Frequently Asked Questions 

SOPREMA PIR insulation products (including SOPRATHERM and THERMACLASS) should not be used within the external wall construction of buildings containing a storey more than 11 metres above ground level.

Approved Document B (England) requires that materials forming part of the external wall construction of such buildings achieve a reaction to fire classification of Euroclass A1 or A2-s1,d0 (i.e. non-combustible or of limited combustibility). PIR insulation does not achieve this classification and is therefore not suitable for use in these applications.

It remains the responsibility of the designer, installer, or end user to ensure that the proposed specification complies with all relevant national Building Regulations and statutory guidance, together with any other applicable local or project-specific requirements. This includes consideration of fire performance and any relevant height restrictions.

Our general recommendation is to maintain a 50mm clear residual cavity between the insulation and the outer leaf of masonry.

In certain circumstances, it may be possible to reduce the residual cavity to an absolute minimum of 25mm. However, this should only be considered where permitted by relevant guidance, certification, and project-specific requirements.

A copy of the relevant BBA certificate for partial fill cavity wall applications is available on our website.

From a thermal performance perspective, there is no material difference between a 25mm and 50mm clear residual cavity.

Prior to specification, the designer should confirm acceptance of any reduced cavity width with the relevant building control body and any applicable third-party warranty provider. Where NHBC standards apply, a residual cavity of less than 50mm is unlikely to be accepted.

At present, we are unable to provide thermal bridge calculations where rainscreen support brackets are incorporated within the build-up. These junctions require numerical modelling in accordance with recognised standards, including BS EN ISO 10211 (2D/3D thermal bridge modelling) and BR 497 (Conventions for calculating linear thermal transmittance and temperature factors).

This is not currently a service that SOPREMA provides.

The necessary calculations may be available directly from the bracket manufacturer (for example, NVELOPE or other specialist façade support system suppliers), who may be able to provide project-specific psi-values or modelling data.

Please also note that SOPREMA products must not be used within the external wall construction of buildings containing a storey more than 11 metres above ground level. Approved Document B requires materials forming part of the external wall construction of such buildings to achieve Euroclass A1 or A2-s1,d0. PIR insulation does not achieve this classification and is therefore not suitable for use in these applications.

It remains the responsibility of the designer to ensure compliance with all relevant Building Regulations and project-specific requirements.

Yes. Both THERMACLASS Cavity Wall 21 or SOPRATHERM CW4000 should be installed tight against the internal blockwork.

Approved Document Part L states under “Continuity of insulation” (4.15 e) that insulation should be fitted without air gaps and tight to the structure, cavity closers, lintels, and cavity trays. Mortar snots should be removed to ensure a tight fit with the structure and cavities cleared of debris. Where fire-stopping socks are required, these should fully fill the areas where they are fitted.

From a thermal calculation perspective, small sealed air voids (i.e. trapped air pockets without air movement) would not materially affect the calculated U-value of the wall build-up, as they behave as still air.

However, installing the insulation tight to the inner leaf reduces the risk of gaps forming around structural elements and junctions, which could otherwise permit uncontrolled air movement and adversely affect airtightness and overall energy efficiency.

For this reason, installation tight against the internal blockwork represents best practice and aligns with the requirements of Approved Document Part L.

Any residual cavity would be on the external side of the insulation.

No. Partial fill cavity wall or Full-Fill insulation boards do not require taping at board joints.

Within a masonry cavity wall construction, the insulation boards are installed within the cavity and are not intended to provide the primary air barrier. Airtightness is typically achieved at the inner leaf (for example, through plastered blockwork or an internal lining system).

Provided the boards are correctly installed in accordance with the relevant specification, tight-butt jointed, properly supported by wall ties, and positioned to maintain the required residual cavity, taping of board joints is not required.

Good workmanship remains essential to minimise gaps and maintain continuity of insulation in accordance with Approved Document Part L.

For existing solid masonry walls, insulating the external face is generally the preferred method of thermal upgrade, as it helps to manage thermal bridging and moisture risk more effectively. However, this is not always feasible due to planning, heritage, or site constraints.

For retrofit projects where internal wall insulation (IWI) is proposed, a holistic approach must be adopted. This should consider airtightness, ventilation strategy, and overall moisture management to minimise condensation risk. Assessment in accordance with PAS 2035 by a qualified Retrofit Assessor or Coordinator is recommended prior to specifying internal wall insulation to solid masonry walls. Guidance can be found in publications such as Retrofit internal wall insulation: best practice.

