## 1 Introduction
### Cross Laminated Timber (CLT) and Its Development as a Sustainable Material
#### CLT as a Sustainable Building Material  
The capital advantage of the CLT construction system is the low environmental impact of the material. In comparison with conventional building materials, CLT is a less energy-intensive construction material. [PERSON] et al indicates that the increased usage of multi-layer timber  
Figure 1: Cross-laminated timber (CLT) construction in five layers. construction may significantly reduce the environmental impact of buildings during their life cycle [40]. Separate research in New Zealand shows that the environment impact of a residential building with a light wooden structure is much less than one with concrete structures. Over the life cycle of the building the total cost of the house with the light wooden construction is 11% [41]. At the end of 20 th century, [PERSON] et al revealed that the energy consumption of a multi-family house with a concrete structure is around 8-10 GJ/m\({}^{2}\) in comparison with approximately 3 GJ/m\({}^{2}\) for a wooden structure and 4.5 GJ/m\({}^{2}\) for a light-weight steel structure. Research also shows great potential for CO\({}_{2}\) reduction achievable by wood structure buildings. The CO\({}_{2}\) emissions of houses with a concrete structure are 850 kg/m\({}^{2}\), compared with 250 kg/m\({}^{2}\) for wooden structures and 450 kg/m\({}^{2}\) for light-weight steel structures [42]. [PERSON] shows that one cubic meter laminated wood can absorb 582 kg CO\({}_{2}\) in consideration of the reuse of the material. The absorbed atmospheric CO\({}_{2}\) is locked in the body of the timber structure. This data can be compared with the CO\({}_{2}\) emissions by concrete (458 kg/m\({}^{3}\)) and steel (12.087 kg/m\({}^{3}\)) [43]. [PERSON] et al reveal that in the US, the energy consumption of wooden construction is 15-16% less than that of steel or concrete construction, without taking into account heating or cooling energy. The GHG emission of a wood structure is 20 to 50% lower than that of steel or concrete construction over a 100-year life cycle [44]. [PERSON] et al analyze the future development of timber constructions and related GHG effects in the EU [45]. Their research shows that residential buildings with CLT structures have better material efficiency than conventional building types [45]. The increased use of CLT can lower GHG emissions and increase carbon storage in the building stock [45]. Research by [PERSON] et al. compares the operational energy use of a reference residential building with structures in reinforced concrete and CLT in 31 cities in China. By replacing the concrete by CLT, the energy consumption of the reference building becomes 29.4% lower than that of the conventional structure. A 24.6% reduction in carbon emissions during the operational stage can be achieved when all concrete structures are replaced by CLT [46]. However, CLT construction also has some disadvantages as an energy efficient building material. Due to is low thermal mass and low U values, there is a higher possibility of summer overheating when used instead of conventional materials [47]. [PERSON]'s research in Sweden found a higher overheating risk from timber-frame buildings than from concrete-frame buildings in the same climate conditions. This overheating in the building could not be remedied by solar shading or ventilation [48]. Lightweight timber construction is more suitable for regions with milder summer temperatures rather than hot climates. It should be encouraged that the material of the lightweight construction is characterized by high thermal mass, high thermal effectivity [49]. In addition, compared to the lightweight wood construction, CLT system is not efficient enough to carry the lateral forces and to dissipate in seismic activity due to its heavier weight. Specific systems need to be set up when designing the CLT constructions. [PERSON] et al.