CPD 7 2020: Building Regs Parts L and F: Planned Changes

This Kingspan-sponsored CPD explains the proposed changes to Parts L and F of the Building Regulations for England and Wales, on energy performance and ventilation.


The way we design and construct our built environment will need to change significantly over the course of this decade if the UK is to meet its commitment to achieve net-zero carbon emissions by 2050. With the construction industry already facing a significant challenge to upgrade the energy performance of the country’s historic building stock, it is important that new buildings do not add to this burden.

By upskilling now with a best practice fabric-first approach, project teams can get ahead of tougher regulations due in 2025

Later this year, in one of the first meaningful legislative steps since committing to reaching net-zero emissions by 2050 (95% in Wales), the UK government and the Welsh parliament are set to publish revised versions of Part L and F of the Building Regulations for new dwellings in England and Wales. Coming into force this year, these documents set the requirements for property energy performance and ventilation. They are designed to provide a meaningful stepping stone towards the considerably more arduous standards set to come into force in 2025. With this in mind, it will be important to consider not only how to build in compliance to meet the 2020 requirements, but also what steps need to be taken to prepare for the 2025 standards.

In this CPD, we look at some of the key proposals within the recent Parts L and F consultations in England and Wales, their limitations, and what steps construction practitioners should be taking to be ready for the 2025 standards.

Performance targets

It is important to note that control of the Building Regulations is devolved within the UK. As such, while the recent consultations in England and Wales both deal with Parts L and F of the Building Regulations for domestic properties, these are entirely separate documents with different requirements. Each is supported by guidance documents that provide recommendations on how to reach compliance. At this stage, only the requirements for new domestic properties have been consulted on, with non-domestic and refurbishment standards expected to follow soon.

The requirements within Part L are built around a principal performance metric, which each property must achieve. Historically, this has been a carbon emissions target for the property. The consultations provided two options for reducing this emissions target further. In the English consultation, these were for a 20% or a 31% (preferred) reduction over the existing requirements, while in Wales more ambitious targets of 37% (preferred) or 56% were set.

Although these options are expressed in terms of a reduction in property carbon emissions, they were actually developed around a new principal metric – called primary energy – with carbon emissions retained as a secondary metric. The reason for this change is that while controlling these emissions is important for the UK’s overall net-zero carbon programme, they are not a direct measure of energy efficiency. As work continues to decarbonise fuel, and ultimately to power homes solely through zero-carbon electricity, a low emissions figure will tell us little about how efficiently a property uses energy in operation.

The primary energy metric has been developed to provide a more accurate measure of overall energy usage, considering both the energy needed to prepare a fuel for use and the final energy demand of the property.

How primary energy is calculated

A primary energy factor (PEF) is first generated for each fuel type based on the energy used in upstream production activities. These can include:

  • Planting and cultivation of biofuel sources
  • Extraction
  • Processing and transformation
  • Transportation
  • Transmission (including distribution losses)

These PEF factors have been pre-calculated and are provided within the Standard Assessment Procedure (SAP) specification document. The property energy demand is then calculated for each specific use, including space heating, water heating and lighting, factoring in the efficiency of the technologies used in the home. The energy for each use can then be multiplied by the PEF of its fuel type. These figures are added together to give the total primary energy demand for the dwelling.

For example, primary energy demand for heating is calculated through the following equation:

(property energy demand / efficiency of heating technology) x PEF

Let’s assume the property is heated by 100% efficient electric panel heaters with an overall heating demand of 10,000kWh. The fuel factor for electricity is 1.501, so primary energy demand is calculated as follows:

(10,000kWh / 1) x 1.501 = 15,010kWh

Importantly, the primary energy calculation allows any energy generated by on-site renewable technologies, such as photovoltaic (PV) panels, to be subtracted from the overall energy demand. For example, if we included a PV array generating 1,500kWh specifically for use within the property described above, then the calculation would be changed as follows:

([10,000kWh – 1,500kWh] / 1) x 1.501 = 12,759kWh

Structural insulated panels provide excellent thermal performance, while their offsite manufacture makes them compatible with a fast-track build programme

Removal of the Fabric Energy Efficiency Standard

There is little question that primary energy should provide a more accurate estimate of property energy efficiency than reliance on carbon emissions. However, since the publication of the consultations there has been considerable criticism of the way it is implemented within the English draft.

