The issue of climate change has become highly important over the last decades. All sectors need to find solutions in order to reduce the carbon footprint and the built environment sector is definitely not an exception to this. One of the solutions is to use more biobased materials, such as timber. The use in timber is not only limited to low-rise buildings, more mid- and high-rise buildings are being built as well. The number of buildings including timber with at least six stories was increased from 32 in 2015 to 115 in 2023.

Even though the number of timber buildings is increasing, the actual environmental impact is largely unknown. This is because there is no commonly-used method to calculate the carbon footprint of a building which is complete and correct. Currently, the MPG (MilieuPrestatie Gebouwen) and the method described in the Paris Proof Agreement are mainly used in the Netherlands. The MPG uses the Environmental Product Declarations (EPDs) from the database from the NMD viewer, but this viewer does not provide the essential information for the user to choose the proper EPD. The Paris Proof Agreement disregards the end-of-life phases, which leads to incomplete results. Therefore, another method was developed to calculate the carbon footprint and to determine the difference in carbon footprint between a concrete and/or steel structure with a timber-hybrid structure. This method uses two sets of phases: phases A-C and phases A1-A5. The former uses all relevant LCA phases and will provide the most accurate results, while the latter can be used to compare the results to the Paris Proof Agreement limits. Phases A4 and C2 (transport to and from the building site) and modified to fit the current projects.

Two case studies were used for this research. The first case study is KasseNova aan de Vaart, a seven-story residential building made of concrete. The carbon footprint is calculated for this concrete design, as well as three design variants including CLT floors, CLT walls and a combination of the two. The second case study is Apollolaan 171, a six-storey office building made of steel and concrete. The design variants include glulam columns, CLT floors and beams and a combination. For all variants, design calculations of the timber elements were performed. For CLT elements, two end-of-life scenarios were considered: 100% recycling and 100% incineration. The current design of KasseNova aan de Vaart has a carbon footprint of 3,145 ton CO2-eq. when phases A-C are considered. In the scenario that the floors and walls are replaced by CLT, the carbon footprint is decreased by 63.1% when the CLT is recycled, while this is decrease is 12.7% when the CLT is incinerated. When phases A1-A5 are considered, the current design has a carbon footprint of 222.46 kg CO2-eq./m² (above the 2021 Paris Proof limit of 220 kg/m2) and the CLT floor and wall design variant has a carbon footprint of 35.12 kg CO2-eq./m² (below the 2050 limit of 50 kg/m2), including biogenic carbon uptake. The current design of Apollolaan 171 has a carbon footprint of 1,671 ton CO2-eq. (phases A-C). For the scenario in which the floors, beams and columns are replaced by CLT and glulam, the carbon footprint is reduced by 89.2% (CLT recycled) and 49.8% (CLT incinerated). Considering phases A1-A5, the carbon footprint of the current structure is 255.17 kg CO2-eq./m2 (above the 2021 limit of 250 kg/m²), while the carbon footprint of the structure including CLT and glulam elements is -44.93 kg CO2-eq./m² (below the 2050 of 50 kg/m²).

These results show that an increase in timber in the structure leads to a reduction in carbon footprint. This is because timber takes up carbon in the product stage. However, the amount of reduction depends on several factors. If the CLT is recycled at the end of its lifetime, the reduction will be more significant than if the CLT is incinerated. When the structure includes concrete, the carbon footprint depends on the type of cement being used in the concrete. CEM I will lead to a higher carbon footprint than CEM III. For structures including steel, a higher percentage of recycled steel leads to a lower carbon footprint. Also, in general, the country of production influences the carbon footprint. If a country has a high percentage of coal in its energy mix, the carbon footprint will be higher.