Insulation’s role in the Future Homes and Building Standard (FHBS)

The Future Homes Standard was first proposed by government in 2019. Despite plans for implementation in 2025, as of the time of writing, the final details of the Future Homes and Buildings Standard (FHBS) have yet to be confirmed.

Compared to a home constructed to Part L 2013, the Future Homes Standard should deliver new homes that emit 80% less carbon emissions compared to homes built to Part L 2013 regulations. These homes are intended to be built to a performance level that will not require further retrofitting to meet operational net zero targets as the electricity grid continues to decarbonise. Parallel proposals are being developed for new non-domestic buildings.

Is the Future Homes and Building Standard going to happen?

“What do we know about the timing of the Future Homes Standard?” asked the Future Homes Hub in January 2025¹ . Based on various official statements, the FHBS will happen – with expectations that it will still be in 2025. Current consultation documents provide a strong indication of the likely scope of the standard.

In an October 2024 blog post rebutting claims that low carbon building standards would be watered down, MHCLG said: “The final standard has yet to be determined but will be shaped to ensure that new homes can be designed in keeping with local architectural practices, whilst also balancing energy bills, emissions, energy demand and construction costs.”²

The road to the FHBS standard started with a 2019 consultation on ‘interim’ changes to the Building Regulations. These changes were subsequently introduced in 2021, in the form of revised Parts L and F, and a new Part O. As part of the move towards the electrification of buildings, Part S for EV charging infrastructure was also implemented.

A further consultation took place in 2023 to formally seek views on the final form of the FHBS. The Labour government elected in 2024 has indicated it does not intend to re-consult on the FHBS, hence the expectation that what is finally introduced will closely resemble the consultation proposals.

What does the Future Homes and Building Standard include?

The question of what makes a building sustainable, net zero and/or fit for the future, is a complex one.

Even restricting the definition to energy efficiency and carbon emissions is not clear-cut. Does it mean upfront carbon, operational carbon or whole life carbon? Is the way building occupants use the building (‘unregulated energy use’) included in the definition?

And then there is the question of climate adaptability and whether buildings will be useable in the face of rising temperatures and more extreme weather.

The 2023 Future Homes Standard consultation³ outlined what was in and out of the scope of the proposed regulation changes. Embodied carbon was not part of the scope; nor was unregulated energy use. Likewise, water efficiency, biodiversity and nature recovery – all elements of a ‘more sustainable’ built environment – were all out of scope.

Changes already implemented via the 2021 Building Regulations updates included improved building fabric (including insulation), higher-specification windows to reduce heat loss, lower air permeability, and the introduction of low-carbon heating and hot water systems such as heat pumps5. The proposed FHBS consultation builds on these measures.

As well as improving energy efficiency regulations, 2021 also saw improvements made to Part F (Ventilation), and the introduction of Part O (Overheating). These concurrent changes were designed to improve the holistic performance of new buildings, and the 2023 FHBS consultation featured relatively little further updating to either.

Alongside the content of the FHBS, another significant change is the way in which dwellings will be assessed. The Home Energy model (HEM), which is proposed to replace SAP 10, has been designed to improve the accuracy of assumptions and input data at design stage. As a result, it has around 500⁴ data inputs, compared to between 80 and 100 used now, and aims to produce compliance calculations that are a better reflection of how the finished building will perform in use.

Despite the extent of changes introduced in 2021, and expected to be introduced in the near future, there are concerns with the FHBS. With regard to the residential proposals, the UK Green Building Council describes the options as “weak” and says they cannot “be considered genuinely future.”⁵ CIBSE, meanwhile, has expressed concerns around airtightness, ventilation, compliance targets and measures to address the performance gap.⁶

The proposed key requirements for meeting the Future Homes and Buildings Standard (as of April 2025)

The key requirements for meeting the Future Homes and Buildings Standard below are taken from what has been proposed so far by the government. The key requirements may be subject to change once the FHBS officially launches.

Building fabric efficiency

• Higher insulation standards: Walls, floors and roofs to achieve improved U-values.
• Improved airtightness: Reducing draughts by sealing gaps and cracks in the building fabric will be key to minimising heat losses.
• Reducing thermal bridging: Structural weaknesses where heat can escape must be addressed to maintain energy performance.

