Figure 1 . Illustration of a double window defined by ISO 10077‐1
ISO 10077 includes the procedure for calculating thermal transmittance , UU � , for a double window , which is a system consisting of two separate windows , as shown Figure 1 . Using this method , it is necessary to calculate the thermal transmittances of the internal and external windows , UU �� and UU �� , respectively . The internal surface resistance of the external window when used alone , RR �� , the external surface resistance of the internal window when used alone , RR �� , and the thermal resistance of the space between the glazing in the two windows , RR � , are also calculated according to the equations . The
thermal transmittance of a double window shall be calculated using the following formula : 1
UU � � UU ��� �RR �� �RR � �RR �� �UU ���
4 . Results and Discussion
Table 4 denotes the thermal transmittance ( U‐value ) of seven double windows according to the procedures conducted in the laboratory and via international standards . The method in ISO 10077 can calculate the thermal transmittance of a double window with values more similar to the experimental results compared to the results using ISO 15099 . When the method in ISO 10077 calculates the U‐value , the linear thermal transmittance of a double window needs to be calculated according to ISO 10077‐2 . The difference between the laboratory results and the calculated results in Case 2 vary widely . Case 2 uses the same frame as Case 1 does , but it has good thermal performance because of the glazing system that has high performance . This means that the calculating procedure should consider the effect of glazing on the overall thermal performance of a double window in future studies .
Table 4 . The thermal transmittance of double windows according to the procedures used in the laboratory and the calculation methods
Case
U‐value in the laboratory ( W∙m ‐2 ∙K ‐1 )
1 1.220
2 0.737
Calculation for the U‐value of the window Thermal resistance ( Rs ) of the air cavity Linear thermal transmittance
ISO 15099 ISO 10077 ISO 10077
ISO 15099 CFD CFD
N / A
1.216 ( 0.3 %) 0.915 ( ‐24.2 %)
ISO 10077‐1 ( table ) 1.147 ( 6.0 %) 0.872 ( ‐18.3 %)
ISO 10077‐2 ( calculation ) 1.134 ( 7.0 %) 0.845 ( ‐14.7 %)
Improving the Calculation Method for the Thermal Performance of Double Windows in Korea 166