Small carbon footprint for timber multi-storey buildings

Currently green building rating systems like Green Star focus on the environmental impact of buildings in respect to their operations during use. For example, how much energy is used to heat them, or to light them. Although the choice of construction materials has significant impact on the carbon footprint of the building, it is not yet recognised by most rating systems. This deficiency must be addressed; green rating systems should adopt a more holistic and life-cycle approach.

Buchanan, John and Love in their article on this topic in the recent New Zealand Journal of Forestry (Feb 2013) point out that 85% of the total green gas emissions of a building (over its whole life-cycle) comes from its operations. For a typical building, operational energy consists mainly of energy for heating and cooling (65%), energy for lighting (16%), hot water (15%) and electrical appliances (6%). Most buildings, as long as they are appropriately insulated, perform similarly amongst the alternative building materials of concrete, steel and timber. It is also noted that there is a strong trend of reducing operational energy as modern buildings become more energy efficient.

The same authors also point out that for a life-cycle approach, the environmental impact of the construction and ultimate demolition and disposal/recycling of the building and its materials must be included. The life of the building is assumed to be 60 years. Using the authors' data a graphic of the global warming potential (emissions/ sequestration of CO2 [equivalent]) of the life-cycle of a multi-storey building is presented in the chart below.

Note the dominance of the operational energy and the relative insignificance of the embedded CO2 [equivalent] of the materials (in blue). Note also the emissions from timber at the end of building's life (burning and decomposition) versus the "negative" emissions from recycling some of the steel.

The limitation with this approach is that it could be argued that abatement and/or sequestration of green house gasses is more valuable now than in the future. In the future, other technologies may be introduced that significantly reduce the cost of abatement and sequestration or produce low, or no, emissions energy. An alternative way to look at this is to say if you had to purchase carbon credits to account for a building's emissions then in an economic analysis you would discount the cost of those carbon credits in the future.

What would happen if you discounted the carbon as you would other costs? This is presented in the chart below.

The impact of the embodied green-house gasses becomes much more prominent as this occurs at the beginning of the building's life. The operational energy and maintenance components reduce in absolute amount and proportionally as they are discounted effectively over 30 years. The end of life component becomes substantially lower being discounted for the full 60 years. In terms of an investor seeking a rate of return on investment in a building, this discounting approach (whether applied to carbon credits or cash flows) will better mirror the commercial reality of the choice among different building materials.

This analysis is a more realistic scenario that demonstrates the significant environmental benefit of timber buildings over concrete and steel. On this basis, green-house gas emissions from the timber multi-storey building are less than one half of those of concrete and nearly one third those of steel. This is surely a compelling reason to favour timber buildings, not to mention timber's more favourable seismic and fire resistance. It is also a compelling reason to have building materials included in green building rating systems.