Comparing insulation: which one is right for your project?

Effective thermal insulation is an important part of building design. It plays a crucial role in reducing energy consumption, improving occupant comfort, supporting sustainability goals and meeting regulatory requirements. Understanding how insulation works, the various types available and how performance is measured can help architects, specifiers and builders compare insulation types effectively and make informed choices.

In this guide, we will explore some of the key principles of thermal insulation, explain common performance terminology and highlight the main considerations involved in choosing an insulation product for a specific project.

How does thermal insulation work?

Heat is transferred via conduction, radiation and convection. These three processes can be used to heat buildings, but they are also responsible for heat loss through the building fabric. Insulation reduces these effects, helping to keep heat inside the structure.

Conduction

Conduction is the transfer of heat through solid materials. In buildings, this is a major cause of heat loss. Heat moves from the warm interior to the cooler exterior via direct contact between solid materials. For example, in an uninsulated wall, heat could travel through the internal lining, into the structure and then out through the external wall and cladding.

Mike Vaczi, Technical Director at Celotex, explains that the main role of thermal insulation is to interrupt this process. “Most insulation materials contain tiny pockets of air or gas with low thermal conductivity. Because heat moves much more slowly through gas than it does through solids, these materials slow the rate of heat transfer,” he explains. “By placing insulation between solid layers – such as wall linings and external walls – the effects of conduction can be reduced,” he adds.

Conduction also causes thermal bridging where heat travels through other more conductive building materials, such as metal or dense structural materials, bypassing the insulation layer. This makes careful detailing and installation essential to achieving thermal performance requirements.

Radiation

Thermal radiation is the transfer of heat in the form of electromagnetic waves, typically infrared. Radiation can travel through air or a vacuum, which is why we feel the sun’s warmth from millions of miles away. In buildings, radiant heat is emitted by warm internal surfaces – such as walls, floors, furniture and even people and equipment. It is then absorbed or reflected by cooler surfaces like windows and external walls, contributing to heat loss.

Mike says that to help reduce radiant heat transfer across cavities, some insulation panels have low-emissivity (low-e) surfaces: “These are typically shiny materials, such as foil, that reflect infrared radiation back towards the interior, reducing radiant heat loss.”

Convection

Convection is the transfer of heat through the movement of fluids or air. Warm air is less dense than cool air so it rises, while cooler air sinks. This creates convective air currents, which can lead to heat stratification. Warmer air gathers near the ceiling, while cooler air settles at occupant level.

In terms of thermal insulation, Mike explains that convection is a concern when air can circulate within the building envelope. “Gaps between insulation panels or poorly installed insulation can lead to convective currents that carry heat away from warm areas,” he warns. “Good insulation installation practices help to limit air movement and minimise heat transfer.”

By addressing all three forms of heat transfer, insulation improves a building’s thermal performance, enhancing energy efficiency, reducing energy demand and increasing occupant comfort. As well as keeping heat in during the winter, insulation can slow heat transfer from outside to inside during the summer. But managing overheating typically requires additional measures such as shading and ventilation.

What insulation types are there?

Thermal insulation comes in a wide range of types, each with its own with different characteristics relevant to specific applications. These include rigid foam insulation boards, flexible mats, natural materials and loose-fill products. The most suitable choice of insulation will depend on project-specific requirements. The following table provides a comparison of common insulation types, their format, typical fire classifications, thermal performance and moisture resistance.

Other factors to consider include compressive strength, dimensional stability, weight, moisture performance, environmental impact, cost and ease of installation. These can vary significantly between manufacturers and are often measured using different methods for different product types, making side-by-side comparisons impractical. Please refer to individual product datasheets for accurate information.

What do insulation performance terms mean?

When choosing an insulation product, there are several key performance terms that describe how the product performs.

