Review of the Experimental Methods for Evaluation of Windows’ Thermal Transmittance: From Standardized Tests to New Possibilities

Abstract

One of the most important parameters that indicate the energy performance of a window system is the thermal transmittance (U-value). Many research studies that deal with numerical methods of determining a window’s U-value have been carried out. However, the possible assumptions and simplifications associated with numerical methods and simulation tools could increase the risk of under- or over-estimation of the U-value. For this reason, several experimental methods for investigating the U-value of windows have been developed to be used either alone or as a supplementary method for validation purposes. This review aims to analyze the main experimental methods for assessing the U-value of windows that have been published by national and international standards or as scientific papers. The analysis criteria include the type of the test in terms of boundary conditions (laboratory or in situ), the part of the window that was tested (only the center of glazing or the entire window), and the data analysis method (steady-state or dynamic). The experimental methods include the heat flow meter (HFM) method, guarded hot plate (GHP) method, hot box (HB) method, infrared thermography (IRT) method, and the so-called rapid U-value metering method. This review has been set out to give insights into the procedure, the necessary equipment units, the required length of time, the accuracy, the advantages and disadvantages, new possibilities, and the gaps associated with each method. In the end, it describes a set of challenges that are designed to provide more comprehensive, realistic, and reliable tests.

Comprehensive Review and Analysis of Glazing Systems towards Nearly Zero-Energy Buildings: Energy Performance, Thermal Comfort, Cost-Effectiveness, and Environmental Impact Perspectives

“…These values are not material constants and can be affected by the boundary conditions of the glazing systems [49]. As stated by Simões et al [11] and Moghaddam et al [11], three main methods may be employed to accurately determine the U-value and g-value of a glazing system: Numerical methods (or simulation tools), laboratory tests with boundary conditions close to the operational ones, and in situ tests. When considering the use of numerical methods, these should be validated against a reliable experimental method.…”

“…Otherwise, there is a risk of under-or over-estimation of the results. By developing reliable in situ methods that consider real operational conditions, U-values and g-values could be evaluated more accurately, faster, and at a relatively cheap cost in the future [11]. Reproduced from [45].…”

“…The literature review demonstrated that providing general recommendations for the selection of glazing solutions may not be reasonable. The thermal and optical properties of a glazing system, such as U-value and g-value, are heavily influenced by climatic conditions, orientation, window frame, and size [11]. When considering environmental and economic factors, other facade components, type, regional energy prices, energy accessibility, economic conditions, and more can also play a significant role in choosing an optimum glazing solution.…”

“…Among the various components that make up a building's facade, a lot of attention has been paid to the window and glazing systems in recent years. Windows provide natural light, natural ventilation, favorable solar gain (in cold regions in the winter), aesthetics, and positive psychological impacts, making their use in buildings inevitable [11][12][13]. However, windows are among the most energy-inefficient parts of a building's envelope, particularly due to their weakness in terms of thermal transmittance (U-value) [14].…”

“…The U-values of windows are normally much higher than those of other elements, such as walls and horizontal elements. In fact, in the newest building codes introduced by most of the EU members, the U-value of windows is allowed to be four to six times the U-value of the opaque elements [11]. In addition, a high g-value for window glazing can allow a large portion of solar radiation to enter the building, resulting in an increase in cooling demands [3,15,16,[18][19][20].…”