Bridge over troubled water

The correct lintel specification can help eliminate thermal bridges, reducing energy consumption and carbon emissions, says Keystone Lintels.

For specifiers, it is essential to understand the impact of thermal bridges if buildings are designed to meet Part L regulations and carbon reduction targets. With a steel lintel interrupting the insulation layer and creating a major thermal bridge in a building, its specification can compromise a building’s fabric thermal performance. This feature looks at the detailing around this junction, and why this important structural element can be designed to be more thermally efficient and offer better buildability.

As the UK continues on its path towards more energy-efficient homes and achieving its net zero targets, preventing heat loss by addressing thermal bridges has become increasingly important. In fact, research undertaken by the Building Research Establishment (BRE) has found that thermal bridging can account for up to 30% of heat loss from buildings. To address this, architectural detailing and on-site construction practices have become the focus, but, in the first instance, it is important to understand what a thermal bridge actually means.

In short, a thermal bridge is a localised area in the thermal envelope of a building, where there is increased heat loss compared with the surrounding area. For example, a thermal bridge is created when a steel lintel spans between the inner and outer leaf of a cavity wall, providing a clear path for heat to escape by bypassing the insulation layer.

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 thermal transmittance or U-value (ie 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 heat loss or Psi values (pronounced Si with a 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 impact on the building’s energy performance must be calculated independently in addition to U-values.

Heat loss and lintels

Today, almost all lintels in domestic-scale dwellings are made from steel for a number of reasons, such as providing more design flexibility and easier on-site handling than other alternatives. However, steel has a high conductivity value, so, with lintels typically spanning long lengths, it is no surprise, when you add them all up in a building, that they are considered one of the worst contributors to heat loss via thermal bridging. Therefore, considering the thermal performance of lintels at design and specification stage is more important than ever.  

A lintel design that incorporates a thermal break will outperform and be much more thermally efficient than a standard lintel. For instance, Hi-therm+ lintels use a patented combination of a polymer isolater and galvanised steel to bond the internal and external walls 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 0.03-0.06W/mK, making it the ideal low-cost and sustainable solution for specifiers aiming to achieve building regulations with the fabric-first approach.

Fabric first

The fabric-first approach has, for some time, been supported by the housebuilding industry. It is an approach to building design that looks at maximising the energy performance of the structure itself through the components and materials making up the building envelope. This is a first step before renewable, mechanical or electrical building services or technologies are considered to further reduce CO₂ emissions. A well-designed fabric can, on its own, reduce energy consumption and, therefore, lessen bills in any building type. This approach factors in a number of aspects, such as a high level of thermal insulation, designing out thermal bridges, excellent levels of air tightness and a design that maximises elements such as building orientation, making the most of solar gains from the sun.

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 CO₂ emissions achieved through fabric measures is built in for the life of the building, and, therefore, can ensure that the energy demand and CO₂ emissions of a site remain low. Renewable technologies, on the other hand, have a more limited lifespan than building fabric and may give rise to further embodied CO₂ emissions from a development once they reach end of life if they are not maintained or replaced at a cost to the homeowner.

With junctions above openings in buildings so vulnerable to heat loss through thermal bridging, paying close attention to details and structural elements such as lintels is a key piece of the puzzle to ensuring energy-efficient buildings. Adopting a fabric-first approach will ensure buildings continue to perform as designed and will go some way to maximising the overall efficiency of UK homes, ensuring they are well positioned for future regulatory changes.    

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Image credit | Keystone Lintels



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