Hygrothermal Performance And Optimization Of Light-Framed Timber Structure Buildings In Hot Summer And Cold Winter Zone

Wooden structure buildings have a good potential for energy saving and emission reduction.The building envelope of this type of building has good thermal insulation performance and performs well in the adjustment of the indoor environment.Moreover,the components of wooden structure buildings have extremely low carbon emissions in the production process,and can also be recycled after being discarded.With the consensus reached by countries around the world that energy consumption should be reduced,wood structure buildings have gradually attracted more attention.And the light-framed timber structure(LTS)building is the most recognized structural system for wooden buildings since it could be built quickly.However,the application of the LTS buildings is extremely limited by the risks of mould and rot for wooden materials.Besides,the indoor environment is also complicated by the intermittent energy-consumption modes supplied with natural ventilation adopted by occupants in the hot summer and cold winter(HSCW)zone.Therefore,it has great significance to study the hygrothermal performance of the LTS buildings with the distinctive modes of intermittent energy consumption and ventilation,and further to select an appropriate building envelope.The methodology of theoretical arithmetic,lab test,in-situ test,and hygrothermal simulation were adopted for the research.In this study,the hygrothermal properties of several common wooden materials were tested.The hygrothermal performance of the LTS building envelope in the steady state was calculated and compared with the result of the experiment.Further,the thermal performance and the potential for energy saving were evaluated compared with the limitation of the energy saving standard of the building.Moreover,in-situ experiments were carried out to analyze the characteristic of the hygrothermal environment as well as the process of heat and moisture transfer.Finally,simulation was used to evaluate the performance of LTS residential buildings under different modes and to select a better construction of the building envelope.The main conclusions are listed as follows:(1)Wooden materials have the characteristic of porous and low density.They have good thermal insulation performance,and also have a strong ability for moisture storage and humidity control.Besides,they are anisotropy in heat and moisture transfer.It is easier for heat and moisture to transfer in the direction that parallels the wood fiber bundle.Different materials have little difference in properties of heat and moisture storage(such as specific heat capacity and curve of equilibrium moisture content).But properties vary on the speed of heat and moisture transfer(such as thermal conductivity and water vapour transfer coefficient).(2)Based on the result of the calculation,the difference in heat transfer coefficient between the two LTS walls is small.The heat transfer coefficients of the two LTS walls all can meet the limit value of the thermal design the Standard for energy efficiency design of residential buildings in hot summer and cold winter regions and the General code for energy efficiency and renewable energy application in buildings.Both of them have good thermal insulation performance.Compared with the reference building that the building envelopes just meet the requirement of the thermal design in the standards mentioned above,the buildings that adopted the LTS envelopes consume less energy.The two LTS walls are different in terms of wet performance,and the total water vapour diffusion resistance of structure 2 is 21.39 times that of structure 1.(3)Under the condition of intermittent energy consumption supplemented by ventilation,the indoor hygrothermal environment of the LTS buildings is greatly affected by the ventilation and the operation of air conditioning.In summer,the characteristics of the inverter air-conditioner that the compressor would change its frequency makes different dehumidification capacities of air-conditioner over time.The dehumidification capacity when the airconditioner is just turned on is much stronger compared with the dehumidification capacity when the air-conditioner is operated for a long time.The relative humidity drops sharply when the air conditioner is operated for a short time,but the indoor relative humidity will rise to 60%-70% after a period of operation.Structure 1 with gypsum board finishing is suitable under intermittent energy consumption or mild external conditions for its quick response to the regulation of air conditioning and ventilation.Whether structure 2 can maintain a stable and good indoor environment under harsh external conditions or under the situation with less operation time for air-conditioning and ventilation.(4)Due to the sudden change of water vapour transfer coefficient,high relative humidity is prone to occur on both sides close to the mineral wool layer of the LTS walls.This phenomenon occurs in the outer material of the mineral wool in winter while in summer it occurs in the inner material of the mineral wool.In addition,the temperature and humidity at different positions of the same wall are affected by air stratification,heat and moisture transfer from the attic as well as structural thermal bridges.The humidity at the height position of structure 2 is high,which may increase the risk of mold growth.(5)From the simulation of the LTS residential buildings under annual typical modes in the HSCW zone,it can be found that daytime ventilation has a better effect on the improvement of indoor hygrothermal environment and mildew prevention.However,night ventilation will cause a greater risk of mold growth.The higher ventilation rate makes the cooling energy consumption significantly lower than the heating energy consumption.Appropriately increasing the cooling time greatly improves indoor hygrothermal comfort without increasing energy consumption in a large proportion.(6)Under the mode of daytime ventilation,the LTS wall adopts the calcium silicate board with high permeability as the finishing has the best performance.Although the wooden panel finishing can alleviate the fluctuation of indoor humidity and store moisture,the drying effect of ventilation on it is not as good as that of permeable finishing such as gypsum board and calcium silicate board.In the meantime,the moisture released when there is no ventilation will increase indoor humidity.Further,wooden materials also have a lower critical relative humidity for mold growth.So,in the simulation of intermittent conditions,the risk of mold is serious when wooden materials were adopted as the finishing,and it is not suitable for application in this area.This study enriches the domestic database of hygrothermal properties of wooden materials and makes up for the deficiencies in the study of hygric properties and anisotropic hygrothermal properties.This paper makes a comprehensive analysis and evaluation of the LTS buildings from different perspectives,such as the basic thermal performance of LTS building envelope and the dynamic hygrothermal environment of the LTS building under intermittent energy consumption modes,and makes suggestions and optimization based on the actual situation,which provides strong technical support for creating a good indoor hygrothermal environment and as well as the mildew prevention of the LTS building.