Abstract
The calculation method for the thermal transmittance (U-value) of double windows as specified by the Korean government (ISO 15099) is often inappropriate. To develop a more suitable calculation method, the thermal properties of the air cavity between the internal and external windows should be considered. Herein, seven cases of double windows were set up. The air cavities were designed in accordance with international standards and computational fluid dynamics (CFD) and used for the calculation of the U-values of the double windows according to ISO 15099 and 10077. All the calculated U-values were compared with experimentally obtained values. In accordance with the ISO 10077-1 method, the thermal resistance of the air cavity calculated using CFD could produce double window U-values that are similar to the experimentally obtained values. In most cases, the difference between the theoretical and experimental U-values was 5% and less than 0.14 W·m−2·K−1, implying that the U-values calculated using CFD and the ISO 10077-1 method are approximately equal to the experimentally obtained U-values. Korean regulations do not include ISO 10077-1 for double-window assessment. However, these criteria can provide a solution in improving the accuracy of the calculation of the overall thermal transmittance of double windows.
Highlights
Since 2012, the Korean government has operated the Energy Efficiency Standards and Labeling Program for windows, which requires window companies to provide the energy ratings for their products prior to sale
Calculation method A based on International Organization for Standardization (ISO) 15099 exhibited a difference between experimental and theoretical U-values of 10% to 24%, the Korean government operates the regulation to ensure the use of this method
According to Korean regulations, the procedure provided in ISO 15099 is used in the calculation of the thermal transmittance of double windows
Summary
Since 2012, the Korean government has operated the Energy Efficiency Standards and Labeling Program for windows, which requires window companies to provide the energy ratings for their products prior to sale. The government has suggested a simulation method in the program for the determination of the thermal performance of windows This method provides an alternative procedure by which window companies can save time and money on laboratory testing, which is necessary for the determination of energy ratings. The thermal performance of a single window can be calculated according to ISO 15099 such that it does not vary much from the test value [7] This method cannot be used for a double window because of the thickness of the air cavity in the direction of heat flow. The thickness of the air cavity is usually 70−120 mm, which means the length of the heat flow direction If this thickness exceeds 50 mm, ISO 15099 requires that another calculation method should be used to determine the thermal properties of the air cavity, for example, performing laboratory tests. Window companies suspect the reliability of the ISO 15099 method and require a more suitable method for calculating the U-values of double windows
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