We have undertaken a desktop study to test the significance of lintel performance on the outcome of Part L assessment. The study was based on the 2010 version of the Building Regulations for new homes. We conclude that selecting lintels with a view to their thermal performance can be of real advantage to designers seeking to meet strict carbon targets.
In our recent article on thermal bridging (March 2012), we considered the benefit that could be gained by considering the performance of improved building junctions. Now we consider the worst performing of all junctions – the window lintel.
Worst Case: If no attempt is made to consider the thermal performance of lintels, then we must use a default worst case in the SAP calculation. This would be in keeping with a general assumption that no effort has been made to reduce the heat loss due to bridging elements (In SAP jargon, an overall Y value of 0.15, a lintel psi-value of 1.0W/m.k)
Approved Construction Details (ACD): It is possible for the house builder to sign-up to an improved set of details, including a lintel detail. This gives an enhanced performance in the SAP. The national approved details (ACD details) describe measures to improve lintel performance when they are installed in masonry, timber or steel frame external walls. (In SAP, this would tend to give an overall Y value closer to 0.08, though results vary).
The ACD details distinguish between two lintel types. Those with a perforated baseplate have an assumed performance that is twice as good as the worst case (a lintel psi-value of 0.5W/m.K). All other lintel types without base plates are treated yet more favourably than this (lintel psi-value = 0.3W/m.K).
Manufacturer Details: Can we go better than the national approved details when specifying lintels? Upon request, manufactures can supply psi-values for their lintel products. These must be treated carefullybecause the detail of the surrounding wall is also a significant factor.
Some manufacturer’s psi-values reviewed by Richard Hodkinson Consultancy are a close match with the ACD values. However, some were discovered to be much better. To illustrate this, we carried out a deskstop study that included a lintel with half again the heat loss of the ACD lintel (a lintel psi-value of 0.15W/m.K).
Our desktop study applied these performance values to four different houses types (A,B,C,D), in configurations that include both mid terraces and end terraces. The results are visualised in Table 1.
We found that by adopting a nationally approved lintel detail, overall CO2 emissions can be reduced by approximately 2.5%. And a further 2% reduction might be obtained by selection of a best practice manufactured lintel.
In the context of current Building Regulations, a reduction of greater than 4% is not insignificant, and we conclude that this is definitely an area of building construction that is worth more consideration.
TABLE 1: Building Regulation CO2 Savings associated with improved lintel psi-value: