CPD 06 2021: Specifying timber to target Net Zero

This CPD sponsored by TG Escapes Eco-Buildings looks at how timber can make its mark in greening the delivery of the built environment, as construction embarks on a step change in techniques

Facing up to the carbon problem

The UK construction industry contributes around 6% of the UK’s GDP (according to a parliamentary research briefing), while the UK Green Building Council (UKGBC) estimates that 10% of the UK’s carbon emissions can be directly attributed to construction.

Taking into account buildings’ lifetime operational outputs, the built environment sector is responsible for up to 45% of UK emissions. The construction sector therefore causes proportionately far more carbon emissions than it contributes to GDP.

The education sector makes up 17% of all construction projects, split 65% on school and college projects and 35% on university projects, with an estimated 35% of the total being publicly funded. These figures suggest publicly funded educational projects directly contribute 0.36% to the UK’s GDP and 0.6% of the country’s carbon emissions.

If we accept that public money should be used for the public good, ought not publicly funded projects be required to meet lifecycle net zero standard PAS 2060 to help reduce the UK’s carbon emissions?

PAS 2060 uses BS EN 15978 lifecycle stages (see diagram overleaf) to record carbon contributions. The lifecycle is split into several stages: product, construction phase, in use and end of life. Most of the embodied carbon in the whole-life carbon process comes from raw materials extraction, the manufacturing process and the transport of construction products, which collectively contribute about 50% of the embodied carbon.

The next most intensive stage is “in use”, with 20% of embodied carbon coming from periodic maintenance, repair, replacement and refurbishment, and another 23% coming from operational carbon due to energy and water usage. This leaves the construction and decommissioning phases contributing only 7% of the embodied carbon of a traditional build.

By 2030 all buildings will be expected to achieve net zero in operation, which has the potential to reduce the whole-life carbon of a project by around 20%. Realistically, however, this will be much less than 20% due to the embodied carbon of the measures used to lower operational energy. For instance, technologies used for renewable energy (essential in reducing operational carbon) bring an additional embodied carbon cost in the product, maintenance and replacement stages during their lifetime.

Achieving – and defining – net zero

The term “net zero” can be used in different ways. In the context of the built environment, it is used to describe both buildings that are net zero carbon in their day-to-day operation, and those that are net zero carbon in their construction.

The UKGBC has played a vital role in developing a workable definition to help the construction industry in its development of buildings that achieve the objective of net zero carbon. It has adopted a two-pronged approach to defining a net zero building, comprising operational use and construction practice.

The starting point for a net zero construction definition is: “when the amount of carbon emissions associated with a building’s product and construction stages up to practical completion is zero or negative, through the use of offsets or the net export of on-site renewable energy”.

Likewise, the current definition for a building’s in-use energy is: “when the amount of carbon emissions associated with the building’s operational energy on an annual basis is zero or negative”.

Operational energy includes heating and cooling systems, cooking, lighting and small power. Construction energy should incorporate the potential costs of building in flexibility, actual later adaptation and the impact of deconstruction. This considers the total carbon emissions created through construction, adaption and deconstruction over the building’s lifetime.

Only when both elements satisfy a net zero ideal can a building be deemed to fully comply with the aims of the Climate Change Act and fit within the government-set target of 2030 for operational net zero and the 2050 embodied carbon target.

The disciplines necessary to meet the net zero goal target in construction are still in the early stages of development. However, there is likely to be a rapid, iterative evolution of the principles and metrics. These will become subject to sharper definition, allowing for a tighter set of guidelines.

Collaboration and consensus within the construction sector – including builders, designers and policy-makers – is more likely to lead to the resulting system of recommended building methods, practices and policies.

Lifecycle carbon emissions under BD EN 15978

Rules, regulations and responsibility

The government has initiated this process with specifications PAS 2050 for assessing the lifecycle greenhouse gas emissions, and PAS 2060 to demonstrate carbon neutrality for certification.

Until such time as a legally binding set of net zero building regulations is created, the general principles that the UKGBC is encouraging the construction industry to adopt are threefold.

Firstly, the polluter pays and any emissions made should, ideally, be measured and offset as they occur. Secondly, measurement of emissions should be accurate and not estimated, and the data collected must be made available transparently and publicly. Finally, action should take place now, before the formulation of prescriptive requirements.

At present, the UKGBC stipulates that the primary priority in achieving net-zero operational energy efficiency is to reduce both the demand for and the consumption of energy: that which is used should be calculable and disclosed.

