The building envelope with the openings play an important role in the process energy performance and ensure the well-being in a working space because of the impact of the openings to the visual and thermal comfort of the occupants. When designing buildings and structures, the standards for permissible safety should be taken into account, which are expressed by the value of critical duration of fire and the explosive load. The aim of the study is to propose the integrated approach in determining the rational Window to Floor Ratio to provide the target Daylight Factor, energy efficiency for the tropical climate, in consider to fire and the explosive load using the graphic of the relationship of the Window to Floor Ratio to the target minimum Daylight Factor. The result was shown for tropical of Vietnam, the Window to Floor Ratio was recommended of 15.2% to 18.5% corresponds to recommended target Daylight Factor of 1.35%.

The energy used in the buildings in subtropical and tropical countries is mainly for the cooling load requirements for air conditioning in summer, which is evaluated by OTTV value. To ensure a satisfactory OTTV value, the top priority of building envelope designing is reasonable WWR with appropriate sun shading devices to reduce the heat penetrated through window glass into the buildings. Therefore, the accurate calculation of solar radiation through window glass with shading devices is very important. The research aims to calibrate the reduced solar radiation coefficient of the shading device (β) from the original method, thereby completing the OTTV evaluation method with high accuracy. The calculation is based on the sunlight projection to determine the window’s shaded area created by shading devices of any shapes with consideration of diffuse radiation diminished. The results of β calculated showed that in tropical climate, the total solar radiation on the window glass is much lower than the previous method. A computer software built from the proposed theory helps the designer easily choose the optimal sun shading solutions. The research conducts the calculation as an example for horizontal-inclined and the combined of horizontal and left/right vertical shading devices.

Horizontal illuminance is an important indicator, which shows the resources of the light climate of each region. Meticulous consideration of the light-climate data allows us to develop graphs of diffuse and global illuminance (E_D and E_G) and go to the territorial norms of daylight spaces at the modern level. Measurements of horizontal illuminance in Vietnam are not available; therefore, the calculation of the determination horizontal illuminance from solar radiation by light equivalents using the values of the luminous efficacy is widely used now. The paper contains calculation of the luminous efficacy for Vietnam, the results are graphs of horizontal illuminance for Ho Chi Minh. Comparative analysis with horizontal natural illuminance of Moscow shows a high level and uniform distribution of outdoor illuminance in the tropical sky in Vietnam. The maximum values of E_D and E_G for Ho Chi Minh City are 42.01 Klux and 87.17 Klux; for Moscow these values are equal to 28.3 Klux and 53.1 Klux, respectively. In addition, the levels of illuminance in the winter months in Ho Chi Minh are much higher than in Moscow, which is explained by the latitudinal difference in the location of these cities and the corresponding movement of the Sun.