006 BP 1016

BP 10 October 2016

COVER STORY | CATNIC | ADVERTORIAL THERMAL THINKING Thanks to the flexibility afforded through steel construction, steel lintels have become the byword for architectural achievement across the industry. Yet increasingly stringent energy efficiency legislation, combined with sophisticated measurement of energy wastage through the building envelope, has brought to light the need for improvements in the psi values of lintels. Here, Richard Price, Technical Director at Catnic explores the issue. Steel provides a material that is relatively lightweight, enabling greater possibilities in construction with steel components safer to handle and use onsite too. What’s more, because steel is recyclable, components can be recovered from a building at the end of its useful life or during a phase of regeneration and renewal, and used again. Essentially, whatever is designed or aspired to, the architect, engineer or specifier can be confident that the result will be achieved effectively, seamlessly, soundly, legally and sustainably. Crossing the thermal bridge While steel lintels have created opportunities in architectural achievement and structural performance, over more recent years the increasingly stringent Building Regulations and Part L in particular has thrown the spotlight on the potential for energy in the form of heat to be lost through structural details. Any material, component or system that forms a bridge linking two structural features – such as roofs and walls, or walls and floors, together - stands to risk the transfer and dissipation of heat. The thermal performance of a building takes into account heat loss through both the main 6 BUILDING PRODUCTS | OCTOBER 2016 fabric of the building and the junctions between building elements. Heat lost through the building fabric, for example through walls, roofs, floors and doors and windows, called U values, is measured in W/m2K. Heat lost at the junctions between these elements, for example at window heads, window jambs, window sills, corners, walls and floors and the walls and roof of a building at its eaves, called psi values, is measured in W/mK. The total fabric heat loss of the structure is calculated as the sum of the U value multiplied by the area plus the sum of the psi value multiplied by the length of the building. Of course the challenge for modern building specifiers is the unequivocal need for steel lintels. Most structures would not be built safely, compliantly or cost effectively without them. But how to avoid the potential for heat loss through the thermal bridge they might effectively create? All lintels from a reputable manufacturer can be used and will comply with the Building Regulations. However the psi value of all lintels specified must be taken into account and obviously, the lower the overall psi value, the better the resulting total fabric heat loss of the building will be. Where lintels with higher psi values are used, there will be a pay off; the specifier needs to calculate where compensations can be made elsewhere. So for example cavity sizes might be wider, more insulation may be needed, additional insulation might be required at the window and door heads. At greater cost but also resulting in a reduction to the overall impact of a higher total fabric heat loss might be the specification of solar PV, heat recovery systems and so on. Regulations and standards The dialogue cannot eschew the Fabric Energy Efficiency Standard (FEES). This forms the foundations of the Building Regulations Part L 2013. It specifically focuses on the thermal performance of walls, roofs and floors. In turn, without improvement, the proportion of heat lost through thermal bridges in the building fabric, such as the lintels, is therefore emphasised. Part L of the Building Regulations has become progressively more complicated, though there is an optional ‘off the peg’ solution, based on the Part L 2013 Notional Dwelling. This is fully detailed in SAP 2012 Appendix R and demands a lintel Psi value (the heat lost through a linear


BP 10 October 2016
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