With regard to construction, a whole-life carbon assessment should be undertaken (and disclosed) and all embodied carbon impacts from the products and construction methods used must be measured and offset.

In both the use and construction of a net zero building, every effort must be made to utilise renewable energy supplies (both on and off site) and any remaining carbon should be offset using a recognised framework, again to be publicly disclosed.

In August 2020, the government’s Property Agency issued a paper, Net Zero and Sustainability: Design Guide, which while aimed specifically at the government’s own building estate has drawn heavily on the UKGBC framework to identify the steps and processes that a project team should undertake to deliver a net zero building.

Using timber for schools

As timber is a natural product that sequesters carbon as it grows, it is a highly suitable material for reducing carbon during the product stage. Each square metre of timber frame removes and stores 12.5kg of CO2 during stages A1-A3, and 12.1kg of CO2 for stages A1-A5, which end with practical completion. While it is unlikely that the whole build could become negative in terms of carbon just by using a timber frame, it is an essential factor in achieving a calculated net zero build.

However, timber has some considerable secondary benefits. It is a truly sustainable product when sourced from forests certified by the Forest Stewardship Council or the Programme for the Endorsement of Forest Certification. It also has well-known biophilic properties which help enhance a learning environment.

Timber classrooms have been proven to reduce stress among students versus traditional classrooms, as evidenced by a 2007 study, Schools without Stress, by Weitzer Parkett and proHolz of Austria. As well as reducing stress, biophilic design can enhance sensory and motor development by elements from the living, natural environment which can help inspire curiosity, imagination and discovery in students. Furthermore, the use of natural materials can reduce fatigue, while cognitive ability and emotional wellbeing can be increased by the inclusion of nature in learning environments.

Classrooms that incorporate biophilic design can offer significant benefits to learning, including increased attendance, higher test scores, improved behaviour, reduced stress and increased focus, while reducing problems around attention spans and ADHD behaviours.

Prices and budgets

Theconstructionindex.co.uk shows the £/m² price for timber frame and masonry construction are similar at £1,148.38/m2 and £1,180.34/m2 respectively: a difference of £31.96/m2 in favour of timber.

If we take an average 250m2 building, we can expect 300m2 of external wall area and 150m2 of internal walls and partitions. Assuming we have a cavity wall construction for the external walls, this gives a product stage embodied carbon of 54kg CO2/m2 and for the internal walls 16kg CO2/m2. See table 1.

If we only look at the pure wall structure, excluding finishes, on a 250m2 build, the masonry solution would have produced 18,600kg CO2 and timber -5,625kg, a difference of 24,225kg CO2. Carbon offsetting prices are relatively low at the moment: some schemes are pricing a tonne of carbon at as little as £10/tonne, while others price it as high as £70/tonne to a retail customer.

Using table 1 to calculate the difference in carbon, and converting to per m2 of total floor area, we can deduce that a masonry wall structure will contribute 74.4kg CO2/m², whereas the timber frame will sequester 22.5kg CO2/m². Using the cheapest rates available, offsetting the masonry construction would cost an estimated extra £0.75/m², and using timber would save £0.22/m². So the saving in offsetting costs by using timber is roughly £0.97/m², rising to £6.97/m² if using the highest retail rates.

However, whole lifecycle analysis requires us to also record the in-use and decommissioning stages. The above figures look at the product and construction process only. If we assume comparable in-use cost, we must next assess the difference between the two construction methods at end of life stage C (see table 2).

End of life stage C carbon cost

As it is assumed that the majority of the timber in the future would be incinerated for power, timber performs poorly at stage C, releasing twice as much carbon as the masonry build. However, when we take the three stages together, we find that timber is still significantly better, as can be seen in table 3.


Overall a timber construction will always be more carbon efficient than a traditional masonry build. At the moment, there is little benefit from a cost perspective. However, as the carbon trading market grows, it is expected that prices will rise as people move to offset the emissions of their products, improving the economic case.

As we move to net zero in the operation economy from 2030, with utility providers offering offsetting, we expect to see prices rise as demand in offsetting credits increase. This is likely to result in a boom in renewables. These have an embodied carbon element, and the next focus will be the embodied element in the structure and services.

The use of a timber structure makes achieving net zero easier and more economical, which is often the critical factor in the educational market.

Education accounts for just 15% of public spending in the construction sector, while total public sector spending accounts for 25% of all construction sector spend. Therefore, if public sector projects were required to achieve net zero, theoretically we could reduce the construction carbon output of the UK by 25%, or 2.5% of the UK’s total carbon output.

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