Keystone: lintels key to tackling thermal bridging

According to Keystone Lintels Ltd: “Lintels are in most cases the most significant non-repeat thermal bridge in a dwelling, as traditional style cavity wall lintels interrupt the line of insulation with a continuous piece of highly conductive steel. The default or accredited Psi-value for a typical steel lintel is between 0.3 W/mK and 0.5 W/mK thus accounting for more than a third of the total heat loss through non repeating thermal bridges [in a typical case].”

The company said the award-winning Keystone Hi-therm lintel virtually eliminates thermal bridging across the lintel. The Hitherm lintel utilizes a two piece construction, with a galvanized steel component supporting the inner blockwork connected to a GRP component supporting the outer brickwork.

The GRP component has much lower thermal conductivity therefore reducing total heat loss substantially, resulting with a very low Psi-value of 0.05W/m.K or lower depending on block/ insulation conductivities. The Hitherm lintel can provide in excess of an 80% improvement over the default or accredited lintel Psi-value, offering a significant advantage within Sap or Deap calculations.

Thermal bridging is factored into building regulations compliance in both the UK and Ireland, with the industry given various options, such as the use of punitive default values or inputting calculated values. A common alternative to using default values is to source calculated psi-values created by a person with suitable experience and expertise or a third party accredited assessor. Many leading building product suppliers such as Keystone Lintels now produce these bespoke Psi-values for their customers at no charge.

The accredited assessor will use specialist thermal modelling software to draw the construction detail in the software package and then add in the material conductivities. This software then calculates the heat loss at the junction, subsequently producing an accurate Psi-value. The bespoke Psi-value can often be significantly better than the default or accredited value, thus having a dramatic improvement within the SAP/ Deap calculations.