Sustainable wooden building concept for central Japan

In light of the high probability of global warming, the mitigation of climate change is one of the most urgent issues. As long as human society is expected to grow further, the fundamental philosophy of technology development must be based on sustainability. Developing technologies should not deteriorate the equal opportunity for people to continue growing through time and space. Under such global agreement, the major issues are resource depletion and the rapid growth of developing regions. Even though the rational use of resources has been intensively discussed, the utilization of renewable resources has not been fully implemented yet. Among many renewable resources, wood is of high importance because of its diverse usages and its ability to absorb and store CO2. Also very related to the issue of resource consumption, the growth of developing regions has a significant impact on the environment and human society on a global scale. These urban developments need to be supported by sustainable measures. It should be noted that large parts of such emerging economies are situated in subtropical regions. Hence, solutions to deal with subtropical conditions are increasingly needed. Today it is widely recognized that the construction industry is playing a key role in both resource depletion and urbanization of developing regions. Rational management of construction activities and buildings may have significant impact on mitigating climate change. Among other aspects, the selection of construction methods and materials and the advancement of energy efficiency are key points to consider. In the field of energy efficiency of buildings, there have been numerous studies and many technologies have already been in practical use for some time. The state-of-art building technology is super-insulated and airtight buildings, which are often a hybrid of passive (driven by no energy consumption) and active (driven by active energy consumption) measures. Those technologies have been developed based on experiences in primarily cold/mild climatic regions. Considering the issues addressed above and the proportion of the country characterized by a subtropical climate, Japan has great potential for improving its construction industry from a sustainability viewpoint, especially given its market size (e.g. 1 000 000 newly-built houses in 2008) and its rather backward energy standard. When developing new construction methods, the following points should be considered thoroughly. Japan has diverse climatic conditions due to its geographic characteristics. The northern regions have a continental climate similar to central/northern European conditions, while the central and southern regions have a subtropical climate. It is divided into six climate-zones in accordance with the Japanese law on the rational use of energy in the housing sector. This climatic diversity should be carefully taken into account as much as possible when discussing the design of a building. The use of domestic forest resources is another important issue for Japan. Although 93% of the detached houses in the housing stock (2008) are wooden constructions and 66% of the land is covered by forest, the self-sufficiency rating of wood (including paper/pulp and others) is only around 26% (2010). An extensive forestation program was carried out as a national policy after the Second World War in order to secure the supply of the raw material. Nowadays those densely planted forests are causing environmental damage to the ecosystem such as decreasing the capacity of CO2 storage, increasing the risk of landslides and so on. At the same time the economy of domestic forestry has been going through long-term depression due to the low cost of imported timber. For conducting sustainable forest management, it is absolutely necessary to come up with measures which stimulate the forestry economy by creating profit. The key is to add value even to the unprofitable forest products such as the thin timbers. The latest effort in the Japanese housing industry can be characterized by two measures, namely employing foreign technologies especially for energy efficiency and high-tech housing services, such as sophisticated heat pumps, fuel cells and so on. As for the implementation of foreign technologies, the rather rapid change by direct transfer without sufficient consideration on the local climatic conditions as well as socio-cultural aspects have resulted in problems regarding building physics and conflicts in terms of social acceptance. As the Japanese government plans to implement obligatory regulation on energy efficiency for all buildings in the coming decade, there is a strong need to develop building technologies which soundly deal with both the subtropical climate and socio-cultural aspects. Considering the need for utilizing domestic timber and the already well-developed housing service technologies, a novel wooden building envelope, which ensures the rational use of resource and energy efficiency, may be one such sound solution. In order to deal with the issues, a vapor-open wood-based building envelope system was developed within the research and development project funded by the Commission for Technology and Innovation (grant number: 9755.1 PFIW-IW). The envelope consists mainly of layers of natural materials, namely an external insulation layer of wood fiber board, a structural layer of cross laminated timber consisting of slats with small section and an interior finishing layer composed of a wood and clay composite. Each component is made of hygroscopic material with moderate vapor permeability. Therefore the system allows the moisture flux to move through the wall in both directions. Also the thickness of each layer can be determined regardless of the other layers. These features solve the moisture related problems inside exterior walls under subtropical conditions. Besides the considerations regarding building physics, the design philosophy of the envelope also comprises ecological, economic and social aspects. The components are based on natural materials, and so it may be produced using local resources. Local production also promotes the local economy creating a local value chain. The local climatic conditions and socio-cultural aspects, such as user behaviors, can be taken into account by the flexibility assured by the layered structure. In this thesis the building physics, economic and ecological performance of the envelope system under Japanese conditions was quantitatively assessed. The specific purpose was to evaluate the feasibility of the envelope system for implementation in the Japanese market. At the same time, the application of the evaluation method developed within this dissertation on other construction methods and in other regions was also considered. The verification of the system’s overall performance was carried out within an interdisciplinary framework. Firstly, in order to investigate the durability, the hygrothermal performance of the envelope was analyzed with a transient heat and moisture transfer model. This model was validated by experiments with full-scale wall specimens. Secondly, in order to investigate the energetic performance of a building with this envelope system, a model was created to simulate the indoor temperature, relative humidity and the heating/cooling demand, combining the hygrothermal model of the envelope itself, heat balance model of the whole building and the moisture balance model, taking into account the interaction of the envelope and ambient air. Finally, in order