ZEMCH 2019 International Conference Proceedings April.2020 | Page 173
1. Introduction
The South Korean government has operated the Energy Efficiency Standards and Labeling
Program for windows since 2012, which requires that window companies develop energy ratings for
their products prior to making sales. Grades 1–5 are decided by the test results of thermal performance
and airtightness according to Korean Standards, KS F 2278 [1] and KS F 2292 [2]. The government also
has operated a simulation system for the thermal performance of windows in the program, which
provides an alternative procedure for window companies to save time and money on laboratory testing
that are necessary for energy ratings. Obviously, a window (a base model) should provide the test
results of the thermal performance and airtightness, as tested by a certified laboratory earlier. Series
models can be modified in the base model only in terms of the glazing system used or for thermal
breaks of the frame [3]. If the difference between the experimental and calculated results of a base model
does not exceed a range specified in the operational regulations on equipment for efficiency
management [4], the tested window frame can be used to calculate a series model. Usually, a series
model changes the glazing system compared to the base model, and the glazing system should be the
same thickness as that applied to the base model. The Korean government allows companies to use the
calculation method specified in the International Organization for Standardization (ISO) standard
15099 [5], so WINDOW/THERM [6] is mainly used as the simulation program to evaluate the thermal
performance of the window; however, the thermal performance of a single window can be calculated
according to ISO 15099 to not vary much from the test value [7], but this method cannot be used for a
double window because of the thickness of the air cavity in the direction of the heat flow [8]. A double
window has a large air cavity that is formed by the interior and exterior windows, and the thickness of
the air cavity in the direction of the heat flow is generally 70 mm to 120 mm. If this thickness exceeds
50 mm, ISO 15099 requires the use of another calculation method for the thermal characteristics of an
air cavity to be analyzed by laboratory tests. In previous work, the calculated results according to
computational fluid dynamics (CFD) are more similar to the experimental results than to the calculated
results according to ISO 15099, indicating that ISO 15099 is not sufficient for calculating the thermal
characteristics of the air cavity between the external and internal windows in a double window [8]. For
an appropriate calculation method for the thermal transmittance of a double window, the thermal
characteristics of an air cavity between the internal and external windows need to be calculated to
indicate the circumstances of an experimental test.
In this study, the calculation methods for the thermal transmittance of a double window defined
by ISO 15099 and 10077 [9‐10] are analyzed to determine which one is more appropriate. Seven cases
of double windows are considered based on four types of double windows and six types of glazing
systems. The air cavities for the windows in each case are calculated using the international standards
and are simulated by CFD. Finally, the calculated U‐values by ISO 15099 and 10077 are compared with
the experimental results.
2. Methods
2.1. Double window types and cases
Table 1 indicates that six types of glazing systems are established for double windows. The glazing
systems are selected based on the International Glazing Database (IGDB) operated by the National
Fenestration Rating Council (NFRC). These consist of two 5 mm glass panes separated by a 12 mm‐
wide gap filled with air (Air) or argon gas (Ar). In Table 1, CL is a glass pane with no low‐emissivity
coating, LE is a glass pane with low‐emissivity coating, and the number before each abbreviation means
the thickness of a glass pane or wide gap between glass panes. Glazing system C is 25.76 mm because
it has one laminated glass pane. It consists of 5 and 3 mm glass pane, and in between, there is a 0.76
mm PVB coating. Glazing system F is 25 mm and consists of a 14 mm‐wide gap filled with argon gas
between one 6 mm‐coating glass pane and a 5 mm glass pane.
Improving the Calculation Method for the Thermal Performance of Double Windows in Korea
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