to reduce overheating risk and protect the health and welfare of occupants. Part O came into force in June alongside the latest changes to Part L and replaces SAP Appendix P as the means of demonstrating overheating compliance. It provides two main routes:

1. Simplified method

Project risk is considered at the most basic level and is used to set limits for solar control and ventilation. The maximum area of glazing is determined in response to floor area, orientation, location and whether or not cross-ventilation is achievable. To ensure adequate ventilation, the simplified method also uses the basic design parameters to determine the minimum opening (referred to as free area) that must be provided. In high-risk locations, there is a further requirement to provide shading on glazed areas facing north-east to northwest (via south).

2. Dynamic thermal modelling

This provides a sitespecific assessment and can be used to demonstrate compliance as long as it is in accordance with CIBSE Technical Memorandum 59 (TM59). This provides a method for assessing overheating in homes using dynamic thermal modelling and includes a compliance criteria. Essentially, it offers a broader range of strategies for limiting solar gain and greater overall design flexibility for reducing overheating. This includes the use of thermal mass. In addition to reducing overheating risk, this may enable greater flexibility with other aspects of the design – for example, by allowing slightly larger windows for improved daylighting and passive solar gain during the heating season.

Dynamic thermal modelling also allows the use of mechanical ventilation and, in situations where passive options are insufficient, mechanical cooling is permitted. Whichever route to compliance is used, Part O includes provisions to ensure the overheating mitigation strategy is usable. These relate to noise, pollution, security and safety issues. Where these performance criteria cannot be met using the simplified method (for example where occupants are unlikely to open windows due to external noise), then dynamic thermal modelling will need to be used and a more tailored design approach taken.

The highlighted Part L revisions clearly show the government’s direction of travel towards zero carbon and the planned 2025 Future Homes and Future Buildings Standards. The broad objective of these standards has already been established, but many of the details are yet to be developed, awaiting further consultation in 2023. Technical updates to SAP and SBEM are also anticipated in readiness for the new standards. There is no doubt that technology has an increasingly important role to play, but the fabric-first approach still underpins the zero-carbon strategy. For 2025, there is likely to be more of a focus on the dynamic nature of thermal performance, and less on the traditional steady-state approach.

This could provide new opportunities to use concrete to reduce peak heating and cooling loads and shift some of the daily energy demand away from periods of high grid-carbon intensity – something that the next major update to SAP (SAP 11) should help unlock. For housebuilders, detailed technical guidance and examples can be found in Part L 2021 – Where to Start, published by the Future Homes Hub.

Photos: Hufton + Crow, Dr Jerry Harrall, Willmott Dixon, Hawkins\Brown, Gusto Homes

Image above

Howgate Close in Nottinghamshire is a collection of nine lowenergy rural homes developed by landowner Dr Chris Parsons. About 50,000kWh of renewable electricity will be generated annually from 138 roof-mounted photovoltaic panels. The concrete structure helps to store heat and regulate the internal temperature, and has achieved exceptionally low air pressure tests of 0.35-0.77m3/(h.m2) @ 50Pa

Image above

Willmott Dixon’s Aurora office development in Bristol, designed by Bush Consultancy. When completed in 2018, the building achieved a 37% energy saving against Part L targets. The energy strategy involved watersaving technologies, a rooftop PV array and using the thermal mass of the concrete frame to minimise peak loads

Image above

South-facing balconies at Hawkins\Brown’s Agar Grove building in north London, part of the largest Passivhaus development in the UK. The balconies provide shading in the summer while allowing some benefit from solar gain in the winter

Image above

Gusto Homes’ Woodlands Edge development near Lincoln, where the glazing strategy minimises heat loss from north-facing elevations (pictured) and maximises winter heating from south-facing windows. Under Part O, dynamic thermal modeling will enable such approaches to balance passive solar gain during the heating season with the overheating requirements set out in CIBSE TM59