Net zero homes: the future is timber

Finding a way to build homes that are low in both operational and embodied carbon is vital if the UK is to meet its carbon reduction commitments. We already have the tools and technology to do this. (Spoiler alert: timber-based systems make it really easy). All we need now is for regulation to catch up…

Housing’s carbon challenge

According to the UK Green Building Council (UKGBC), the UK’s built environment is directly responsible for 25% of greenhouse gas emissions, operational and embodied. Of that, just under 59% is due to domestic buildings.

The UK has committed to reducing our carbon equivalent emissions by 100% compared to 1990 levels by 2050 and at COP26 last year in Glasgow, the Government signed up to meeting a 68% reduction by 2030. As of 2020, emissions were down by just below 50% according to the latest figures from National Statistics. So, there is still a long way to go.

Changes to the Building Regulations, which came into force in June this year, will go some way towards reducing carbon emissions from housing. The new Approved Document L (Part L) which deals with energy emissions, will lead to a 30% reduction compared to the 2013 version of Part L.

However, tightening up Part L will only impact on operational carbon. Many, including the House of Commons Environmental Audit Committee, believe that the embodied carbon which goes into creating a new home should be reduced as well. In its May 2022 report, Building to net zero: costing carbon in construction, the committee, which monitors how Government policies impact on the environment and sustainable development, says:

“The embodied carbon cost of the construction is not required by current policy to be assessed or controlled, other than on a voluntary basis. As a result, no progress has been made in reducing these emissions within the built environment…If the UK continues to drag its feet on embodied carbon, it will not meet net zero or its carbon budgets.”

What is net zero, anyway?

‘Net zero’ has become an everyday term, applied liberally to news stories and releases about construction projects. But it is not always clear exactly what it means.

The UKGBC set out an explanation of net zero for the built environment in its 2019 Net Zero Carbon Building Framework Definition, acknowledging that this was a high-level view that would develop over time. The definition sets out frameworks for net zero in construction and operation and promises a more detailed explanation of net zero whole life carbon in the future.

Net zero carbon in construction covers materials and products from their extraction, manufacture, transport and installation up to practical completion (stages A1 to A5 of a life cycle analysis). Net zero carbon in operation covers operational energy, such as for heating and lighting (stage B6). Whole life carbon would encompass both construction and operational carbon as well as maintenance and refurbishment over a building’s life, demolition and reclamation.

UKGBC proposes a ‘fabric first’ approach which means that for net zero in construction, the embodied carbon of a building’s components should be reduced as much as possible with the remaining carbon offset using a recognised framework at practical completion and publicly disclosed – something that is rarely done in today’s ‘net zero’ carbon stories. Net zero in operation should prioritise on-site renewables, followed by offsite renewables with any remaining carbon to be offset and disclosed.

What do the Building Regs say?

The changes to Part L, which ramp up the energy efficiency requirements for buildings, aim to pave the way for a further improvements in 2025 when the Future Homes Standard will be introduced, leading to emissions that are between 75% and 80% lower than with Part L 2013. The Standard will also say that from 2025 homes must be ‘zero carbon ready’ so that no retrofit work is required for them to be heated by electricity as the grid is decarbonised.

As well as ramping up energy efficiency requirements for the fabric of a home, the new Part L has a greater emphasis on eliminating thermal bridging which usually occurs at junctions between elements. This means that construction joints must be properly designed and executed, with on-site inspections and photos required.

Alongside the new Part L comes Part O which covers overheating. Part O says that homes should be designed to limit unwanted solar gain in the summer and with adequate means to remove heat from the indoor environment.

In July 2021, the construction industry proposed a new Part Z for the Building Regulations which would require whole life carbon assessments and embodied carbon limits for projects with more than 10 dwellings. On 25 November 2022, a private members’ bill, Carbon Emissions (Buildings) which is based on Part Z is to be reintroduced by Jerome Mayhew MP, having been withdrawn by its previous backer.

How can timber help?

High levels of insulation, and elimination of thermal bridging, can be achieved through a variety of wall and roof constructions. However, that does require a step-change in on-site practices to a much more rigorous and regulated approach to ensure that every interface and detail is delivered as designed.

Designers often turn to offsite closed panel timber systems inclusive of insulation when designing ultra-energy efficient homes. These systems are manufactured offsite in a factory setting with precision, they are resource efficient, can be erected very quickly, they are timber rich and offer excellent energy performance.

Such a system was used for the Athlete’s Village in the East End of Glasgow. The developer Legacy, a consortium of local companies working with the City Council, employed CCG to design and deliver 700 homes using its iQ closed panel system which incorporates insulation, service zones, pre-installed windows and doors.

Glasgow City Council is not alone in its desire for low-energy, affordable housing. Local authority and housing association developers, such as Exeter City Living and Cambridge City Council, are turning to a Passivhaus-style approach which involves a highly energy efficient fabric, extremely high levels of air tightness, little or no heating and mechanical ventilation. One such scheme is Heathcott Road in Leicester a Passivhaus scheme of 68 affordable homes, designed by rg+p for developer emh group and deploying Westleigh Partnerships’ Westframe PassiPlus+ timber framing.

As embodied carbon becomes more of a consideration, timber-based construction such as offsite closed panel timber systems offer advantages too. Traditional materials such as concrete blocks, bricks and masonry have significantly higher embodied carbon than timber. And making the panels offsite also means that material usage is optimised, reducing waste – and hence, carbon.

Even considering a traditional cavity wall construction, timber systems offer a lower embodied carbon solution. The designers of the UK’s very first certified Passivhaus with a cavity wall, Denby Dale, completed in 2010, recently commissioned calculations to compare the embodied carbon of different wall types for that house. Timber I-beam walls with render came out with the lowest embodied carbon, followed by timber I-beams and stone.

Some designers choose to go further to reduce embodied carbon – and to use natural, breathable materials – by selecting wood fibre insulation. For instance, Trunk CLT chose to use wood fibre insulation with cross-laminated timber (CLT) for Benslow Lane, a detached house in Hertfordshire. Available as rigid boards, semi-rigid boards or flexible quilts, wood fibre insulation also has the benefit of being non-toxic and safe to handle and work with.

Changing markets

Why aren’t we using more timber to build our homes? This is a question asked by the House of Commons Environmental Audit Committee in its report, in which it reports that multiple experts recognised that “the use of timber (subject to forestry management) in place of concrete, masonry and steel was one of the most successful strategies to reduce embodied carbon.”

There are, of course, issues to overcome. Not least, insurance companies and their approach to timber construction and the lack of a coherent policy to increase our home-grown supplies of timber products.

But while we wait for regulation to catch up, the market is moving on its own. With the growth of Passivhaus schemes, local authorities such as Greater London Authority setting their own net zero carbon targets for new developments, and companies setting their own carbon reduction agendas, some forward-thinking housebuilders have seen the writing on the wall and are already switching to timber frame and offsite. These are all steps in the right direction, but if we are to meet our net zero targets, a combination of market demand, industry action and government policy is needed today.