SOPREMA offer various vapour open or closed solutions on our solid wall application pages, and our online u-value calculator.

SOPRATHERM XR4000 and GA4000 have a declared compressive strength of ≥ 140 kPa, determined in accordance with EN 826. The declared value reflects either the compressive stress at 10% deformation or the compressive strength, whichever is the lower figure.

Our BBA Certificate covers the use of these products in domestic floor applications, which it defines as a uniformly distributed load of up to 1.5 kN/m².

SOPREMA products may be considered for use in situations with higher floor loadings. In such cases, the suitability of the floor construction should be assessed by a suitably qualified person, taking into account the full floor build-up and the declared compressive strength of the insulation.

The ability of a floor to withstand a given loading is not determined solely by the insulation properties, but by the performance of the floor construction as a whole. This includes factors such as the thickness and strength of the screed or structural layer, and its ability to distribute loads across the insulation.

Where higher compressive strength is required, SOPREMA XPS insulation may be more suitable. Please refer to the relevant XPS BBA Certificate for further details.

Section 1 of the relevant BBA Certificate provides information on floor loadings and confirms the compressive strength classification of SOPRA XPS products.

SOPRA XPS boards are available in different compressive strength grades:

• SOPRA XPS 300 ≥ 300 kPa
• SOPRA XPS 500 ≥ 500 kPa
• SOPRA XPS 700 ≥ 700 kPa

These values are determined in accordance with EN 826 and represent the compressive stress at 10% deformation.

The appropriate grade should be selected based on the design loading and the overall floor construction.

The ability of a floor to withstand a given loading is not determined solely by the compressive strength of the insulation, but by the performance of the floor build-up as a whole. This includes the thickness and strength of the screed or structural slab and its ability to distribute loads across the insulation layer.

For applications involving higher point loads, heavy-duty floor use, or specialist requirements (e.g. industrial floors), the floor construction should be assessed by a suitably qualified person to confirm suitability of the selected XPS grade.

No. Condensation risk analysis for solid ground-bearing floors cannot be assessed using the standard Glaser method set out in BS EN ISO 13788.

The BS EN ISO 13788 methodology relies on defined internal and external air temperatures and vapour pressures. In the case of a ground-bearing floor, there is no external air boundary condition on the ground side of the construction. As a result, the standard interstitial condensation calculation method is not applicable.

Assessment of moisture risk in ground-bearing floor constructions should instead consider ground moisture conditions, the damp proof membrane (DPM) strategy, and the overall moisture management approach for the building.

SOPREMA PIR insulation is designed to remain dry during storage and installation, despite its closed-cell structure.

We are unable to assess the extent of any potential damage to boards that have been exposed to water.

PIR insulation is closed-cell and foil-faced, which provides a degree of resistance to moisture. Where water exposure has been minimal in both duration and intensity, and the boards are allowed to dry thoroughly before installation, it is unlikely that thermal performance will be significantly affected.

However, prolonged or substantial exposure to water may introduce contaminants that could compromise the insulation material. Water ingress may also cause the foil facers to delaminate from the insulation core. Where boards have been flooded or show visible signs of water damage, replacement is recommended, as performance cannot be guaranteed once the product has been materially affected.

A P/A ratio is the Perimeter-to-Area ratio of a floor. It is an important factor in determining the heat loss through the ground floor of a building.

The P/A ratio is calculated as:

P/A = Exposed perimeter (m) ÷ Floor area (m²)

In ground-bearing floor constructions, heat loss is greater at the exposed edges of the floor than at the centre. The P/A ratio reflects how much exposed perimeter exists relative to the total floor area and therefore influences the calculated U-value. Typically this will be a number between 1 and 0.

The P/A ratio is used in ground heat transfer calculations in accordance with BS EN ISO 13370 and forms part of SAP and basement U-value assessments.

Insulation used beneath a raft foundation must be capable of withstanding sustained compressive loads and potential ground moisture exposure.

Extruded polystyrene (XPS) insulation, such as SOPRA XPS, is typically suitable for this application due to its high compressive strength, low water absorption, and long-term dimensional stability.

SOPRA XPS boards are available in different compressive strength grades (e.g. 300, 500 and 700 kPa), allowing selection to suit the specific structural loading requirements of the project. The appropriate grade should be determined based on the calculated design loads and overall foundation design.

The suitability of the insulation must be considered as part of the foundation system as a whole. Structural assessment by a suitably qualified engineer is required to confirm that the selected insulation grade is appropriate for the imposed loads and ground conditions.

