Understanding psi values - the thermal performance of lintels

Psi values are a measure of the thermal performance of junction details. Junctions within buildings are areas where different construction materials meet, such as at the intersections of walls with floors or walls with roofs. In the case of lintels, the supporting steel lintel can interact with a variety of materials such as bricks, blocks and door or window frames, depending on its position and function within the building.

Additional heat can be lost at junctions compared to the surrounding building fabric. These additional heat flows are often referred to as thermal bridges.

Psi values for junction details are calculated using specialist software. Psi value units are W/mK (Watts per metre Kelvin) which is a measure of the heat flow per unit length of the junction for a temperate difference of 1 Kevin or 1oC. Just as with U-values, when looking for good thermal performance figures for psi values, the lower the number the better.

Psi values are so important because they provide a way to measure just how much heat will be lost from individual junctions within a building.

Heat loss at junctions within a building can account for around 30% of the overall total heat loss. So careful consideration of these values allows the specifier and designer to make an informed choice on which materials to use when looking to minimise heat loss and importantly reduce the environmental impact of a building.

Lintels are an essential requirement for a building from a structural perspective and found over openings, such as doors and windows, and in buildings with masonry walls. They are therefore present in significant numbers depending on the size and design of the building.

Specifying lintels based on their psi value to improve the thermal performance of the junction can bring cost benefits elsewhere in the design when meeting the thermal performance targets within the current SAP calculation requirements.

Assessing the impact of the psi value of a lintel can also be a way to explore pushing the building fabric beyond current compliance targets and on towards the new requirements of Part L 2021.

The new Approved Document came into play on 15th June 2022 and requires an overall 31% improvement in CO2 emissions over current levels for dwellings. This will put extra emphasis on all areas of fabric performance. The thermal contribution of lintels in general, and in particular, Thermally Broken Lintels (TBL), that can offer lower psi values, will become a key area to focus upon when specifiers look for ways to enhance the thermal performance of buildings.

The separation of the inner and outer sections of steel in a TBL disrupts the ability of heat to transfer across the cavity. The addition of insulation between the two steel sections further reduces the ability for heat to cross over from one side to the other. This enables a TBL to achieve lower psi values and makes it the perfect choice when looking for specification options that will have a significant and lasting impact on overall fabric thermal efficiency.

Catnic has a range of lintels with a range of psi values to suit any project. For information about any lintel in the Catnic range use the product selector, download technical literature, or contact us to discuss your project requirements.