Thermal bridging in buildings is known to compromise the performance of the building’s fabric, which affects the ability to meet Part L regulations and carbon reduction targets. Lintels and detailing around junctions are key to properties performing as-designed. Ben Cheetham, National Specification Manager at Keystone Lintels, considers how these vital structural elements can be designed with thermal efficiency in mind.
Preventing heat loss by addressing thermal bridging is growing in importance; particularly with the UK government aiming to create more energy efficient homes and attain its ultimate goal: carbon-neutrality by 2050. Indeed, research carried out by the BRE found that thermal bridging can account for up to 30% of heat loss from buildings. To address this, architectural detailing and onsite construction practices have become the focus, but in the first instance it’s important to understand what a thermal bridge actually means.
Explaining ‘the bridge’
A thermal bridge is a localised area in the thermal envelope of a building where there is increased heat loss compared to the surrounding area. A building envelope will have two types of thermal bridge: repeating thermal bridges and non-repeating thermal bridges. Repeating thermal bridges are accounted for in the calculation of a building element’s U value (i.e. an external wall). Any material that interrupts the insulation layer in a repeatable and predictable way would be classed as a repeating thermal bridge. Examples of these would be steel wall-ties in masonry construction or timber or steel studs in framed construction.
The remaining non-repeating thermal bridges are typically found in junctions located within the external envelope of the building and are dealt with by “Psi values” – pronounced ‘Si’ (silent p). Examples of non-repeating thermal bridges would be a steel lintel above a window or door opening. Junctions such as these are assessed using thermal modelling software and their impacts on the building’s energy performance must be calculated independently in addition to U-values.
Heat loss and lintels
Today, the majority of lintels in domestic-scale dwellings are made from steel for a number of reasons, such as providing more design flexibility and easier onsite handling than other alternatives. However, steel has a high conductivity value and with lintels typically spanning across long lengths when you add them all up in a building, it’s no surprise they contribute significantly to heat loss via thermal bridging. Therefore, taking into account the thermal
performance of lintels at design and specification stage is more important than ever.
A lintel design which incorporates a thermal break will outperform and be much more thermally efficient than a standard lintel. For instance, Keystone’s award-winning Hi-therm+ lintels offer an innovative solution to improving SAP energy calculations, meeting Part L regulations and reducing thermal bridging by up to 80%. Their all-round performance is due to a patented combination of a polymer isolater and galvanised steel to bond the internal and
external leafs together by spanning the intervening gap. The polymer isolater provides a powerful thermal break in the lintel and virtually eliminates this key thermal bridge. As a result, Hi-therm+ Lintels are up to five times more thermally efficient than a standard steel lintel; they are also available in the same lengths, sizes and loading capacities as the standard range of lintels.
The Hi-therm Lintel has an impressively low thermal conductivity with a Psi value of 0.03- 0.06 W/m.K, making it the ideal low cost and sustainable solution for specifiers aiming to reduce energy demand through a fabric first approach.
Part L 2021?
As the government considers two options for an uplift in energy efficiency standards in Part L – now expected to be brought in during 2021 – the challenge for architects is to meet these ever more stringent energy ratings in a cost-effective manner. Both options suggest additional uplifts in fabric targets to further maximise energy savings that can be built-in for the lifespan of the building.
Option 1 (‘Future Homes Fabric’) is intended to deliver a 20% improvement on the current Part L standard. This is expected to be delivered predominantly by very high fabric standards, which means lower levels of heat loss from windows, walls, floors and ceilings.
Option 2 (‘Fabric plus technology’) is intended to deliver a 31% improvement on the current standard. It’s expected that this would typically be delivered through a more minor increase to fabric standards alongside use of low-carbon heating and/or renewables technologies
Fabric first
Whatever the outcome, it’s clear that improving the building fabric will be key to meeting these new regulations. A fabric-first approach to property design concentrates finance and efforts on improving fabric U-values, reducing thermal bridging and improving airtightness. It is a first step before renewable, mechanical or electrical building services or technologies are considered to further reduce CO 2 emissions. A well-designed fabric can, on its own, reduce energy consumption and therefore lessen bills in any building type.
The benefits of this approach are increasingly widely realised and ongoing research continues to reinforce the significant positive impact this approach can have economically, environmentally and socially. The reduction in CO 2 emissions achieved through fabric measures is built-in for the life of the building to therefore ensure that the energy demand and CO 2 emissions of a site remains low. Renewable technologies, on the other hand, have a limited lifespan and risk a significant increase in emissions from a development once they reach end-of-life, if not maintained or replaced at a cost to the homeowner.
With junctions above openings in buildings particularly vulnerable to heat loss through thermal bridging, paying close attention to the details and structural elements such as lintels is key to ensuring energy efficient buildings. A fabric-first approach to property design will provide the all-important framework to ensure ‘as-designed’ performance is achieved whilst futureproofing homes against regulatory changes.