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Technical review by: Y.Z. ZHOU of Warmframe Technology Corporation, Beijing, China, and Richard LI of AECOM Building Engineering (Façade), Hong Kong

ABSTRACT from The Façade, the official publication of Hong Kong Façade Association, Issue #33:

Thermally broken windows consist of thermally broken frames and insulated glass, and have superior thermal performance compared to regular windows. Their characteristics of low U-value and low shading coefficient contribute to reduce both the heating and cooling loads of buildings in Hong Kong, thereby significantly reducing carbon emissions.

Thermally broken windows are globally recognized as one of the most important technologies for achieving a safer and greener society, and have been widely used around the world for three decades. Shenzhen, China has been using thermally broken windows since the new national code changed the maximum allowable window U-value from 3.5 W/(m²·K) to 2.4 W/(m²·K). However, Hong Kong hardly uses any thermally broken windows, which is worth discussing.

This paper discusses the barriers to the adoption of thermally broken windows in Hong Kong. The first issue is the shortcoming of the current OTTV method, which was obtained based on a set of conditions that are no longer valid. The new conditions lead to a different result, which leads to the overestimation of the building energy performance. The second issue is about the current practice of LEED in Hong Kong. Engineers use the glass center U-values as the window U-values, which doesn’t follow the actual requirement of LEED. This discrepancy means that the actual energy performance of the buildings is unlikely to meet the design requirements. The third issue is that the Buildings Department (BD) does not approve thermally broken windows due to structural safety concerns, and a simple and mature solution to address BD concerns is presented. The paper also discusses its limitations and the future plan for the comparable energy simulation and experimental testing.

Keywords: thermally broken window, U-value, OTTV, building envelope

1. BACKGROUND

In the 19th century, monolithic glass was used in windows. To improve the thermal performance of windows, American inventor Thomas D. Stetson patented an insulated glass unit (IGU) in 1865. However, it wasn’t until the oil shortages of the 1970s that IGUs became more widely used. At that time, glass manufacturers began to develop low-emissivity (low-E) coatings to improve energy efficiency, nearly doubling the insulating performance and reducing the U-value by 50% of a conventional IGU with uncoated clear glass. 40 years ago, a German company developed the first thermal break for windows, the plastic bar installed between the outer and inner parts of the metal window frame. Today, although modern thermally broken windows consist of different glass and frame profiles, the original concept of using a low-conductivity material as the thermal break has remained unchanged (Figure 1).

Figure 1: Modern Thermally Broken Windows

One of the key parameters for differentiating windows is the U-value, which may be described as the speed of heat transfer through the outer and inner surfaces of the window. The lower the U-value, the better. For the whole window, the best U-value is 0.8 W/(m²·K) or less, which consists of well-insulated frames and high-performance IGUs; while the worst U-value is 6.8 W/(m²·K) or more, which consists of regular frames and single glass. It should be noted that the window U-value is the area-weighted average of the frame and glass, not just the glass itself, as specified in the relevant standards, i.e. ISO 15099, ASHRAE 90.1, etc.

Germany was one of the early adopters of thermally broken windows, switching from regular windows in the 1990s when the window U-value limit changed from 3.1 W/(m²·K) to 1.8 W/(m²·K). The United Kingdom completed a similar upgrade in the 2000s. The United States and China began using thermally broken windows in the 2000s, but only for buildings in cold climates back then; buildings in hot climates didn’t use thermally broken windows until 2020. In Shenzhen, almost all the buildings used regular windows in the past, but since the new code, GB55015-2021: General Code for Energy Efficiency and Renewable Energy Application in Buildings, was introduced in 2021, the maximum allowable window U-value was changed from 3.5 W/(m²·K) to 2.4 W/(m²·K), requiring thermally broken windows. Hong Kong doesn’t have a window U-value requirement. Wong (2017) showed that the most popular configuration of windows for commercial buildings in Hong Kong were non-thermally broken (regular) frames with low-e IGUs [1]. The typical window U-value in Hong Kong is estimated to be an average of 3.6 W/(m²·K) (Figure 2).

Figure 2: Window U-value Requirements Since 1990

• INTRODUCTION

The appearance of thermally broken windows can be exactly the same as regular windows, seen from the inside or outside. It is the thermal breaks that make the thermal performance of thermally broken windows exceed that of regular windows. THERM software was used to quantify and visualize this performance. Developed by the Lawrence Berkeley National Laboratory (LBNL), THERM is one of the most widely used thermal simulation software programs in the world. Simulation results show that the benefits of thermally broken windows include:

Higher energy efficiency and less carbon footprint

U-Value and Shading Coefficient (SC) are the main characteristics related to the energy efficiency of windows. Table 1 shows the U-value and SHGC of a state-of-the-art thermally broken window and a regular window with the same low-e IGU.

