Sponsored by Rockwool, this CPD module explores how evolving building regulations are driving improved thermal performance and how stone wool insulation can help narrow the performance gap between design intent and real-world outcomes.
Deadline for completion Friday 27 June 2025.
Despite decades of innovation in building materials, energy modelling and regulations, one stubborn truth remains: too many buildings still underperform. The design promises one level of energy efficiency – on paper – but the built reality often tells another story. This so-called performance gap is not just a technical oversight; it is a systemic issue that impacts energy bills, occupant comfort and the UK’s ability to meet its net zero goals.
Whether due to rushed installations, inconsistent workmanship or unforeseen site conditions, even small flaws – like an overlooked air gap or a poorly insulated junction – can seriously compromise thermal performance. In a world where every kilowatt counts, closing this gap has become one of the most urgent and impactful challenges for the built environment sector.
The stakes are higher than ever. The UK government has set legally binding targets to achieve net zero carbon emissions by 2050. Achieving this relies heavily on reducing energy use in buildings – which in turn demands a shift from design intent to actual performance. Developers, contractors and specifiers all have a part to play in ensuring that new buildings are not only well-designed but also executed with care and technical precision.
This module explores how regulation, material choice and best-practice installation can work together to ensure that buildings not only meet but maintain their performance targets – both today and decades into the future.
Objectives
- Understand the evolution of UK thermal performance regulations, including the latest updates under the Future Homes Standard and Approved Document L and how these impact design and construction.
- Identify common causes of the performance gap between designed and as-built energy efficiency and evaluate how design simplicity and installation quality can mitigate these issues.
- Evaluate the role of stone wool insulation in improving building performance, including its durability, thermal resilience and suitability for fabric-first and Passivhaus design approaches.
- Apply best-practice principles for insulation installation to ensure continuity, minimise thermal bridging and support long-term compliance with energy efficiency standards.
Understanding thermal regulation
Thermal performance in UK buildings has long been shaped by regulation. U-value limits were first introduced in the 1965 Ð԰ɵç̨ Regulations for England and Wales, applying initially to dwellings and establishing minimum thermal insulation standards that have driven industry practice ever since.
In the post-war years, thermal performance standards were minimal. For example, the maximum allowable U-value for lofts was 1.4 W/m²K. However, due to the urgency of the national reconstruction effort, these standards were often ignored in favour of speed and cost, and by 1967 the National Housing Survey found that much of the UK’s housing stock was in worse condition than anticipated.
A shift began in 1976 when minimum insulation requirements were formally introduced, marking the start of more enforceable thermal regulations. From that point onwards, standards became progressively stricter, with U-value limits for cavity walls being tightened in 1985, in 1990 and again in 2002 as energy efficiency became a growing national priority.
However, these were not just regulatory ticks – each update represented a growing awareness that the building fabric has a profound and lasting impact on energy use. As heating technology evolved and fuel prices rose, thermal comfort and sustainability expectations also shifted.
The 2006 revision marked a move away from purely elemental requirements to whole-building performance. For the first time, buildings were required to meet carbon dioxide emission targets, assessed using a standard energy model. Limits on U-values and air permeability rates were also introduced.
These new metrics aimed to encourage more holistic thinking, where insulation, heating systems, airtightness and ventilation all worked together as part of a co-ordinated strategy for energy reduction.
The performance gap emerges
Despite tightening standards, a growing body of evidence showed that as-built homes often failed to meet their designed energy performance. In 2012, the government commissioned the Zero Carbon Hub to investigate. Its 2014 report revealed a consistent gap between predicted and actual energy use.
Among the contributing factors were inconsistent installation, poor detailing and thermal bypass. Research by Belgian professor J Lecompte in 1990 showed that even a 6mm air gap could increase heat transfer by over 150% – a striking illustration of how on-site errors can undermine design intent.
The report also noted that many project teams lacked clarity on who was responsible for energy performance at each stage, from design to handover. This fragmentation led to weak accountability and inconsistent quality checks.
Since then, the performance gap has been recognised not as a single issue with a single fix but as a result of many small failures across a project lifecycle. Addressing this has meant rethinking how products are selected, how site teams are trained and how performance is verified in practice.
