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Building regs what’s changed: Part L, F and introducing Part 0 on ventilation

Stephen Cousins

A mandatory 30 per cent cut in carbon for all new homes is among a range of changes to the Building Regs. RIBA Journal summarises the key points.

In the shift towards a Future Buildings Standard, the government has introduced a range of changes to the Building Regulations, including a mandatory 30 per cent cut in carbon for all new homes and a 27 per cent cut for other buildings, including offices and shops.

The Department for Levelling-Up, Housing and Communities (DLUHC) set out the changes, which include interim uplifts to Part L and Part F, and the introduction of Part O, in its response to a public consultation on the Future Buildings Standard, concluded in April.

Approved Documents F (volume 2), L (volumes 1 and 2) and O will come into effect on 15 June 2022. However they will not apply in relation to building work on a particular building, where a building notice or an initial notice has been given to, or full plans have been deposited with, a local authority, in respect of that building, before 15th June 2022, provided that the building work on that building is started before 15th June 2023.

Alongside amendments to the Building Regs, five new approved documents are published:

Printed versions of the new approved documents can be purchased from

Part L - Interim uplifts to standards for non-domestic buildings 

Non-domestic buildings must achieve an average of 27 per cent reduction in CO2, relative to 2013 standards. This is the more ambitious of two options put forward by the government in its consultation, the first was for a 22 per cent reduction.

The government said the mandate would achieve a balance between making progress towards the Future Homes Standard, due in 2025, while providing the industry 'with the time it needs to develop the supply chains and skills that will be necessary and accounting for market factors'.

Part L amendments introduce a new principal performance metric measuring energy efficiency. ‘Primary energy’ will be used in combination with COmetrics to assess compliance with Part L. Primary energy calculations take into account factors such as the efficiency of the building’s heating system; power station efficiency for electricity; and energy used to produce fuel and deliver it to the building. 

The majority of respondents to the consultation (62.2 per cent) said they disagreed with using primary energy as the main performance metric and the government has said the approach will be reviewed before implementation of the full Future Buildings Standard.

The new uplifts introduce new minimum efficiency standards for both new and replacement thermal elements, windows and doors. In most cases these will be set at levels proposed in the consultation. For example, the U-value of new walls is 0.26W/m2 K, compared to 0.35W/m2 K previously. Most types of new window, roof window and curtain walling must achieve a U value of 1.6, versus 2.2 previously.

Most respondents to the consultation said they disagreed with using primary energy as the main performance metric and the government says it will review the approach

Turning to building services in new non-domestic buildings, the minimum efficacy of lighting installations in new non-domestic buildings has been increased to 95 luminaire lumens per circuit watt for general lighting and 80 luminaire lumens per circuit watt for display lighting. Lower efficacies in some rooms can be offset by higher efficacies in others.

A separate standard for lighting that requires a high level of optical control, including innovative high excitation purity lighting, has been introduced.

New non-domestic buildings now require a building automation and control system if they include a heating or air-conditioning system of 180kW or over, rather than 290kW originally proposed. 

A new minimum standard is introduced to ensure that wet space heating systems in new buildings are designed to operate with a maximum flow temperature of 55°C – considered important for system efficiency.

All space heating and domestic hot water boiler installations in existing non-domestic buildings must now include controls to improve the effective efficiency of the system. The minimum standards for air distribution, comfort cooling systems and lighting will also apply in existing non-domestic buildings. BACs must also have a maximum flow temperature of 55°C.

Part L adopts CIBSE’s TM23 as the single approved methodology for testing airtightness for non-domestic buildings to avoid 'practical difficulties' of using multiple testing methodologies.

Part F - Interim uplifts to standards for non-domestic buildings

New guidance includes standards on minimising the ingress of external pollutants and the proper installation of ventilation systems.

New guidance on performance-based ventilation standards will allow designers to assess ventilation strategies against individual volatile organic compounds, based on data from Public Health England, as an alternative route to using a total VOC limit. 

Part F recommends that all replacement windows in non-domestic buildings are fitted with background trickle ventilators unless it can be shown that replacement windows would not reduce useful ventilation or that a mechanical ventilation system is present. Where outside noise is an issue, attenuating background ventilators should be fitted.

Regarding transmission of infection via aerosols, there is a new requirement for the installation of CO2 monitors in offices and specifically in ‘high risk’ rooms where there may be a risk of airborne infection.

New guidance recommends that mechanically ventilated common spaces in offices have a minimum air supply rate of 0.5 l/s.m2, which is below the 1 l/s.m2 outlined in the government’s proposals.

Amended guidance on recirculating systems states systems should be capable of operating in a mode that prevents the recirculation of air within spaces or between different spaces, rooms or zones within offices, unless suitable filtering or cleaning systems are in place.

Government proposals to increase required ventilation capacity in offices and specify ventilation rates in ‘high risk’ rooms in response to Covid-19 have not been adopted. This is in anticipation of more concrete evidence on the impacts.

