Scientists from Kyushu University and Sugiyama Jogakuen University have identified the types of Japanese residential buildings that produce the lowest CO₂ emissions during construction. In a large-scale study, the researchers compared the carbon footprint of various housing construction technologies, taking into account the number of floors in houses made of wood, reinforced concrete and cast-in-place concrete, as well as steel-frame buildings. Rather than housing management, the study focused on emissions generated before occupancy, namely, during the production of construction materials, their transportation and the erection of the buildings.
Although Japan’s construction sector accounts for about 30% of the country’s power consumption, it was unclear until recently which types of housing generate the most CO₂ emissions during construction. In order to obtain a comprehensive picture, the researchers used the method of environmentally enhanced input-output analysis. This approach views the economy as a single system of interconnected industries and makes it possible to consider not just individual materials or construction operations but the entire set of production chains.
The analysis was based on data from 2015, the most recent data with a detailed breakdown of the construction sector by technology (wood, reinforced concrete, steel) and links to industry-specific emission factors. Although industrial energy efficiency has improved over the past decade, the fundamental differences between construction technologies have essentially remained unchanged.
The scientists combined economic flows with data on direct CO₂ emissions in each sector. As a result, they obtained a model that takes into account not only emissions generated directly at the construction site, i.e., from equipment operation, but also the entire hidden carbon footprint of construction, from iron ore mining and cement production to power generation and material transportation.
Particular attention in the study was given to wood. Unlike concrete and steel, whose production is associated with high emissions, trees absorb carbon dioxide from the atmosphere while growing and internally accumulate it. The researchers quantified this temporary carbon storage effect and took it into account when calculating the net carbon footprint of wooden houses during construction.
A comparison of the four main technologies revealed a stark contrast.
Wooden houses demonstrated the lowest net carbon dioxide emissions per square meter of floor area at about 195 kg CO₂ per m2. As a point of comparison, reinforced concrete buildings produced 1,109 kg CO₂ per m², while concrete and steel-frame buildings generated 857 kg CO₂ per m² and 803 kg CO₂ per m², respectively. This means that wooden houses were 4–5 times less carbon-intensive.
The researchers also analyzed the impact made by the number of floors on the distribution of construction emissions. A significant portion of the aggregate emissions comes from low-rise housing ranging from one to three floors. This is primarily due to the scale of construction: low-rise buildings make up the majority of new housing, and their low specific emissions are offset by mass construction. While the share of aggregate emissions is significantly smaller in the segment of four-to-nine-floors-tall houses, each of these buildings is highly carbon-intensive due to the heavy use of concrete and steel, which makes this segment critically important from a climate policy perspective.
The researchers believe that urban planning authorities will need to significantly expand the use of wood in construction, including in mid-rise buildings, in order to successfully decarbonize the economy.