Progress has been made – but the problem persists, particularly in volume housing where build speed and cost efficiency can still outweigh quality assurance.
| Part L1 2021 (England) | |
|---|---|
| Wall | &²Ô²ú²õ±è;0.18°Â/³¾Â²° |
| Roof | &²Ô²ú²õ±è;0.11°Â/³¾Â²° |
| Floor | &²Ô²ú²õ±è;0.13°Â/³¾Â²° |
| Party wall | &²Ô²ú²õ±è;0.00°Â/³¾Â²° |
| Part L1 2022 (Wales) | |
| Wall | &²Ô²ú²õ±è;0.13°Â/³¾Â²° |
| Roof | &²Ô²ú²õ±è;0.11°Â/³¾Â²° |
| Floor | &²Ô²ú²õ±è;0.11°Â/³¾Â²° |
| Party wall | &²Ô²ú²õ±è;0.00°Â/³¾Â²° |
| Section 6 2022 (Scotland) | |
| Wall | &²Ô²ú²õ±è;0.15°Â/³¾Â²° |
| Roof | &²Ô²ú²õ±è;0.09°Â/³¾Â²° |
| Floor | &²Ô²ú²õ±è;0.12°Â/³¾Â²° |
| Party wall | &²Ô²ú²õ±è;0.00°Â/³¾Â²° |
| Technical Guidance Document L 2022 (Ireland) | |
| Wall | &²Ô²ú²õ±è;0.13°Â/³¾Â²° |
| Floor | &²Ô²ú²õ±è;0.11°Â/³¾Â²° |
| Roof | &²Ô²ú²õ±è;0.14°Â/³¾Â²° |
| Party wall | - |
| Technical Booklet F1 2022 (Northern Ireland) | |
| Wall | &²Ô²ú²õ±è;0.15°Â/³¾Â²° |
| Floor | &²Ô²ú²õ±è;0.13°Â/³¾Â²° |
| Roof | &²Ô²ú²õ±è;0.13°Â/³¾Â²° |
| Party wall | &²Ô²ú²õ±è;0.02°Â/³¾Â²° |
Raising the bar
An uplift to Part L of the Ð԰ɵç̨ Regulations, in force since June 2022, has brought further changes across the UK. These updates are part of the roadmap toward the Future Homes Standard in England, due to take effect in 2025.
Key changes include:
- Compared with the previous standard, a 30% reduction in carbon dioxide emissions for new homes and 27% for non-domestic buildings
- Introduction of a primary energy metric to assess energy use
- New minimum efficiency standards for thermal elements, windows and doors
- A maximum flow temperature of 55ºC for wet heating systems
- Greater emphasis on compliance evidence, including photographic records.
Photographic documentation, as outlined in Appendix B of Approved Document L, must show:
- Type and continuity of insulation
- Airtightness and thermal bridge detailing
- Insulated pipework and building services.
These images must be reviewed by the energy assessor during construction and provided to the building owner at handover, alongside the energy performance certificate (EPC) and Ð԰ɵç̨ Regulations England Part L (BREL) report.
These regulatory changes mark a significant shift in accountability for both design and construction teams. The increased emphasis on insitu verification is intended to ensure that what is specified on paper is properly executed on site. As a result, project teams must now adopt a more co-ordinated approach, involving quality checks at every stage of the build.
This shift also underscores the growing need to upskill site operatives and installers to understand and deliver thermal detailing correctly. Consistent, robust installation practices are no longer optional – they are fundamental to meeting today’s performance benchmarks and building long-term confidence in the finished product.
Fabric-first design
With around 40% of the UK’s greenhouse gas emissions linked to the built environment – half from homes – improving building performance is essential for achieving net zero targets.
The Future Homes Standard is designed to reduce emissions from new homes through a combination of low carbon heating (such as air-source heat pumps) and high-performance building fabric. In England, this will be mandatory from 2025. While devolved administrations are pursuing similar outcomes, their approaches and performance benchmarks will differ slightly.
Improved insulation is central to these goals, helping to retain heat, cut energy demand and reduce running costs – all while delivering comfortable, resilient homes.
The increasing push for performance-focused design has placed a spotlight on fabric-first principles. This approach focuses on reducing the building’s energy demand through superior insulation and airtightness, before introducing renewable energy technologies. Such methods prioritise the building’s structure and material properties, ensuring it functions as efficiently as possible throughout its lifespan, irrespective of the heating or cooling systems installed.
Closing the gap on site
Correct installation is just as important as good design. According to tests by BRE Group, poor workmanship can increase a U-value by as much as 310%. Common issues include:
- Gaps between insulation layers
- Poor continuity at junctions
- Loft insulation poorly installed or displaced
- Debris in cavity walls
- Incorrect window and door positioning.
These faults are rarely visible post-completion but can significantly undermine thermal performance over time. To reduce this risk, designs should prioritise simplicity, minimising thermal bridges and potential air leakage points.
