The thermal conductivity of a material is known as its ‘Lambda’ value and is measured in Watts per metre per Kelvin (W/_mK). By looking at the Lambda values of the materials that make up an insulated roof panel, it is possible to calculate its thermal resistance value, known as the ‘R-value’.

These R-values can then be used to calculate the thermal transmittance, or ‘U-value’, of the insulated roof panels. U-values are a measure of the thermal performance of building elements, and they are entered into SAP or SBEM software to help determine the energy efficiency of the building

How do you calculate the R-Value and U-value of an insulated roof panel?

The R-value, or thermal resistance, of a material is determined by its thickness and thermal conductivity. The R-values of all the individual materials within the panels can be added together and, along with other factors, used to calculate the U-value of the panel.

If we first look thermal resistance, the R value is measured in m²K/W, and is equal to the thickness of the material divided by its thermal conductivity or Lambda value:

R-value = thickness (m)
              --------------------
             Lambda (W/_mK)

The R-value of a material is therefore thickness dependent – the thicker it is, the more resistance it will offer, whereas the Lambda value of a material will not change with thickness.

Once the R-value of each of the components that make up the insulated roof panel is known, the U-value of the panel can be calculated. However, these R-values alone will not be enough to give the true picture – other factors that influence thermal performance, such as joint geometry, surface resistances and fixings, must be included before the final U-value of the insulated roof panel can be determined.

There are rules that cover how U-values should be calculated within a document produced by the BRE, called BR 443 ‘Conventions for U-value calculations’. This references several calculation, measurement and product standards to ensure that U-value calculations are an accurate reflection of thermal transmittance across a range of products in a variety of combinations.

The U-value is important as it indicates the rate of heat loss through a structure such as a wall, floor, or roof. The lower the U-value the better insulated the structural element and less heat will be lost, making the building envelope more energy efficient.

How thermally efficient are insulated roof panels?

A wide range of thermally efficient insulated roof panels are available to match the U-value requirements of the latest building regulations and meet the thermal demands of modern building envelope design. A combination of different insulation types and thicknesses allows insulated roof panels to offer very low U-values in the region of 0.20 W/_m²K to 0.15 W/_m²K.

The engineered nature of insulated panel construction ensures consistent insulation performance. It also enables precise installation of panels on site to create high levels of airtightness that also contribute to providing a thermally efficient building envelope.

At Tata Steel our Trisobuild^® insulated roofing systems and Trisomet^® insulated roof panels provide our customers with a choice of thermally efficient solutions. They can be constructed on-site or arrive as factory made panels and can be tailored to match specific project U-value requirements. If you would like to check that the building envelope on your project has the lowest possible carbon footprint, we have a useful online Building operational CO₂ tool.

If you would like to discuss any aspect of the thermal performance of our insulated roof panels, please contact the Tata Steel building envelope team for more information.