PIR insulation is not generally suitable for use beneath raft foundations due to its lower compressive strength and moisture exposure limitations.

SOPREMA does not support the use of PIR insulation in hybrid flat roof constructions where insulation is installed both between and above timber joists.

BS 5250 defines a cold roof as one requiring ventilation above insulation installed within the structure, and a warm roof as one where all insulation is positioned above the structural deck. Hybrid arrangements fall outside these standard definitions and can introduce increased condensation risk.

Where insulation is installed between or below joists in a flat roof, ventilation would normally be required in accordance with BS 5250.

For existing structures, any proposed hybrid or altered build-up should be assessed at specification stage as part of a complete roof system, taking into account moisture risk, ventilation provision and overall condensation strategy. In some cases, project-specific hygrothermal assessment may be required.

BROOF(t4) is a system-level fire classification relating to the external fire performance of a complete roof build-up. It does not apply to individual products in isolation.

As such, a BROOF(t4) classification can only be achieved and declared for a tested and classified roofing system, rather than a standalone insulation board.

BROOF(t4) classified constructions are available through complete SOPREMA flat roofing systems. Please contact us for further information on tested system build-ups and applicable certifications at techncial@soprema.co.uk

BROOF(t4) is required where a roof is within a specified distance of a relevant boundary, in order to limit the risk of external fire spread over the roof surface.

Approved Document B requires roof coverings to achieve an appropriate external fire classification. BROOF(t4) is the highest performance classification under BS EN 13501-5 when tested to exposure condition t4.

In practice, this classification is commonly required where the roof covering is within 6 metres of a boundary, although the specific requirement should be confirmed against the applicable Building Regulations and project circumstances.

Since the introduction of the Building Safety Act, there has been increased scrutiny of fire compliance, including verification of roof covering classifications.

As BROOF(t4) is a system classification, compliance must be demonstrated for the complete tested roof build-up rather than individual components.

Internal battens to form a service void, is generally recommended where PIR insulation thicknesses exceed 50mm.

In flat roof applications, introducing a service void provides a practical means of supporting thicker insulation layers and creates a suitable fixing zone for plasterboard using standard-length fixings. Reliance on very long plasterboard fixings can be more difficult on site and may not always provide adequate support to the insulation layer.

A service void can also accommodate a low-emissivity air space where appropriate, which may contribute to improved thermal performance when designed correctly.

In addition, the void provides a convenient zone for routing services or installing downlights, helping to minimise penetrations through the air and vapour control layer (AVCL) and thereby supporting airtightness.

Ventilated roofs are required where there is a high vapour resistance layer above the insulation, such as sarking felt or sarking boards. Ventilation gaps at the eaves and ridge are essential to permit airflow between the felt and insulation to allow moisture vapour to escape, preventing condensation and maintaining a dry roof structure. A 40mm is typically required for this ventilation void in accordance with BS 5250.

Unventilated roofs with a breather membrane are a sealed roofing system that relies on a breathable underlay to allow moisture to escape while preventing water ingress. It eliminates the need for traditional ventilation by permitting vapour diffusion through the membrane to reduce condensation risk within the roof structure. A drape space into the rafter of 15-25mm is required between the insulation and the breather membrane. Insulation can fully fill the rafter void only if counter battens are installed to form a drainage path.

Warm pitched roofs position the primary insulation layer above the rafters, beneath a vapour-permeable (breather) membrane and the roof covering, so that the structural timber remains on the warm side of the insulation. This helps to minimise thermal bridging and reduce condensation risk within the roof structure.

Where additional insulation is installed between the rafters, the insulation layer above the rafters should provide the majority of the thermal resistance. As a general principle, the insulation above the rafters should be equal to or greater than the insulation between the rafters to ensure the timber structure remains sufficiently warm and condensation risk is controlled.

Internal battens to form a service void, is generally recommended where PIR insulation thicknesses exceed 50mm.

In pitched roof applications, introducing a service void provides a practical means of supporting thicker insulation layers and creates a suitable fixing zone for plasterboard using standard-length fixings. Reliance on very long plasterboard fixings can be more difficult on site and may not always provide adequate support to the insulation layer.

A service void can also accommodate a low-emissivity air space where appropriate, which may contribute to improved thermal performance when designed correctly.

In addition, the void provides a convenient zone for routing services or installing downlights, helping to minimise penetrations through the air and vapour control layer (AVCL) and thereby supporting airtightness.

Basement constructions may incorporate insulation either externally or internally.

Extruded polystyrene (XPS) insulation such as SOPRA XPS may be suitable for use externally where it is exposed to high moisture conditions, subject to appropriate specification and detailing.

