Thermal bridging and the Future Homes Standard

[edit] Introduction

The Government has responded to the Future Homes Standard Part L consultation, and new standards and compliance metrics to be adopted have been confirmed. There is a need for the industry to step up to ensure we meet this new performance measure. Being delivered as part of the new Future Homes Standard, the proposals are ambitious – but they need to be if we are to hit our net zero emissions target by 2050.

Whilst new targets and regulations seem onerous, the ability for us to create thermally efficient homes is not something new. Ensuring the fabric of our homes is well designed is a critical first step, and by paying careful attention to detailing we can eliminate some of the issues that create poorly performing homes – one culprit being thermal bridging.

[edit] Confirmed changes

Following a consultation on proposed changes to Part L (Conservation of fuel and power) and Part F (Ventilation) for new homes, proposed changes have now been confirmed and will be the first step in achieving the ambitious new Future Homes Standard. The second stage of this consultation, known as the Future Buildings Standard, will set out energy and ventilation standards for non-domestic buildings and existing homes. It will also include proposals to mitigate against overheating in residential buildings.

The October 2019 Part L consultation proposed two potential carbon reduction targets for the interim 2020 date – a reduction of 20% or 31% in carbon emissions compared with 2013 Part L. The Government has chosen to introduce its preferred 31% option. The finalised detail of Part L is set to be published in December 2021 and come into force from June 2022. This gives the industry around 18 months to prepare for the changes.

All homes will have to meet tightened energy efficiency standards to lower energy consumption and bills, helping to protect the environment. The 31% target will provide a meaningful and achievable increase to the energy efficiency of new homes compared to current levels.

This will mean developers have to upgrade their current specifications through a combination of more thermally efficient materials and introduction of low carbon technologies. Designers will retain the flexibility they need to use the materials and technologies that suit the circumstances of a site and their business (e.g. PV and gas boilers or air source heat pumps).

These changes are essential if we are to meet the Government’s plans to radically improve the energy performance of new homes - with all new homes being highly energy efficient and equipped with low carbon heating and being zero carbon ready by 2025.

[edit] Getting the fabric right

How are we going to achieve this standard? One key area will be addressing the thermal performance of a building envelope through a fabric first approach to building design.

The secret to thermally efficient building envelopes, fabric first focuses on materials and components that make up the building fabric before considering costly renewable technologies. It is an approach that will enable specifiers to meet - and even exceed - regulatory performance criteria, whether it is for large scale social housing or a much smaller residential property.

Whilst a reduction in CO2 emissions is one consideration when designing thermally efficient housing, an improvement in thermal comfort can also have a positive impact on occupants – adding to their thermal comfort, productivity and wellbeing.

[edit] Thermal bridging

A critical element of the fabric first approach will be addressing the issue of thermal bridging, which can be responsible for up to 30% of a home’s heat loss. Eliminating thermal bridging through good design and correct product specification will be essential if we are to ensure we meet these ambitious new regulations.

A localised area in the thermal envelope of a building, a thermal bridge is where there is increased heat loss compared to the surrounding area. For example, a thermal bridge is created when a traditional steel lintel spans between the inner and outer leaf of a cavity wall, providing a clear path for heat to bypass the insulation and escape to the outside environment. Weak spots in the continuity of insulation such as this can have a significant impact on a building’s heat loss and have a detrimental effect on the overall fabric efficiency of the external wall.

We need to first define the two different types of thermal bridges in a building envelope: repeating thermal bridges and non-repeating thermal bridges.

Repeating thermal bridges are accounted for in the calculation of a building's U-values (U-values measure how effective a material is as an insulator). Any material that penetrates the insulation layer repeatedly and predictably is classed as a repeating thermal bridge. For example, this could be where timber studs bridge a layer of insulation in a wall or wall ties between skins in traditional construction. The heat loss from these elements is accounted for in the overall U-value calculation for the element they sit within, i.e. external wall in this example.

Non-repeating thermal bridges occur where building elements meet. Examples include where walls join with floors, where walls join with the roof and around openings such as windows and doors.

Psi values are the thermal units used to measure the amount of heat loss from these junctions. Each junction must be assessed independently in the SAP calculation. Specifying materials and products with lower psi values will ensure an efficient, cost effective fabric is achieved for the building. It is important for designers to obtain independently calculated psi values from manufacturers or energy assessors to ensure they are accurate.

[edit] Thermally efficient lintels

Traditional steel lintels can create a significant thermal bridge in homes due to the high thermal conductivity of steel and because they span over long lengths in a typical build. In addition to considering the wall construction, the length of the lintel and the load supported by the lintel, a lintel design which incorporates a thermal break will outperform and be more thermally efficient than a standard lintel.

For instance, hi-therm+ lintels use a patented combination of a polymer isolater and galvanised steel. The polymer section provides a powerful thermal break in the lintel and significantly reduces its conductivity value, with a psi value of between 0.03 and 0.06 W/m.k. As a result, hi-therm+ lintels are up to five times more thermally efficient and are also available in the same lengths, sizes and loading capacities as the standard range of lintels.

The importance of lintels should not be understated. The hi-therm lintel has an impressively low thermal conductivity performance which contributes towards its psi value of between 0.03 and 0.06 W/m.k., making it the ideal low cost and sustainable solution for specifiers aiming to achieve building regulations with the fabric first approach.

When you consider the BRE has found that thermal bridging can account for up to 30% of heat loss from buildings, then paying close attention to the details and structural elements such as lintels can have a huge impact on the overall thermal performance of a building.

In both theory and reality, the fabric first approach will ultimately bring us closer to the carbon reduction targets proposed under Building Regulations Part L and the 2025 Future Homes Standard. If a building’s fabric is designed and built as efficiently as possible in the first instance it will continue to perform as intended and save energy for years to come.

This article originally appeared under the headline, 'Meeting the Future Homes Standard: Eliminating thermal bridging will be critical' in the Architectural Technology Journal (at) issue 137 published by CIAT in spring 2021. It was written by Keystone Lintels Limited.

--CIAT

[edit] Related articles on Designing Buildings Wiki

• Approved document L.
• Building regulations.
• CIAT articles.
• Fabric first.
• Future Homes Standard.
• Lintel.
• Thermal bridging in buildings.
• U-values.