to investigate the feasibility of applying the envelope system to Japanese conditions, a model was created to define the economic and ecological optimal insulation thickness, using the transmission heat loss model described above, simplified Life Cycle Assessment (LCA) and Life Cycle Cost Assessment (LCCA) methods. In the optimization model several scenarios were considered for taking the uncertainty of the future economic situation into account. This thesis is a cumulative dissertation, and consists of six chapters. Chapter 1 presents the overall background and the goal of the study. In section 1.1, a general concept of sustainability in technology development is presented as an orientation with which to begin the study, and then global issues in the construction sector and the rapid economic growth in subtropical regions are introduced. In section 1.2, the specific conditions of the Japanese housing industry are elaborated upon in order to highlight particular problems. In Chapter 2, the detailed design philosophy of the envelope system is introduced, and finally the methodologies for evaluating its performance from the viewpoints of building physics, economics and ecology are introduced by reviewing past studies. The three following chapters (3, 4 and 5) consist of three research papers (Paper I, II and III). Chapter 3 presents Paper I, which was published in a peer-reviewed scientific journal (Paper I: Building and Environment - Preliminary Investigation of a Vapor-open Envelope Tailored for Subtropical Climate). Chapter 4 presents Paper II, which was published in a peer-reviewed scientific journal (Paper II: Building Simulation - Heat and Moisture Balance Simulation of a Building with Vapor-open Envelope System for Subtropical Regions). Chapter 5 presents Paper III, which was published in a peer-reviewed scientific journal (Paper III: Energy and Buildings - Economic, Ecological and Thermo-hygric Optimization of a Vapor-open Envelope for Subtropical Climates). In Chapter 3 (Paper I), the hygrothermal performance of the envelope system, investigated by means of testing full-scale walls in a climate chamber, and the corresponding one-dimensional transient heat and transfer simulation are presented. In order to maintain consistency between calculation and measurement, the individual materials were tested for their hygric and thermal properties. The results in Chapter 3 show that the experiment and the numerical simulation corresponded to each other with high accuracy. Based on these findings, attempts were made to calculate the behavior of a wall assembly under real climatic conditions of central Japan, Kyoto. As a result, it was shown that no risk of interstitial condensation and mold growth was predicted under the real climatic conditions of Kyoto. In Chapter 4 (Paper II), the heat and moisture balance model of a building with the envelope system is proposed. In the moisture balance model the moisture buffering by the interior materials was taken into account in addition to the standard factors, such as air infiltration, indoor moisture load due to human activities, etc. The prediction of the moisture buffer value (MBV) of the interior finishing materials was attempted and validated by empirical data. Subsequently, the whole building calculation was carried out under the condition of a Japanese city, Hikone, and energy consumption and the contribution of the moisture buffering to indoor comfort was investigated. The results of Chapter 4 are as follows. The MBVs of the mineral-based materials were predicted with high accuracy. However the MBVs of the wood-based composite were much higher than the experimental value. In order to create a more accurate MBV prediction model, nonlinear moisture conductance on the fiber scale should be taken into account when modeling wood-based materials. The heating and cooling demand of a test house was 9.4 kWh/m2 and 14.5 kWh/m2 respectively. The moisture buffering contributed to a significant reduction of humidity fluctuation. It was concluded that the envelope system can be implemented to provide highly energy efficient buildings under subtropical conditions. In order to enhance both energy efficiency and indoor comfort of buildings in subtropical regions, there is still a strong need to develop a more holistic method for finding the optimum building design, considering not only moisture buffering by interior materials but also all the relevant factors, such as shading, active dehumidification and so on. In Chapter 5 (Paper III), the optimal insulation thickness of a building with the envelope system was investigated under the conditions of eight cities in Japan by an economic and ecological model, taking into account both initial and running cost. The basic intention was to define the optimal insulation thickness of a building dealing with the trade-off between economic and ecological performance (“the thicker the insulation is, the less heating energy is consumed and the more material is used”). The thermo-hygric minimum thickness was also determined in order to ensure the longevity of the buildings. The transmission heat loss model for those simulations was based on the whole building model proposed in Chapter 4. Consequently, the following main findings were made: 1) the ecological optimal thickness was larger than the economic optimal thickness, 2) the thermo-hygric minimum was within the economic optimal range in most of the cases, and 3) the interest rate of the currency and the electricity price increase have a significant influence on the result of the optimization analysis. With the aid of the optimization model, it was shown that applying this envelope system is feasible in Japan, especially in the central and southern regions. Finally, the overall conclusion of the thesis is given in Chapter 6, summarizing and discussing the findings and discussions in Chapter 3 to 5. The main conclusions were made as follows: - The hygrothermal performance of the envelope system was modeled and validated successfully. - With the aid of the heat and moisture balance model suggested, it was made possible to predict indoor temperature, relative humidity and heating/cooling demand, and the envelope system can be used for providing highly energy efficient building under subtropical conditions. - With the aid of the insulation optimization model proposed, it was shown that implementing the envelope system is feasible in Japan, especially in the central and southern regions. However, there remain several deficits to be improved upon in the models, such as the inadequacy of the hygrothermal model with wind driven rain, the limitation of the modeled volume in the whole building simulation and limited parameters in the optimization model on local conditions (material price, energy mix, etc.). There is potential for expanding the overall model by optimizing the insulation thickness and housing service setting for dealing with the trade-off between the energy consumption and indoor comfort. It was concluded that the envelope system proposed in this study has a high potential for implementation in central Japan to enhance the sustainability of the Japanese housing industry. It is also applicable to other regions that have similar climatic conditions. The modeling methods (the whole building hygrothermal model and the insulation thickness optimization model) can be applied to other construction methods with minor adjustments. The whole building hygrothermal model will be validated by empirical data from a test house in the future (anticipated construction in Ohmihachiman in May 2013).