Ben Cheetham of Keystone Lintels considers how housebuilders can, by pursuing a fabric-first approach to energy efficiency and careful specification, avoid the problems of thermal bridging
Preventing heat loss by addressing thermal bridging is growing in importance – particularly with the Government aiming to create more energy efficient homes and attain its ultimate goal: i.e. carbon neutrality by 2050.
Indeed, research carried out by the BRE found that thermal bridging can account for up to 30 per cent of heat loss from buildings. To address this, architectural detailing and onsite construction practices have become the focus. In the first instance, however, it’s important to understand what a thermal bridge actually is.
EXPLAINING ‘THE BRIDGE’
A thermal bridge is a localised area in the building envelope where there is increased heat loss compared to the surrounding area. Two types of thermal bridge can occur: 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 measured in “Psi’ values. Examples of non-repeating thermal bridges would include 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 domesticscale 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 the 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, and so housebuilders should look to achieve this from the outset.
PART L CHANGES DRIVING EFFICIENCY
The Government is considering two options for an uplift in energy efficiency standards in Part L – now expected to be brought in during 2021 as part of the Future Homes Standard which comes into force in 2025. It will be a challenge for builders 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 – with thermal bridging again a pertinent issue.
Option 1 (‘Future Homes Fabric’) is intended to deliver a 20 per cent 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 per cent improvement on the current standard. It’s expected that this would typically be delivered through a lesser increase to fabric standards, alongside use of low-carbon heating and/or renewables technologies.
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 CO2 emissions. A well-designed fabric can, on its own, reduce energy consumption and therefore reduce bills in any building type.
The benefits of this approach are increasingly widely recognised, 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, to therefore ensure that the energy demand and CO2 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, while also futureproofing homes against regulatory changes.
Ben Cheetham is national specification manager at Keystone Lintels