• Thermal conductivity: Also known as the lambda value or λ value, thermal conductivity is the rate at which heat is conducted through a specific product, such as an insulation panel. It is measured in watts per metre-kelvin (W/m·K). The lower the number, the better the thermal performance.
• R-value: This indicates how well insulation resists heat flow – that is, its thermal resistance. It is calculated by dividing the thickness of the material by its thermal conductivity and is expressed in square metres kelvin per watt (m²·K/W). A higher R-value means better insulation performance.
• U-values: U-values are related to thermal conductivity, but the two differ in scope. While thermal conductivity applies to a single building product, U-values measure how much heat passes through an entire building element, such as a wall or roof. U-values are calculated by considering the thermal resistance of all layers in a building element, including insulation, structural materials and finishes. U-value calculations follow the BRE document BR 443 ‘Conventions for U-value calculations’ and also take into account thermal bridging, the resistance of airspaces, and corrections for thermal losses due to air gaps. They are measured in watts per square metre kelvin (W/m²·K). The lower the U-value, the better the thermal insulation of the construction element.
• Euroclass fire ratings: These ratings indicate how a product reacts in a fire. For example, in a fire rating of A1, s2, d0:
  • A1 means the product is non-combustible. Ratings range from A1 (non-combustible) to F, which indicates limited fire performance.
  • ○ s2 indicates moderate smoke production (s1 means minimal or no smoke, while s3 means substantial smoke)
  • ○ d0 means no flaming droplets or particles are produced in the first ten minutes (this can range up to d2, meaning many flaming droplets).

Sometimes, the Euroclass fire rating is simplified to just the first letter rating (A1 to F, as in the table above), but it’s important to remember that the single-letter rating does not reflect the smoke or flaming droplet production. The testing procedure for Euroclass ratings is defined in BS EN 13501-1.

• Compressive strength: This is a measurement of how much pressure a material can withstand before being crushed. Expressed in kilopascals (kPa), it’s the maximum amount of compressive strength that a material can bear at 10% deformation. The testing procedure for insulation compressive strength is defined in BS EN 826:2013.
• Dimensional stability: Describing how much insulation changes shape or size when exposed to heat or moisture, dimensional stability is measured as the percentage change under specific temperature and humidity conditions. The testing procedure for insulation products is set out in BS EN 1604:2013.

How does thermal insulation performance affect SAP ratings?

SAP, or the Standard Assessment Procedure, is a method for assessing the energy performance of residential buildings. It provides a score based on the building’s energy use and carbon emissions. SAP is used in England and Wales; Northern Ireland and Scotland have similar systems. For commercial buildings, SBEM – the Simplified Building Energy Model – is used.

SAP ratings are used to demonstrate compliance with Part L of the Building Regulations, which sets minimum standards for energy efficiency in new and refurbished homes. The rating is expressed as a number between 1 and 100+, with 1 being an extremely inefficient property, 100 indicating net-zero regulated energy cost and scores over 100 meaning the property is exporting energy.

The thermal performance of the building fabric is a significant component of a SAP rating, so thermal insulation is an important consideration. Other factors, like heating systems, ventilation, lighting and renewables, also contribute to the score.

Celotex PIR insulation and XPS insulation are designed to deliver low thermal conductivity, supporting improved thermal performance across walls, roofs and floors. With typical declared lambda (λ) values of 0.021–0.022 W/m.K, they help achieve lower U-values, reducing heat loss through the building envelope. This contributes to lower energy demand for space heating and supports compliance with Part L of the Building Regulations, as well as improved SAP scores.

Where is thermal insulation used in the building envelope?

Insulation can be used in any part of the building envelope – walls, roofs and floors – but different products may be required for each part of the building. For example, basement walls will have different requirements than the walls on the upper levels of a high-rise building. Thermal performance is an important consideration, along with effective detailing to minimise thermal bridging. The most suitable choice will also depend on factors like substrates, exposure to moisture, loading requirements (particularly for floor and flat roof insulation) and location within the building.

