Roof insulation for thermal efficiency

Celotex roof insulation products are designed to deliver thermal performance, dimensional stability and moisture resistance when specified and installed correctly. Suitable for pitched and flat roof constructions in both commercial and residential applications, Celotex rigid PIR boards can be used in a variety of roof build-ups – from warm pitched roofs and loft conversions to flat roof with insulation between joists or under rafters.

What is roof insulation?

Roof insulation is a thermally efficient material that is installed above the habitable parts of a building to reduce heat loss. By keeping heat in, roof insulation improves energy efficiency, which may contribute to lower energy costs and a more comfortable indoor environment. With the right product selection and proper installation, thermal insulation contributes to compliance with building regulations and the long-term performance of the building.

Celotex products for roof applications

Celotex offers several types of roof insulation, with products suitable for various roof build-ups, including warm roof insulation and cold roof insulation:

• Celotex XR4000 & GA4000 – These rigid PIR insulation boards are available in thicknesses of 110–200mm and 50-100 respectively and are suitable for use in both pitched and flat roof applications. GA4000 is foil faced.
• Celotex PL4000 – Designed for dry lining applications, these thermal laminate boards combine PIR insulation with 12.5mm tapered edge plasterboard. They enable the insulation and internal finish to be installed in a single layer. Choose from a range of thicknesses between 50-100mm.
• Celotex TB4000 – PIR boards that can be used alongside separate plasterboard layers for roof applications. Choose from a range of thicknesses between 20-40mm.

Types of roof insulation

Celotex polyisocyanurate (PIR) insulation is a rigid foam board. Its closed-cell structure contains a low-conductivity gas that slows the transmission of heat. This structure makes PIR insulation boards dimensionally stable and moisture resistant. Weighing around 30 kg/m3, they are also relatively lightweight, simplifying transport and making them easier to position on site.

The range of Celotex roof insulation is available in various thicknesses to suit project requirements, with lambda values of 0.022W/m⋅K. Some products measure just 20mm to keep the roof thickness to a minimum in locations where space is limited. Depending on the product, features may include low-emissivity foil facings for improved thermal performance or integration with plasterboard for simplified installation.

When choosing roof insulation, the right product will depend on the roof type, construction method, loading requirements and project-specific thermal performance requirements. The team at Celotex can help you find a product that suits your project.

Key considerations when specifying roof insulation

In England, roof insulation must achieve the minimum U-values set out in Approved Document L. U-values measure the thermal transmittance of a complete building element, such as the roof, rather than individual materials. Since other materials in the build-up also affect U-values, the insulation may need to be thicker or higher performing to achieve the target for the complete roof.

Volume 1 of Approved Document L sets a notional U-value for roofs in new-build dwellings of 0.11 W/m²·K, with a limiting value of 0.16 W/m²·K.

For refurbishment projects, the limiting U-value for roofs is 0.16 W/m²·K. For buildings other than dwellings, Volume 2 sets a limiting value of 0.18 W/m²·K for flat roofs and 0.16 W/m²·K for pitched roofs – this applies to new or replacement elements in new or existing buildings. Approved Document L applies to England. There are similar requirements for Wales, Scotland and Northern Ireland: 

• Wales: Approved Document L (Wales) – Conservation of fuel and power
• Scotland: Section 6 of the Scottish Building Standards – Conservation of Fuel and Power.
• Northern Ireland: Technical Booklet F – Conservation of Fuel and Power.

Other considerations include:

• Thermal performance – When correctly specified, insulation helps to ensure a building is thermally efficient. In addition to thinking about U-values, be sure to also look at lambda values, which measure thermal conductivity and R-values, which measure thermal resistance.
• Thickness – The correct insulation thickness will depend on the roof type, how it was constructed and thermal performance requirements. Celotex PIR boards offer lambda values as low as 0.021W/m⋅K and can be supplied in thicknesses of just 20mm.
• Dimensional stability – Roof insulation must be capable of withstanding any expected loads, as excessive compression can reduce thermal performance. Celotex rigid insulation boards are available in a variety of compressive strengths to suit different applications.
• Ventilation – Insulation affects the movement of moisture within the building. Adequate airflow is essential to prevent condensation, damp, and mould. Methods may include ridge and eaves ventilation or vapour-permeable membranes, in line with the guidance set out in BS 5250:2015, the code of practice for the management of moisture in buildings.
• Product certification – Celotex roof insulation products are BBA certified for defined applications. This confirms that products have been independently assessed for performance and project suitability.

Roof insulation technical support from Celotex

Celotex provides a range of resources to support designers, specifiers and installers. Try our online U-value calculator to specify suitable roof insulation for your next project.