At present, the English Approved Document for new dwellings (ADL1A) uses the Fabric Energy Efficiency Standard (FEES), which sets a clear minimum energy performance target for the fabric elements of a building. To reduce complexity following the introduction of primary energy targets in the 2020 version of Part L, the consultation argued that FEES should be removed. This would mean that the only direct control on the fabric performance of a building would be the worst-case backstop U-values. This approach is already used in Wales as the FEES was never introduced there. However, the backstop U-values proposed in the English consultation are less demanding than even the current Welsh equivalents (which have been considerably tightened within their own consultation as shown in table 1 below).

Heat loss elementsCurrent England ADL1ACurrent Welsh ADL1AEngland 2020 proposedWales 2020 proposed
External walls0.30W/m²K0.21W/m²K0.26W/m²K0.18W/m²K (flats 0.21W/m²K)
Flat and pitched roofs0.20W/m²K0.15W/m²K0.16W/m²K0.13W/m²K
Table 1: Area weighted worst-case backstop U-values for new domestic buildings

As the primary energy calculation allows the contribution from on-site renewable energy to be subtracted from the building’s overall demand, this could allow homes to be built to these backstop U-values across the board. This means that homes built under the 2020 Part L with on-site renewable generation would have a higher heating demand than those built under the current standards using FEES.

This not only seems at odds with the fundamental purpose of revising Part L but could also exacerbate the performance gap between the designed and actual energy performance of properties because of the adoption of heat pumps for heating and hot water.

During the winter, when demand for heating and lighting is high, air-source heat pumps are less effective because of the low temperatures. Output from PV is less too, because the days are shorter and the sun angles lower. As such, if homes waste energy, carbon intensities could actually be higher than at present during these periods.

This issue will also affect fuel bills. The Committee on Climate Change (CCC) estimated in its consultation submission that the current proposals could lead to household bills that are 50% higher than for homes completed under the existing requirements in England. This figure does not even consider the ongoing maintenance costs of any on-site technologies and could ultimately push many towards fuel poverty. Minimising energy demand through fabric efficiency is the first and arguably most crucial step in affordability, as the best way to save money on energy bills is to not use any at all.

Construction practitioners, in both England and Wales, must also look at these changes in the context of the next five years. The English consultation document states that the 2025 standard is expected to require carbon emissions from domestic properties to be between 75% and 80% lower than at present. At that point, achieving a high level of fabric performance can be expected to be a mandatory requirement. As such, rather than attempting to mask leaky, energy-intensive homes through renewable technologies, it is crucial to use the next five years to raise construction practice and develop supply chains for the solutions that will be eventually required.

To guide this upskilling process, it is useful to look in a little more detail at what the requirements are likely to be in 2025.

The 2025 requirements

Both the English and Welsh consultations stated that homes will have to combine the use of low-carbon heating systems – such as heat pumps, district heating or electric heaters – with a specification that meets very high fabric standards. This is expected to require the use of triple-glazing and for building elements to meet the U-value limits shown in table 2 below.

ElementExpected U-value requirements in Welsh Part L 2025Expected U-value requirements in English Part L 2025
External wall0.13W/m²K0.15W/m²K
Table 2: Expected minimum U-value requirements for Part L 2025 in England and Wales

The CCC has provided further detail on what can be expected as we move towards net-zero homes. It states that, in addition to the above, homes will need to achieve high levels of airtightness supported with mechanical ventilation heat recovery (MVHR), where possible be timber-framed, and have a space heating demand of 15-20kWh/m2/yr.

To place this figure in context, today’s average UK home has been estimated by the Passivhaus Trust to have a space heating demand of around 130-140kWh/m2/yr. While this represents a significant change, it is by no means unachievable. Looking internationally, in Brussels net heating requirements have been limited to 15kWh/m²/yr since 2015, and by 2020 Denmark’s space heating demand in residential buildings is to be limited to 20kWh/m²/yr.

Perhaps more significantly, a space heating demand of 15kWh/m²/yr is also a requirement of the voluntary Passivhaus energy performance standard.

As with the CCC recommendations, Passivhaus requires properties to be highly airtight – achieving an air-leakage rate of just 0.6ach @ 50Pa – with ventilation typically provided through MVHR. In addition to being highly insulated, the property must also be virtually thermal bridge free, requiring close attention to detailing. Primary energy demand must also be limited to ≤ 120kWhm2/yr and specific cooling load to ≤ 10W/m2.

Before a property can be certified, it must be carefully assessed to ensure its actual performance matches expectations. This rigorous approach helps to uphold construction standards and means that Passivhaus properties consistently meet or outperform their energy performance targets.

In the past, the Passivhaus standard was seen as being largely the preserve of self-build projects. Increasingly, however, it is being used on large-scale developments – the Stirling prize-winning Goldsmith Street housing scheme in Norwich being the most obvious example.

These developments not only show that these requirements are achievable at scale but also give some indication of the approaches and methods needed to meet them. For example, offsite solutions such as structural insulated panels (SIPs) are a popular option for Passivhaus, as their modular construction and offsite production supports faster build programmes while also providing excellent thermal performance, airtightness and insulation continuity. This can help to limit the burden on site workers – a key requirement as the skills shortage is set to continue – and to ensure that detailing does not let the overall quality of construction down.

The CCC analysis shows that if action is taken now to start investigating these approaches and upskilling workers, it would be achievable (and beneficial) to limit space heating on all new homes to Passivhaus levels (15 kWh/m2/yr) by 2025.

A workable approach for 2020

To start moving towards these future requirements, it makes sense for project teams to adopt a fabric-first approach, focusing on getting the most from the building envelope. Once these approaches have been standardised, it will be much simpler to upgrade the building construction methods to incorporate future requirements such as the use of low-carbon heat sources or PV generation.

While not compulsory, the suggested fabric parameters provided within the notional dwelling for Option 1 of the English consultation and Option 1 and 2 of the Welsh consultation provide a reasonable starting point, with the U-values matching the expected limiting values for the 2025 standards (see table 3 below). The airtightness requirements within the Welsh Option 2 also offer a sensible step towards 2025, allowing project teams to start familiarising themselves with airtightness approaches and how MVHR systems should be designed and installed effectively.

2020 England proposal
Option 1
2020 Welsh proposal
Option 1
2020 Welsh proposal
Option 2
External walls0.150.130.13
Flat and pitched roofs0.110.110.11
Thermal bridgingGlobal value Y = 0.05; or individual values from SAP table R2 (Option 1 values)Global value Y = 0.05; or individual values from SAP table R2 (Option 1 values)Global value Y = 0.05; or individual values from SAP table R2 (Option 1 values)
Air permeability553 + mechanical ventilation heat recovery
Photovoltaics installesNoYesYes
Table 3: Selected notional dwellings building parameters

It is also worth noting that both consultations identified the need to address detailing to prevent thermal bridges undermining the overall performance of the building envelope. For this reason, they recommend that the Approved Construction Details (ACDs) be removed from the new versions of Part L as these will not work with the new fabric standards. The global backstop thermal bridging level values used within SAP will also worsen. To avoid having to use these, it will be necessary for project teams to either model/calculate their own thermal bridging values or to use model construction details from reputable non-government databases containing independently assessed thermal junction details or following details supplied by manufacturers. This ultimately means that paying attention to details will be crucial to achieving compliance.

Kingspan TEK structural insulated panels were used to build housing at the Rayne Park estate near Norwich, where 112 homes are expected to be certified to the Passivhaus standard

Getting the basics right

The changes to the Part L requirements for new domestic buildings in England and Wales are just the first step, with consultations also due on the requirements for existing dwellings as well as for all non-domestic buildings. It is important for firms to be early adopters and to develop the skills that will help them to handle these more rigorous requirements across a range of projects in the future. By getting the basic fabric of the building right, it should be much simpler to implement any additional requirements and to deliver buildings that consistently perform as expected and can be constructed to a reliable schedule.

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