PIR insulation such as SOPRATHERM GA4000 is suitable only for internal installation and must be positioned on the warm side of the tanking or waterproofing system.

In all cases, the proposed insulation strategy should be coordinated with the project’s waterproofing design and overall moisture management strategy.

To enable us to undertake a basement U-value calculation, the following information is required:

1. Perimeter of the basement floor (m)

2. Area of the basement floor (m²)

3. Depth of the basement floor below external ground level (m)

4. External conditions adjacent to the basement perimeter walls (see below)

5. Proposed insulation type and thickness for the floor, or target U-value (W/m²K)

6. Project name or reference

7. Full construction build-up of the relevant element(s) (wall and/or floor), including material types and thicknesses.

Where the full height of the basement wall is retained by soil, confirmation of this is sufficient for item 4.

However, where varying conditions exist around the perimeter (for example, partially exposed walls or adjoining heated spaces), additional information is required to determine the effective basement depth and boundary conditions for the U-value calculation.

In these cases, please confirm:

A. Percentage of basement wall exposed to soil B. Percentage of basement wall not exposed to soil (A + B = 100%) C. Length of any basement party wall adjoining heated spaces (m), where applicable

This information allows the basement to be modelled in accordance with the relevant ground heat loss methodology.

Thermal resistance (R-value) describes the ability of a material or construction layer to resist heat flow.

It is calculated based on the material’s thickness and its thermal conductivity (lambda value). Thermal resistance is expressed in square metres Kelvin per watt (m²K/W).

A higher R-value indicates greater resistance to heat flow and therefore better insulating performance.

Thermal resistance values are used in U-value calculations, where the total resistance of all layers within a construction determines the overall thermal transmittance.

A U-value (thermal transmittance) measures the rate of heat loss through a building element, such as a wall, roof or floor.

It represents how much heat passes through 1 square metre of a construction for every 1-degree temperature difference between inside and outside. It is expressed in Watts per square metre Kelvin (W/m²K).

A lower U-value indicates lower heat loss and therefore better thermal performance. A higher U-value indicates greater heat transfer and poorer insulating performance.

The U-value is calculated by determining the total thermal resistance (R-value) of all layers within the construction and taking the reciprocal:

U = 1 / Rtotal

U-values are used to demonstrate compliance with Building Regulations, including Approved Document Part L. These form part of the Standard Assessment Procedure (SAP) calculation, which is used to assess the overall energy efficiency and carbon performance of dwellings.

SOPREMA does not declare a specific operating temperature range for SOPRATHERM PIR insulation.

SOPRATHERM PIR is assessed for use within the typical service temperature range expected in standard building applications. It is not intended for use in specialist high-temperature environments or applications outside normal building conditions.

Where the insulation is proposed for use in atypical environments or where elevated service temperatures may occur, the suitability of the product should be assessed at specification stage.

SOPREMA cannot confirm compliance with Building Regulations for completed constructions, nor can we provide retrospective design approval where a build-up has not been installed in accordance with our published guidance or certification.

Where a construction differs from our recommendations, any proposed mitigation or remedial measures should be discussed directly with the relevant Building Control Body and, where applicable, the third-party warranty provider. This enables the regulatory position and any potential moisture, thermal, or fire performance implications to be properly assessed.

Any corrective measures should be determined by a suitably qualified design professional and formally approved by Building Control and/or the warranty provider before implementation.

We can provide product datasheets, Declarations of Performance, and certification documents to support an independent assessment where required.

Our certifications such as ISO 9001:2015 and 14001 can be found here: Certifications & Memberships 

Environmental Performance Declarations (EPDs) can be found here: Document Centre

SOPREMA products disposal information can be found within the product safety data sheets: Document Centre

We are unable to provide design advice for insulating log cabins, sheds, shipping containers, or similar thin-walled or lightweight constructions, particularly where PIR insulation is proposed for internal use.

Such buildings are classified under BS 5250 as non-standard constructions, where moisture behaviour cannot be reliably assessed using standard condensation risk methods (such as BS EN ISO 13788).

The vapour resistance, airtightness, ventilation strategy and overall construction of these buildings can vary significantly. As a result, the suitability and positioning of insulation and any vapour control layer must be determined by the building manufacturer or a suitably qualified building professional who can review the full construction build-up.

BS 5250 advises that non-standard constructions may require project-specific hygrothermal assessment to properly assess moisture risk. We can provide product datasheets, Declarations of Performance and BBA Certificates to support an independent review where required.