Wall insulation types:

• Cavity wall insulation: Typically used in masonry walls, either partially or fully filling the gap between the inner and outer leaf.
• Timber-frame wall insulation: Timber-frame construction is an alternative to traditional masonry walls, with insulation typically installed between the studs.
• External wall insulation (EWI): This is when insulation is fixed to exterior walls and then covered with cladding. It is commonly used for retrofit projects with solid wall construction to increase an existing building’s energy performance. It is also used in new-builds that have concrete or steel frames without a typical wall cavity.
• Internal wall insulation: When insulation is installed on the inside face of an external wall. This is often used in retrofit projects where EWI isn’t suitable.

Roof construction types:

• Ventilated pitched roofs are needed when a high vapour resistance layer (e.g. sarking felt or sarking boards) is used above insulation. Continuous ventilation at eaves and ridge is essential to prevent condensation. A 50 mm ventilation gap between insulation and the sarking layer is typically required.
• Unventilated pitched roofs use a breather membrane above insulation, allowing vapour to diffuse out while preventing water ingress. A 15–25 mm drape space between the membrane and insulation is needed unless counter-battens are used. Rafters can be fully filled with insulation only if a drainage path is maintained.
• Warm pitched roofs position insulation above or around the rafters, keeping the structure warm and reducing condensation risk. Typically, rigid boards are installed above rafters beneath a breather membrane. No ventilation is needed if the layer above the rafters is at least equal in thickness to any between-rafter insulation.
• Cold lofts (traditional pitched roofs) have insulation at ceiling level, leaving the loft space cold. Eaves ventilation is essential to remove moisture and prevent condensation at the roof level.
• Flat roofs are typically designed as warm roofs, with insulation placed above the structural deck and under the waterproofing layer. No ventilation is needed within the structure. Inverted roofs place insulation above the waterproofing and use ballast to hold it in place.

Floor construction types:

• Ground-bearing floors: When the floor construction rests directly on the ground – such as a concrete slab floor – insulation can be incorporated above or below the slab.
• Suspended floors: In suspended timber floors or beam-and-block floor constructions, insulation is typically installed between or above the joists or beams.

Basement insulation:

Basement walls and floors require insulation that can withstand below-ground conditions, including moisture and ground pressure. It may be installed internally, externally or both.

What are the benefits of insulation?

With rising energy costs, environmental concerns, strict building regulations and occupant wellbeing all key factors in new home construction, insulation is a crucial consideration. Specifying the right insulation for a project:

• Reduces heat loss through walls, roofs and floors
• Helps to maintain consistent indoor temperatures
• Lowers energy consumption and heating costs
• Reduces carbon emissions
• Contributes to better SAP scores and compliance with Part L
• Improves occupant comfort
• Can slow heat transfer from outside to inside during the summer

In addition to thermal efficiency, insulation can also contribute to acoustic performance and helps control moisture within the building fabric. This reduces the risks of condensation, damp, mould and mildew.

The Celotex range of insulation products are designed to support all these outcomes, while also helping to streamline the construction process. Being made of foam, Celotex boards are lightweight with a format that supports manual handling on site. They have been designed for efficient installation, supporting streamlined workflows on site and keeping construction timelines on track.

Which type of thermal insulation should I use?

Choosing the most suitable insulation material for a project requires careful consideration of thermal performance, building type, budget, installation methods and compliance requirements. Specifiers will need to evaluate characteristics such as required U-values, vapour control, fire performance, strength and stability to determine the best insulation product for each part of the building.

Celotex offers PIR and XPS rigid thermal insulation boards designed to achieve low thermal conductivity levels, contributing to improved U-values in building elements. These lightweight and dimensionally stable boards are easy to cut and move into place, contributing to a simple and high-quality installation.

All Celotex products are backed by our comprehensive technical support, including an expert team, detailed insulation product datasheets and online specification tools such as our U-value calculator.

For a free consultation or product sample, please get in touch.