Table 1: U-value and SHGC of Windows

Although the ratio of the frame to the window area is only 15% to 25% in regular windows, the influence of the frame on the energy efficiency can be as high as 70%. This is because the frame U-value is about 16 W/(m²·K), which is about 10 times the glass U-value. As a comparison, the frame U-value of the state-of-the-art thermally broken window with Warmframe thermal break blanket and warm-edge glass spacer is only 4 W/(m²·K), which is 75% less than the regular frame (Figure 3). Thanks to the high performance of the thermally broken window, the energy efficiency of the building can be greatly improved. The lower U-value and SC contribute to reducing both the heating and cooling loads of buildings in Hong Kong, thereby significantly reducing carbon emissions.

Figure 3: Window Frame U-values

Warmer in winter and cooler in summer

THERM simulations were performed to evaluate the interior surface temperatures of the windows. The outdoor and indoor environmental conditions refer to Hong Kong, and the details are as follows:

• Winter outdoor nighttime temperature = 7℃
• Summer outdoor daytime temperature = 32℃
• Indoor temperature = 25℃
• Solar irradiance = 783 W/m²

Figure 4: Window Frame Temperatures

Figure 4 shows the interior surface temperatures of the state-of-the-art thermally broken window and a regular window. In winter, the temperatures are 21℃ and 16℃; in summer, the temperatures are 27℃ and 40℃, respectively. The results show that the regular window is either too hot in summer or too cold in winter, while the state-of-the-art thermally broken window shows its excellent resilience to the environment.

By maintaining a stable indoor temperature, thermally broken windows increase the comfort level for the occupants. They eliminate cold spots near windows, contributing to create a more comfortable space throughout the year. Seppänen et al (2006) found that when the indoor temperature is at 20-25℃, the productivity of the occupants will reach over 98% of the best; while the temperature is lower than 15℃ or higher than 30℃, the productivity of the occupant will drop to below 90% [2].

Reduced condensation

According to the temperatures in Figure 4 and the indoor temperature of 25℃, we can predict when there will be condensation. Figure 5 shows the curve of dew point temperature at 25℃. For the regular window, condensation will occur when the indoor relative humidity reaches 58%, which is quite common in a humid winter in Hong Kong. However, for the thermally broken window, the indoor relative humidity must be 79% for the window to condense, which is unlikely to happen.

Figure 5: Dew Point Temperature & Relative Humidity Chart

Lower noise transmission

Because sound vibrations are more easily transmitted within the integral aluminum frame and form the sound bridging, the thermal breaks in thermally broken windows can be treated as sound insulators. Although there is no concrete data to show the difference, it is believed that thermally broken windows perform no worse than regular windows. In fact, there are many commercially available products that have been tested to an STC (Sound Transmission Coefficient) of 45, which means that the noise control of the windows is even better than a 4″ hollow CMU (Concrete Masonry Unit).

• RESEARCH QUESTIONS

It is globally recognized that the window is one of the most important factors in achieving sustainability and carbon neutrality for buildings. For example, a window should have a U-value of 1.4 W/(m²·K) to meet the Low Energy Building (LEB) standard, a U-value of 0.8 W/(m²·K) to meet the Passivhaus standard, and an even lower U-value of 0.7 W/(m²·K) to meet the Nearly Zero Energy Buildings (NZEB) standard. None of these are possible without thermally broken windows. Hong Kong, only 50 km away from Shenzhen, has similar climate and challenges, but hardly uses any thermally broken windows other than the regular ones, which seems odd.

After extensive discussions with building professionals including architects, engineers, consultants and contractors in Hong Kong, the most common comments can be summarized as follows:

• Hong Kong uses the Overall Thermal Transfer Value (OTTV) as the sole indicator for the energy efficiency design of buildings, and there is no window U-value requirement in the code.

• For the project seeking LEED certification from the U.S. Green Building Council (USGBC), when window U-values are required by the ASHRAE standard, the engineers simply treat the glass center U-values as equivalent to the window U-values, neglecting the frames.

• The Hong Kong Buildings Department (BD) hesitates to approve thermally broken windows due to structural safety concerns. If you do want to use the thermally broken window in the project, you have to initiate a review process which will cost extra time and money, with no guarantee of approval.
  

  
  

This paper conducts an in-depth analysis of the above issues based on relevant theories, standards.