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Low Carbon Building Materials

February, 2021 |

Increasingly, building with low embodied carbon materials is being considered an essential component in delivering a sustainable built environment. This is reflected in the RIBA 2030 Challenge – a very demanding maximum allowance for embodied carbon being set out for the first-time last year. This is an overdue signal to seriously reflect on the impact of our material choices. The UK Committee on Climate Change also emphasised the need for lower embodied as well as operational carbon in new housing and encouraged building in timber ‘wherever possible’. But is timber use always sustainable?

The UK Government suggests we need up to 300,000 new homes annually to meet our current and future housing needs – that represents a lot of building materials and potentially a much higher demand for timber products. The adoption of timber as the default ‘low carbon’ building material needs careful consideration. The timber industry cannot be assumed to have excellent green credentials.

Fortunately, there are new carbon calculators being developed and better records for materials with more EPD’s (environmental product declarations) available than ever before. The new carbon calculating software PH RIBBON, which plugs into the Passivhaus PHPP software, will prove to be an invaluable tool for assessing embodied carbon for those already using PHPP to predict operational energy/carbon. But will these auditing tools always guide us in the right direction?

The RIBA 2030 Challenge embodied carbon target is for ‘cradle to grave’ emissions, but given the scale of the climate emergency, the key must be to avoid unnecessary emissions now, the cradle to practical completion stages. We must be careful that our carbon accounting processes are robust, and that we are not making unhelpful assumptions. Assumptions around carbon sequestration, for example (carbon storage in the material), or end of life outcomes are both likely to influence the timber and construction industry as they seek to balance the demands of climate change with the demand for new housing.

Firstly then, assumptions around carbon sequestration and the subsequent offsetting of emissions. Is this carbon credit problematic, given that old growth forest is our best defence against climate change? Carbon is stored not just in the timber but in the soil and the diverse fungi the trees and soil support. In contrast ‘agricultural’ pine and spruce forests are not biodiverse and degrade the soil, often significantly releasing carbon when the land is initially cleared. If we want to increase our carbon storage the ideal and most effective action is to protect and extend our natural forests, restoring and enhancing these biodiverse and mixed forest environments – as the Woodland Trust is doing. Giving over larger and larger areas of land to industrial monocultures to feed our increasing demand for timber products is unlikely to be sustainable. It is therefore not altogether helpful to credit sequestration of carbon in timber products, given that the production process is not carbon neutral and does not produce biodiverse landscapes. The best tree for the environment is an old tree set within an established forest. If you accept the sequestration model of accounting, then you could logically conclude that more timber is better and rack up your carbon credits – exactly the opposite of what we should be doing. Thinking of forestry as a carbon and environmentally positive activity belies the complexity of the issue. A more nuanced discussion about how much land should be given over to timber production is needed, and also how much old forest or woodland we must retain and grow. Simply setting target tree numbers is a very reductive way of approaching the issue. Timber is a precious resource, and we should be looking to use it effectively. The increasing use of CLT where it is not structurally required is, for example, not an effective use. To use timber resources responsibly, we need elegant building solutions that are not wasteful or profligate.

Secondly, there are in-built assumptions within current carbon accounting that favour brick and block over timber. This is nicely illustrated in the graph above that shows the output from PH Ribbon of the carbon cost for one of our recent PH15 houses. We have recalculated the carbon output using equivalent performance in brick and block. The resultant graph shows that over the first ten years the carbon emissions are much lower in the timber version (timber frame with natural insulations 5.6 tonnes carbon versus a brick and block equivalent at 47.4 tonnes, a factor of 7.5X) but strangely the carbon cost over the life of the building is lower in brick and block. This is due to the current RICS assumptions for end of life. This assumption is that timber will be 75% incinerated and 25% will go to landfill, whereas masonry is assumed will be 90% recycled and 10% to landfill. Burning timber is very carbon intense, so you can see in the graph that the carbon emissions dive up at stage C (end of life) and overtake brick and block. In terms of a carbon audit both these options struggle to meet the RIBA 30 Challenge, the PH15 timber system 591 kgCO2e/m2 of net internal floor area, and brick and block 523 kgCO2e/m2.

It is completely unsustainable to incinerate 75% of the timber we will be building with over the next 60 years! If this is the plan we should cease immediately. To use timber as a sustainable building material it must first be used sparingly, then it must be designed for longevity (minimally 100-120 years), and then we must find a way to reuse at end of life or capture the carbon if we incinerate for energy. If we accept the carbon auditing assumptions of today, and maintain the current cradle to grave accounting method, we could be encouraging continuing use of ‘business as usual’ higher embodied carbon materials and thereby emitting many times more carbon into the atmosphere over the next ten years. If we re-run the calculation in PH Ribbon, making different assumptions about end of life (stage C) for timber, then the PH15 timber system drops to 180 kgCO2e/m2 and brick and block to 468 kgCO2e/m2. This calculation is for the thermal shell of the building, excluding the fit-out and assumes use of low cement content foundations (50%). This audit demonstrates that only timber with natural insulations (wood fibre and sheepswool) can meet the 2030 target and only when timber is not incinerated at end of life. The importance of retaining the timber and not burning tells us that timber must be prioritised for long term uses. Building low cost, poor quality timber structures is not a sustainable activity.

The RIBA is doing great work looking at the appropriate embodied carbon targets and the current figures are likely to be adjusted shortly. It will be interesting to see what those adjustments are and if any of the current assumptions are to be amended.

Sustainability is a complex issue, and timber in construction poses some interesting challenges*. If we specify timber, then we must consider some of that complexity. There needs to be more discussion on how we account for embodied carbon, so that we are making fair and real-world assessments with the greatest immediate and long-term benefits in reducing environmental impacts. Using less seems to be a big part of any sustainable future, that challenges everything from the type of timber frame we build with, the amount of land we give over to timber production, the size of the houses we design, and a query over the number of new houses we do in fact need.

This Pete Seeger quote sums up the ethos we all need to adopt and accept-

‘If it can’t be reduced, reused, repaired, rebuilt, refurbished, recycled or composted then it should be restricted or removed from production.’

Anna Carton and Jae Cotterell with carbon auditing support from Tim Martel (PH Ribbon). *See film from Sweden on their forestry industry. 

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Buildoffsite Property Assurance Scheme (BOPAS) was developed to address these concerns and perceived risks associated with innovative construction. BOPAS is recognised by the principal mortgage lenders as providing the necessary assurance underpinned by a warranty provision, that the property will be readily mortgageable for at least 60 years.