Finding the Sweet Spot: Optimal Thresholds for High and Low Embodied Carbon in Building Projects

The carbon footprint of a building is broadly categorised into two main components: embodied carbon and operational carbon. These two metrics play a significant role in assessing the environmental impact of buildings throughout their lifecycle.

Quick recap:

Embodied Carbon:

Embodied carbon refers to the total greenhouse gas emissions (measured in CO2 equivalents, or CO2e) associated with the materials and construction processes throughout the entire lifecycle of a building. This includes the extraction of raw materials, manufacturing of building products, transportation, construction, maintenance, and eventual demolition and disposal. Essentially, it encompasses all the carbon emissions released before the building even begins its operational phase.

Operational Carbon:

Operational carbon, on the other hand, refers to the emissions associated with the energy consumption required to operate the building over its lifetime. This includes heating, cooling, lighting, powering appliances, and any other energy use within the building. Operational carbon is an ongoing expenditure of carbon, accumulating year after year as long as the building is in use.

In this article, we are looking into the intricate balance between embodied and operational carbon. We will explore how factors such as building size, shape, and expected lifetime influence these carbon metrics. Furthermore, we will compare scenarios with high and low embodied and operational carbon values, providing a comprehensive overview of the strategies available to minimise the carbon footprint of buildings using simplified metrics.

Impact of Building Size and Shape (residential)

The size and shape of a building significantly influence both its embodied and operational carbon emissions. Larger buildings generally require more materials, leading to higher embodied carbon. However, the shape of a building, particularly its surface-to-volume ratio, plays a crucial role in determining its energy efficiency and, consequently, its operational carbon emissions.

For this simplified exercise we used the following metrics:

embodied carbon in types of residential buildings - simple metric

High Embodied Carbon Values:

High levels of embodied carbon typically exceed 500 kg CO2e per square meter of building area. This is often associated with the use of conventional, energy-intensive materials such as concrete, steel, and aluminium. For instance, buildings constructed primarily with concrete and steel frames often have high embodied carbon due to the energy-intensive processes involved in producing these materials.

Low Embodied Carbon Values:

Low levels of embodied carbon are generally considered to be below 300 kg CO2e per square meter of building area. This can be achieved through the use of more sustainable materials like timber, recycled materials, and innovative construction techniques that minimise waste and energy consumption. Buildings designed with a focus on sustainability, using materials like cross-laminated timber (CLT), timber windows and other low-carbon alternatives, fall into this category.

Operational Carbon Values:

15 kWh/m²/year ≈ 5.4 kg CO2e/m²/year
30 kWh/m²/year ≈ 10.8 kg CO2e/m²/year
60 kWh/m²/year ≈ 21.6 kg CO2e/m²/year

Total Carbon Emissions:

Simple residential building (Total Area: 268.66 m²):

U-Shaped residential building (Total Area: 294.66 m²):

3 Storey Walk Up (Total Area: 740 m²):

5 storey building (Total Area: 1160 m²):

And last but not least - a 12 storey building (Total Area: 2760 m²):

What can we make of this? Key Insights

1. Impact of Embodied Carbon:

Buildings with low embodied carbon consistently show lower total carbon emissions over time compared to those with high embodied carbon, especially when operational carbon is also low. Buildings with high embodied carbon tend to have higher total carbon emissions, but the difference diminishes over longer periods if operational efficiency is high.

2. Impact of Operational Carbon:

Improving operational efficiency (i.e., reducing operational carbon) significantly reduces the total carbon emissions over the building's lifespan. This impact is particularly noticeable over longer time periods. Buildings with high operational carbon emissions accumulate significantly higher total carbon emissions over time, regardless of their embodied carbon levels.

3. Comparison of High and Low Embodied Carbon:

In the short term (20 years), the choice of low embodied carbon materials makes a noticeable difference in total emissions. Over the long term (80-100 years), the operational carbon becomes increasingly dominant. In some scenarios, buildings with high embodied carbon but very low operational carbon (15 kWh/m²/year) can have comparable or even lower total emissions over 100 years than buildings with low embodied carbon but higher operational carbon (30-60 kWh/m²/year).

4. The Middle Path:

Buildings that balance moderate levels of both embodied and operational carbon can achieve total emissions close to those of the most efficient buildings. For instance, a building with moderate embodied carbon and operational carbon of 15-30 kWh/m²/year can perform nearly as well as the best-case scenarios. Focusing on operational efficiency tends to offer greater long-term benefits, especially as the operational phase extends over many years.