Part L Building Regulations

5 times more thermally efficient than a standard steel lintel.

Part L Answered

Part L 2013 introduces the FEES standard with the requirement that new homes should comply with a mandatory minimum fabric performance standard (Target Fabric Energy Efficiency, TFEE) in addition to the original carbon emissions standard (Target CO2 Emission Rate, TER).

The new regulations put emphasis on heat loss due to thermal bridging at junctions which impacts on both the dwelling CO2 emission rate (DER) and fabric energy efficiency rate (DFEE) of the dwelling which is calculated within SAP.

What is a Psi value?

The thermal performance of a lintel is expressed in terms of a Psi value ( ) i.e. Linear Thermal Transmittance, which is calculated using specialist thermal modelling software.
Y-values account for the heat loss through non-repeating thermal bridges (i.e. lintels, cills, jambs etc) within SAP. Y-values are calculated by measuring the linear length of the thermal bridge and multiplying by the respective Psi value, hence the importance of low Psi values. Using a low Psi value significantly improves the Y-value, which in turn has a positive impact on the overall SAP calculation.

Why Hi-therm?

Lintels over doors and windows account for a significant amount of a buildings non-repeating thermal bridging and thereby associated heat loss. The use of Hi-therm can reduce a dwelling’s thermal bridging by up to 1/3 offering an enhanced fabric first solution to housebuilder’s assisting them with compliance to Part L.


Hi-therm is the only one piece lintel which achieves the Appendix R value for steel lintels in Part L 2013