Good insulation specification is about more than U-values

Thermal performance has always been a fundamental consideration when specifying insulation, but evolving building safety requirements and sustainability targets have added complexity to the process.

The introduction of the Building Safety Act and the gateway regime for higher-risk buildings has placed greater emphasis on making and evidencing key design decisions earlier in the project lifecycle. At the same time, design and construction teams are facing increasing pressure to understand embodied carbon and whole-life performance, alongside requirements relating to fire safety, moisture management, airtightness and energy efficiency.

For insulation, this means moving away from specifying a product based solely on its thermal performance, such as thermal conductivity, towards considering its contribution to the performance of the complete building system.

Most aspects of a building’s performance are determined by how individual products work together as part of a system. This is particularly important when considering the building envelope.

An external wall, for example, contains multiple layers and components, including insulation, cavity barriers and external cladding, all of which contribute to its overall performance. Thermal efficiency, fire safety, moisture control and acoustics must therefore be considered together.

To maximise performance, products need to be specified in relation to one another, rather than in isolation, as a change to one element can have implications elsewhere.

Thermal performance is a good example. A calculated U-value is influenced by more than the declared thermal conductivity of the insulation. The complete build-up, including structural elements, fixings, brackets, junctions and thermal bridges, needs to be considered.

Most aspects of a building’s performance are determined by how individual products work together as part of a system

The same principle applies to moisture management and airtightness. Insulation needs to work as part of a wider building fabric strategy that considers how moisture and air move through the construction. This includes the positioning of vapour control layers and membranes, as well as how junctions between different building elements are detailed. Looking at any one of these elements in isolation can give an incomplete picture of how the building will perform.

This is why system-level thinking needs to be at the core of design and specification from the earliest possible stage. Decisions made during concept and detailed design can influence wall thickness, interfaces, material quantities, construction sequencing and ultimately the performance achieved once the building is complete.

This shift towards earlier decision-making also means challenging traditional assumptions. A product or build-up that has been used successfully on previous projects may not necessarily be the right solution for a different building, location, construction method or set of performance requirements.

Beyond thermal performance, there are several other factors that specifiers need to consider, beginning with fire safety.

Fire performance needs to be considered at both product and system level. At product level, the reaction-to-fire classification of an insulation product is an important part of specification. This is determined in accordance with BS EN 13501-1:2018, which provides the classification system for the reaction-to-fire performance of construction products and building elements. It indicates how a product contributes to the development and spread of fire when exposed to fire.

Fire resistance, however, is a different consideration. It is assessed using different test and assessment methods and relates to the performance of a complete construction. For example, BS EN 1364-1 covers fire resistance tests for non-loadbearing walls. The resulting performance classification is based on how the complete wall construction performs under specified test conditions, rather than the performance of an individual component in isolation.

For specifiers, it is therefore important to understand both the reaction-to-fire classification of the insulation and, where required, the evidence demonstrating how it performs as part of the proposed system. Depending on the application, this may mean considering evidence from testing or assessment of the complete construction, rather than relying solely on the product’s individual classification.

Considering these requirements early can help ensure that the proposed insulation is appropriate for the wider fire strategy and that the evidence needed to support the system’s performance is available before key specification decisions are made.

Alongside fire safety, the environmental impact of materials is increasingly influencing project decisions, particularly where clients are working towards specific sustainability targets, such as reducing embodied carbon.

This shift towards earlier decision-making also means challenging traditional assumptions

Thermal performance tells us something important about how a building will perform in use, but insulation can also make a meaningful contribution to its overall environmental impact through embodied carbon. This should be considered alongside operational energy performance.

Environmental Product Declarations (EPDs) are an important source of information, but they need to be read carefully. Figures may be presented using different declared units, product quantities and assessment boundaries, so simply comparing global warming potential (GWP) figures from different EPDs does not necessarily provide a like-for-like comparison.

Specifiers need to understand what each figure represents, how it has been calculated and how it relates to the quantity and application being specified.

The objective should be to use environmental information as part of an informed whole-life carbon assessment of the building, rather than selecting a product based on a single carbon figure in isolation.

Practical considerations are equally important. The physical characteristics of an insulation product can influence much more than its performance once installed. Weight, density, dimensions, packaging and handling requirements can all affect how materials are transported, stored and installed.

On constrained urban sites, for example, the space required for deliveries and storage can become an important consideration. A product that can be transported and handled efficiently may also help simplify aspects of the construction process, reducing costs and environmental impact.

These considerations demonstrate why product selection needs to be viewed in the context of the wider construction process and building lifecycle. The right solution needs to be technically appropriate, but it must also work in the context of how the building will be constructed.

This reinforces why product selection should not happen independently of the wider design and construction strategy.

The physical characteristics of an insulation product can influence much more than its performance once installed

Although specification is becoming more complex, the answer is not necessarily to provide more information. It is about providing better-quality information and ensuring it is considered at the right stage, with the right people involved.

This is where early engagement with manufacturers can help.

Rather than approaching a manufacturer only once the design is established, project teams can engage technical specialists earlier, sharing the performance and environmental requirements they are trying to achieve. This creates an opportunity to explore different solutions, understand how products will interact with the wider system and identify potential issues while there is still flexibility within the design.

Manufacturers’ technical and specification teams can provide advice on how their products perform within the wider building system, helping architects and specifiers understand how insulation interacts with other components and how this can affect overall performance. This can include technical guidance and supporting evidence to inform specification decisions.

Thermal performance will remain an important part of insulation specification, but it is only one part of a much bigger picture.

As expectations around building safety, sustainability and whole-life performance continue to evolve, successful specification will increasingly depend on understanding how insulation contributes to the complete building system.

For architects and specifiers, that means challenging established assumptions, considering performance in the context of wider systems and engaging with technical expertise early enough to influence the design.

Ultimately, the shift is about redefining what good performance means and ensuring that decisions made at specification stage can deliver it.