INSULATION 67
Thermal efficiency, cost-effectively I
Actis Insulation’s Thomas Wiedmer says that while many specifiers continue to struggle to maximise energy efficiency in dwellings, low carbon results are achievable without adding significantly to the build cost
t is a truism that the fabric of a building is the key to its thermal performance. But it is perhaps surprising that, despite the fact that everyone in the construction industry is fully aware of this, many still struggle to achieve maximum energy efficiency to achieve the required SAP rating on houses.
Once the key criteria in achieving an efficient thermal envelope are mastered, there really should be no reason why all new builds from now on (including those that fall under the more stringent Building Regulations Part L requirements coming into force in June) are not as thermally efficient as humanly possible – without costing much more than building a ‘heat leaching’ alterative.
The key to attaining this state of nirvana is in avoiding thermal bridging; indeed this is an explicit directive in the current Part L, which states: “Insulation should be reasonably continuous over the whole building envelope. The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements such as those around the window and door openings. Reduction in thermal performance can occur where the air barrier and the insulation layer are not contiguous, and the cavity between them is subject to air movement.”
Using flexible insulation, vapour control layers and breather membranes can go a long way towards addressing this, as they help reduce the margin for error, making the chances of thermal bridging very slight. This sort of product, due to its flexible nature can be guided into place rather than having to be cut accurately, can be installed in a continuous layer, thus providing a seamless air barrier and creating a better performing building envelope. New Part L regulations extend the emphasis on thermal bridging further and propose that drawings should be provided for junctions and that before elements are concealed, an onsite audit should be
ADF MARCH 2022
undertaken to confirm that the designed details have been constructed, with particular focus on product substitution. Recognising that construction detailing is one of the biggest issues causing the performance gap. Tying projects up with specific details used is important to close the gap between designed and expected performance.
This might sound onerous, but there is help at hand; You can use junction details from a reputable non-government database containing independently assessed thermal junction details, such as Local Authority Building Control’s Registered Construction Details Library (at
www.labc.co.uk). They offer a combination of specific detail, good practice and points to watch, together with a range of modelled psi-values using different build ups. The LABC RCD are freely available and also accessible on the go and drawings and documents can be fed into specifications for projects.
The key to attaining this state of nirvana is in avoiding thermal bridging
Build tight – insulated right While specifiers are given an element of flexibility on how to balance the efficiency of the building fabric and building services - two elements which are crucial for the SAP rating – the Target Fabric Efficiency Rate (TFEE) limits the U-values of thermal elements, ensuring that an efficient envelope provides the backbone to an energy efficient house.
A 15% leeway given under the FEE is aimed at ensuring developers can’t build homes whose envelopes are inefficiently constructed and make up the deficiency with excessive use of renewable technologies. It doesn’t have to cost any more to insulate a house to an impressively low U-value than to insulate it to the minimum required thermal efficiency. Some insulation materials are easier and quicker
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