Part O - Standards for overheating in new residential buildings 

Part O aims to ensure that new residential buildings, including homes, care homes, student accommodation and children’s homes are designed to reduce overheating. It splits England into areas of 'moderate risk' and 'high risk' of overheating, the latter including urban and some suburban parts of London. 

The regulation adopts a 'simplified' route to compliance based on minimising solar gain and removing excess heat. It sets standards based on whether the house or residential unit is cross-ventilated, considers orientation and introduces a standard for the maximum amount of glazing allowed in a single room. 

Dynamic thermal analysis methods of overheating risk in homes allows more sophisticated analysis of buildings as an alternative route to compliance over the simplified method.

Guidance includes acceptable strategies for limiting unwanted solar gain in the summer through shading and other means. Internal blinds or tree cover must not factor into a dynamic thermal assessment because they can subsequently be removed.

Part O adopts measures to ensure overheating strategies are safe and usable by occupants, taking into account noise and pollution near the home, as well as the safety and usability of the windows and security, which may affect occupant behaviour. Information on overheating strategies must be passed to the building owner in the form of a Home User Guide.

Part L - Standards for domestic buildings 

Minimum new fabric efficiency standards are being introduced for new and replacement thermal elements, windows and doors in existing homes. For example, the U value for walls is tightened, from 0.28W/m2 K to 0.18W/m2; and for windows and rooflights from 1.6, or Window Energy Rating Band C, to 1.4 or Window Energy Rating Band B. The U value for doors is cut from 1.8 to 1.4. However, fire doors are permitted to meet a U-value of 1.8 W/m2K in line with the previous standards.

The government has adopted a 'full fabric specification' for setting the level of the Fabric Energy Efficiency Standard (FEES) in new homes under Part L. This is despite the fact that almost half of respondents to the consultation (49.8 per cent) wanted a higher FEES than either of the options on the table. Extensions to existing properties must now adhere to the SAP method of compliance for metrics of fabric energy efficiency and primary energy. According to the government, this will ensure that 'direct electric heating systems are not used in unsuitable circumstances resulting in high bills for householders. 

In the section on building services, wet-space heating systems in existing domestic buildings must be designed to operate with a maximum flow temperature of 55°C, as with non-domestic buildings.

Part F - Standards for existing domestic buildings

The regulation introduces a new requirement that when energy efficiency work is done in buildings, the ventilation is not made any worse, in line with existing measures for controlled services and fittings. 

A mandated checklist is intended to make it easier for renovators to understand the impact of historic and potential future work to a building and whether the ventilation provision will be sufficient.

Ventilation guidelines will include a recommendation that replacement windows are fitted with a background trickle ventilator, unless it can be proven that the ventilation was not made worse. 

To support homeowners, Part F now recommends that all installations of mechanical extract ventilation and installations of new background ventilators come with guidance on why ventilation is important for the health of buildings and their occupants.

A commissioning sheet and checklist, including design flow rates and maintenance requirements, should also be provided when ventilation systems are installed. 

Looking Ahead: Future Buildings Standard

Implemented in 2025, the Future Buildings Standard will aim to produce non-domestic buildings running on low-carbon heat with the best possible fabric standards. No further energy efficiency retrofit work will be necessary to make buildings zero-carbon as the electricity grid decarbonises.

A full technical consultation on the Future Buildings Standard is planned to start in 2023, including proposals for the technical detail and associated draft guidance.

Without regulation of actual energy use, the built environment will not decarbonise at the rate required

Industry concerns

The interim uplifts to Part L and F of the Building Regulations and the introduction of Part O received a tepid response from built environment professionals.

Reacting to the news, RIBA president Simon Allford, said: 'These uplifts will bring us one step closer to decarbonisation, and we welcome that. The new minimum standards for fabric efficiency and new Part O signal real progress, but without regulation of actual energy use, the built environment will not decarbonise at the rate required. Regulations must continue to tighten. I look forward to seeing the full document and working with government to ensure the 2025 Future Homes and Buildings Standards address the urgency of the task at hand.'

Speaking on behalf of the London Energy Transformation Initiative (LETI), Alex Johnstone, an architect at Haworth Tompkins, told RIBAJ: 'The interim uplift to Part L does not tally with what the industry knows is critical to enable significant reduction in carbon emissions in new buildings. Primary Energy has been introduced as a performance metric. This is a confusing metric whose calculation factors will change over time. LETI propose that Energy Use Intensity (EUI) is used as the actual at-the-meter energy reading. We need clear and meaningful metrics to prevent constructing new buildings that will require retrofit in order to meet net zero targets.'
He added: 'Architects’ decisions for form and orientation of buildings will greatly impact a building’s energy use. The regulation is failing to provide a meaningful framework to drive design changes that will lead to necessary reductions in environmental impact.'

Visit Eco Drive, West Midlands and see the first houses built to the expected baseline of the Future Homes Standard.

Read more about designing for good ventilation. 


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