Thermal conductivity (λ or k-value) is a material property that describes how easily heat flows through a material. It is expressed in watts per metre Kelvin (W/mK).

A lower thermal conductivity (lambda value) indicates that less heat passes through the material, meaning it provides better thermal insulation. Conversely, a higher lambda value indicates greater heat transfer and lower insulating performance.

Thermal conductivity is used in U-value calculations to determine the thermal resistance of individual layers within a construction.

SOPREMA cannot confirm compliance with Building Regulations for completed constructions, nor can we provide retrospective design approval where a build-up has not been installed in accordance with our published guidance or certification.

Where a construction differs from our recommendations, any proposed mitigation or remedial measures should be discussed directly with the relevant Building Control Body and, where applicable, the third-party warranty provider. This enables the regulatory position and any potential moisture, thermal, or fire performance implications to be properly assessed.

Any corrective measures should be determined by a suitably qualified design professional and formally approved by Building Control and/or the warranty provider before implementation.

We can provide product datasheets, Declarations of Performance, and certification documents to support an independent assessment where required.

Selecting the right system depends on several key factors, including structural loading, thermal performance, wind uplift, build-up height, durability, access requirements, rooftop plant, and warranty expectations.

For a compliant, project-specific solution, early consultation with SOPREMA’s technical team is recommended.

SOPREMA specification systems are installed by a network of approved contractors. Each contractor is assessed for technical competence and operational capability, with installers receiving product-specific training to ensure work meets the required standards and qualifies for SOPREMA guarantees.

SOPREMA Trade Solutions products may be installed by competent roofing contractors with the appropriate experience and knowledge. It is the responsibility of the appointing party to ensure installers have the necessary skills to apply these products correctly.

If your existing roof is showing signs of deterioration and water ingress, SOPREMA can support with a detailed condition survey utilising both onsite inspection or drone surveys where appropriate.

A visit can be arranged to assess the current waterproofing system. This may include core sampling to understand the existing build-up, along with identifying the source of leaks and the extent of any damage. This assessment enables informed recommendations for repair or replacement solutions.

The correct solution should be based on the ecological goals set out within the BDAP, such as the habitats or species the roof is intended to support.

Where defined biodiversity targets exist, the system should be designed to suit those specific requirements. If the brief is less prescriptive, biodiversity can be enhanced through the use of pre-grown vegetation blankets or seed mixes suited to local conditions and planning expectations.

Consulting with SOPREMA’s technical team at design stage will help ensure the system supports the intended biodiversity outcomes.

SOPREMA can support the integration of solar PV with non-penetrative mounting solutions, including systems such as SOPRASOLAR and SOLAR CARBON.

These systems are designed to maintain the integrity of the waterproofing layer while allowing efficient installation of PV arrays.

Early engagement with SOPREMA’s technical team is recommended to ensure compatibility with the roof build-up and alignment with project requirements.

SOPREMA Specification System guarantees can cover the performance of the specified products and, where applicable, the design elements provided. Where installation is carried out by an approved contractor, the guarantee may also include workmanship.

The length of the guarantee will depend on the system selected, its expected service life, and project-specific requirements.

SOPREMA Trade Solutions products can be offered with guarantees covering product performance. The duration is based on the system and its intended application.

All guarantees are subject to the terms and conditions detailed within the warranty documentation.

Contact SOPREMA to discuss the most suitable options.

BROOF(t4) is a system-level fire classification relating to the external fire performance of a complete roof build-up. It does not apply to individual products in isolation.

As such, a B_ROOF(t4) classification can only be achieved and declared for a tested and classified roofing system, rather than a standalone product.

B_ROOF(t4) classified constructions are available through complete SOPREMA flat roofing systems. Please contact us for further information on tested system build-ups and applicable certifications at techncial@soprema.co.uk

B_ROOF(t4) is required where a roof is within a specified distance of a relevant boundary, in order to limit the risk of external fire spread over the roof surface.

Approved Document B requires roof coverings to achieve an appropriate external fire classification. B_ROOF(t4) is the highest performance classification under BS EN 13501-5 when tested to exposure condition t4.

In practice, this classification is commonly required where the roof covering is within 6 metres of a boundary, although the specific requirement should be confirmed against the applicable Building Regulations and project circumstances.

Since the introduction of the Building Safety Act, there has been increased scrutiny of fire compliance, including verification of roof covering classifications.

As B_ROOF(t4) is a system classification, compliance must be demonstrated for the complete tested roof build-up rather than individual components.