Low carbon heating and hot water solutions

• The Future Homes Standard (FHS) proposal encourages low-emission alternatives, such as air source and ground source heat pumps, for space heating, hot water and comfort cooling.
• Direct-emission fossil fuel heating systems, such as natural gas boilers, are not expected to meet compliance under the proposals.

Preparedness for renewable technologies

• While not a regulatory requirement, designers may consider incorporating features that enable future installation of renewable technologies such as solar photovoltaic (PV) systems. This could include provision for roof space, suitable structural design, and cable routing to facilitate later system installation, where appropriate

Electric vehicle charging capability

• Under current Part S regulations (introduced in 2021) all new homes must include either a fully installed EV charge point or pre-fitted cabling infrastructure to allow easy future installation.

Efficient ventilation systems

• The use of mechanical extract ventilation (MEV) or mechanical ventilation with heat recovery (MVHR) may be specified to achieve compliance with ventilation requirements while maintaining airtightness.

What is the role of thermal insulation in the Future Homes and Building Standard?

The FHBS sets the minimum level of performance that homes and buildings will need to meet. Clients, designers, specifiers and contractors will be free to exceed those minimums if they wish, but achieving better is impossible without first understanding the starting point.

The 2021 regulatory updates, and the energy efficiency proposals of the FHBS, constitute a more holistic approach to building specification and construction. For many, however, their entry into understanding the standards will be the answers to the questions: What U-values do I need to achieve? What thickness of insulation will those U-values require?

Perhaps surprisingly, the building fabric standards proposed for Part L 2021 were not radically different from what immediately preceded it in Part L 2013. Insulation is inherent to every element of a building, from ground floor to external walls to roof structures. However, beyond certain points, increasing insulation thickness delivers diminishing returns, and regulatory compliance depends on multiple interacting design factors.

Instead, differences lay in other areas, like thermal bridging, airtightness and achieving as-built performance.

To achieve the kind of carbon reductions required in 2021 and 2025, vs the 2013 baseline, renewable technology became a crucial part of performance specifications, in addition to good building fabric standards. Solar PV, air source heat pumps and waste water heat recovery all represent viable routes to making compliance achievable, complementary to familiar U-values.

How does the Future Homes and Building Standard impact new homes and building fabric specification?

The shift to lower U-values from Part L 2013 to Part L 2021 was not as pronounced as might have been expected. The FHBS is not expected to improve them further.

However, the changes to the compliance model under the FHBS are expected to result in tighter fabric specifications in practice. This is because reduced flexibility in trade-offs will require more designs to achieve or approach the notional fabric performance.

As part of that, there is one area in which real-world fabric specifications could change substantially. Despite the notional wall U-value for new dwellings expecting to remain at 0.18 W/m²·K, external walls could see relatively big leaps in U-value in practice.

Walls tend to have the biggest surface area of all the building elements, especially in semi-detached and detached properties. To keep building footprints down and to avoid reducing internal footprint, developers have tended to prefer thinner wall constructions – but the changes in regulations are impacting on insulation specifications for external walls.

This has been borne out by real world efforts to understand the implications of the FHBS.

Project 80 is the name given to Eco Drive, a development of 12 homes in Birmingham for Midland Heart Housing Association⁷ . Started in 2016, the project’s aim was to construct traditionally built homes to the intended FHBS, and see what lessons need to be learned for them to be a success. The project’s name derives from the target to cut emissions by 80%.

Project 80 featured three different house types. The shift in U-values for the ground floors and pitched roofs was not substantial compared to Part L 2021. It is the external walls where the biggest changes in specification occurred.

Performance monitoring and occupant feedback was an essential part of Project 80. The project’s interim report featured many useful recommendations that any designer or specifier looking to work to the FHBS would do well to note⁸. In particular, with regard to building envelope design and construction, it suggests the following.

• Wall compositions need to be viewed as a whole for low U-values, low air permeability and fire resistance. Bricklaying and wall tie requirements need to be considered for different insulation types, wall compositions and thicknesses.
• Good buildable junction details need to be available to ensure airtightness and minimise cold bridging to assist designers, energy assessors and small and medium-sized home builders.
• Quality control of installations for insulation and structure is extremely important and needs to be part of contracts, recognising difficulties of wall composition.

These conclusions, which form part of a much wider-ranging set of recommendations, help to illustrate the role that insulation products, and their manufacturers, have to play in delivering the UK’s future homes. You can also read more about the role of insulation specification in junction details and thermal bridging heat losses, and in reducing condensation risk, in our linked blog posts.