A continuous layer of insulation, effective detailing around eaves and loft hatches, and using breathable membranes with appropriate ventilation can help ensure long-term energy performance.
By addressing these common issues upfront, designers and contractors can ensure a building’s thermal performance remains intact for years, delivering the energy savings and occupant comfort it was designed to. These measures also support the long-term durability of the building fabric, which is essential in the face of increasingly extreme weather events due to climate change.
Learning from Passivhaus
The Passivhaus standard exemplifies a fabric-first approach, focusing on building envelope performance to reduce the energy demand for space heating and cooling. This approach is particularly important in a world where the push for sustainability and reducing carbon footprints is more critical than ever.
Key requirements include:
- Space heating: ≤15kWh/m²/year or ≤10W/m² peak
- Primary energy demand: ≤60kWh/m²/year
- Airtightness: ≤0.6 air changes/hour at 50Pa
- Thermal comfort: ≤10% of occupied hours over 25°C.
These metrics ensure comfort, resilience and energy efficiency, often exceeding standard compliance levels. Ð԰ɵç̨s designed to Passivhaus standards go beyond basic regulations, demonstrating that achieving near-zero energy use is possible with the right focus on thermal performance. The process prioritises airtightness, continuous insulation and the elimination of thermal bridges – ensuring that heat does not escape through poorly insulated areas. This creates a living environment where internal temperatures are maintained with minimal reliance on active heating systems.
By applying these principles, we can reduce energy consumption, enhance occupant comfort and create buildings that are more resilient to the effects of climate change.
Ð԰ɵç̨ for climate resilience
The UK’s changing climate poses a growing risk to homes, with overheating now recognised as a significant challenge in new dwellings. As extreme weather events become more frequent, designing buildings that stay comfortable and resilient will be crucial.
Homes must now withstand fluctuations in temperature, humidity and wind, while still maintaining low operational energy use. Achieving this requires a comprehensive approach to building design, where insulation works alongside other systems such as ventilation, airtightness and shading. High-performance insulation plays a key role in mitigating overheating and energy waste, as it regulates the temperature inside buildings by providing a thermal buffer against external weather extremes.
Long-term performance matters
Stone wool insulation offers long-term benefits beyond just U-values. Its dimensional stability helps prevent gaps forming over time, maintaining performance across decades.
Key indicators of durability include:
- Thermal conductivity
- Compressive and tensile strength
- Fire and acoustic performance
- Water resistance and moisture tolerance.
Durable insulation reduces the need for maintenance and refurbishment, improving whole-life performance. This ensures that buildings maintain their intended energy efficiency for years, reducing the likelihood of performance gaps as materials settle or degrade over time. The increasing focus on whole-life carbon assessments also underscores the importance of considering the longevity of materials used in construction.
Independent testing is crucial. Real-world U-values should be verified before and after service to confirm performance. Reputable third-party validation ensures consistency and accuracy in claims. This guarantees that insulation will perform consistently, even in buildings subjected to constant movement or stress due to temperature changes, moisture or structural settlement.
Proving the performance
To better understand how stone wool insulation performs in real-world conditions, Rockwool collaborated with the University of Salford Thermal Measurement Lab. The study they carried out found that tightly joined stone wool slabs form a continuous insulating layer. The knitted structure traps air and prevents gaps – preserving the designed thermal performance in situ.
Because stone wool maintains its shape even as surrounding materials expand and contract, it supports long-term energy efficiency, especially in buildings subject to movement. This is crucial, as buildings are dynamic, with changes in temperature, humidity and movement over time. The performance of the insulation must not be compromised by these factors.
Copenhagen Airport
A powerful example of long-term insulation performance comes from Copenhagen Airport’s Hangar Four, which was originally built in 1958 with 75mm of stone wool in its facade. During recent renovations, Rockwool and the Danish Technical Institute tested the original insulation – finding that after 65 years, its thermal performance remained unchanged.
This level of stability underscores why building fabric is at the centre of modern regulations. For specifiers looking to futureproof buildings and close the performance gap, stone wool delivers a durable and proven solution.
Final thoughts
Achieving real-world energy efficiency requires more than just compliant designs – it demands consistent execution, durable materials and a deep understanding of how each element interacts within the building fabric. As regulations tighten and expectations rise, the gap between design intent and as-built performance must be closed.
Stone wool insulation offers a robust solution, not only through its inherent thermal and acoustic properties but also through its long-term dimensional stability and resilience to climaterelated stresses. By combining high-quality materials with best-practice installation and a fabric-first mindset, building professionals can deliver homes and buildings that truly perform – now and for decades to come.
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