Lambda values and thermal properties

The Lambda value of a building material is the expression of its thermal conductivity – the lower the value, the greater the thermal efficiency: not to be confused with a product’s Thermal Resistance (R-value). Thermal conductivity is the rate at which heat is transmitted through a material and is measured in watts per square metre of surface area, for a temperature gradient of one Kelvin per metre thickness (W/mK).

Ravatherm XPS X was originally developed to deliver improved thermal performance for inverted flat roof applications. However, the increased thermal performance delivered by the technology is now also available in products for floor applications offering 16% improved thermal performance versus other XPS insulations on the market.

Exceptional thermal performance has been achieved through the addition of infra-red ‘blockers’ or particles. These are finely dispersed in the cell walls which scatter and reflect heat radiation, helping to keep a building warm in winter and cool in summer.

Ravatherm XPS X uses carbon dioxide as a blowing agent, which gives it an ozone-depleting potential (ODP) of zero and a Global Warming Potential (GWP) of less than five.

Converting improved lambda values into cost savings

All Ravatherm XPS X products offer exceptional thermal performance, as demonstrated by their lambda value. This performance means that customers can meet their target U-values with thinner insulation boards, providing for additional design flexibility. Cost savings can also be realised with Ravatherm XPS X, by requiring a lesser quantity of insulation to achieve the desired thermal performance.

Ravatherm XPS X 300 SL has a declared lambda of 0.031 W/m.K. This product is suitable for inverted flat roof applications and is available in thicknesses ranging from 50mm up to 200mm. XPS insulation is sometimes priced by the cubic metre, but pricing in this way does not always provide a true like-for-like comparison. This is because when you have a product with a better lambda value, you may be able to reduce the thickness and therefore enable an overall cost saving. In some instances, this could even be the case if the cubic metre price is higher. We suggest that if doing comparisons, it is best to use square metre price instead.

In addition, alternative XPS products may require multiple layers, essentially doubling or even tripling the time and effort needed to install the product, and thus adding to the cost of labour.

These cost-saving solutions can also be accounted for in flooring applications. Ravatherm XPS X flooring products have a lambda value ranging from 0.030 W/m.K to 0.032 W/m.K, dependent on compressive strength and thickness of product. When using insulation in floors, the loads being placed onto the floor also need to be considered. Ravatherm XPS X has a range of compressive strengths achieving 200kPa, 300kPa, 500kPa and 700kPa with standard thicknesses from 30mm up to 200mm, making it a suitable solution for everything from standard residential buildings to airplane hangars.

A dedicated team of specialists can provide advice and support during the design stages of a project, providing detailed U-value calculations and other technical information. U-value calculation requests can be made via the Ravago Building Solutions website:

Lower Lambda = Reduced Thickness = Lower Cost


Published On: February 10, 2021