Masonry cavity wall insulation solutions to meet the Future Homes and Building Standard

Low building fabric U-values for floors and roofs are current practice, but thicker wall constructions will likely be an outcome of the FHBS’s requirements. How walls are insulated, and how that insulation interacts with adjoining elements, is key to delivering a building that performs to the FHBS – in theory and in practice.

As the predominant form of wall construction in UK housebuilding, specifying and detailing masonry cavity walls to deliver buildable constructions with low U-values and minimal thermal bridging is key.

Indicative compliance modelling suggests that wall thicknesses may increase beyond 350mm, and in some cases approach 400mm, depending on the insulation type, blockwork specification, and design approach adopted to meet or improve upon the notional 2021 U-values.

Consider a project seeking to exceed notional fabric standards in one area to offset design trade-offs elsewhere. Where full fill cavity insulation using mineral wool with a declared thermal conductivity between 0.035 and 0.036 W/m·K is specified alongside lightweight aerated concrete blockwork (with a typical thermal conductivity of 0.15 W/m·K), achieving a U-value of 0.15 W/m²·K may require cavity widths of approximately 200mm or greater. This could result in overall wall thicknesses approaching 400mm, subject to external finishes and specific construction details.

Where maintaining thinner cavity widths remains a design priority, lower thermal conductivity insulation materials such as rigid polyisocyanurate (PIR), Celotex’s Thermaclass Cavity Wall 21, for example, with declared thermal conductivity of 0.021 W/m·K, can enable the same U-value target of 0.15 W/m²·K to be achieved with cavity widths typically around 125mm, depending on the full wall specification.

The BBA-certified product (certificate 24/7312– full fill cavity walls) features a thermal conductivity of 0.021 W/mK. Installation can be with a 10mm residual cavity to accommodate mortar squeeze, or as a full fill solution with no residual cavity, giving project-specific options to suit the requirements of specifiers and installers.

Installation can be visually inspected during construction, allowing photographic evidence to be captured for compliance documentation where required under Part L

The installation can be checked on site to avoid gaps between boards in the main wall areas. Care can also be taken to ensure that junction details, such as where a roof meets the top of a cavity wall, are constructed correctly to deliver continuous insulation – vital for achieving as-built performance.

The nature of blown cavity insulation in new-build homes is that the inner and outer leaf masonry must be complete before the insulation can be installed. Unlike rigid board systems, cavity fill installations occur after both masonry leaves are complete, making visual inspection during installation more challenging.

The FHBS continues to generate significant discussion across the industry, with some stakeholders suggesting that the proposals may not fully address longer-term performance ambitions. Regardless of the final form of the regulations, it remains the responsibility of all parties involved in the design, specification, and construction process to ensure that buildings achieve robust performance outcomes, comply with regulatory requirements, and help to minimise the performance gap.

About Celotex

Celotex delivers technical expertise and digital specification tools with system-wide solutions, such as our online U-value calculator. Alongside, our technical resource centre includes regulatory compliance documentation with performance certificates. Backed by this documented performance data, our technical services team provides verified calculations, construction details and regulatory guidance to help in the creation of accurate specifications for standard and non-standard constructions alike.

¹ https://www.futurehomes.org.uk/what-do-we-know-about-the-future-homes-standard

² https://mhclgmedia.blog.gov.uk/2024/10/24/reporting-on-the-future-homes-standard-and-solar-panels/

³ https://www.gov.uk/government/consultations/the-future-homes-and-buildings-standards-2023-consultation/the-future-homes-and-buildings-standards-2023-consultation

⁴ https://www.linkedin.com/feed/update/urn:li:ugcPost:7302239437260750848/?commentUrn=urn%3Ali%3Acomment%3A%28ugcPost%3A7302239437260750848%2C7302266359432835072%29&dashCommentUrn=urn%3Ali%3Afsd_comment%3A%287302266359432835072%2Curn%3Ali%3AugcPost%3A730223

⁵ https://ukgbc.org/policy-advocacy/new-build-standards/future-homes-standard/

⁶ https://www.cibse.org/policy-insight/news/building-regulations-part-l-f-overheating-future-homes-and-future-buildings-standards

⁷ https://www.midlandheartgroup.org.uk/our-impact/project-80/

⁸ https://www.midlandheartgroup.org.uk/media/5x1m4src/csu_2023_159-public-80-project-report_v13_-interactive_19-07-23.pdf