Image 1 : Thermographic image of low-E glass and normal glass in Malaysian apartment building
Both windows may look the same at first glance, but the thermal image reveals a clear difference. The window on the right is noticeably warmer, showing a significant difference in heat radiation. This juxtaposition of the glazing types came about when one of the original low-E glass windows broke and was replaced with a normal glass pane (SGU, single glazing unit). With the use of a thermographic camera, this side-by-side comparison gives a great visual illustration of how effective low-E glass is at reducing heat radiation into the room.
In order to quantify the energy savings of low-E glass, an annual energy simulation was performed using the IES building simulation software for an 26°C air-conditioned high-rise residential room in Kuala Lumpur, Malaysia:
Image 2 : Room energy model with East facing window
Table 1 : Simulation input values for the two glazing types with identical visible light transmittance of 44%
The above table shows that the low-E window, as a lower emissivity value (0.26) on the inside pane of the glazing compared to the normal window (0.84). The low-emissivity reduces the quantity of heat radiated into the room, hence, lowering both the U-value and the Solar Heat Gain Coefficient of the low-E glass compared to the normal glazing.
Image 3 : Simulated annual solar heat gain reduction with low-E glass
The graph above shows that using a low-emissivity window reduces the annual solar heat gain by 50% compared to a normal window. This reduction in solar gain decreases the need for air conditioning, leading to lower energy costs and a more sustainable living, Moreover, the low-E glazing also enhances thermal comfort in the room, because the heat radiation from the hot window is reduced, particularly during the morning hours, when the window is exposed to direct solar radiation. Refer to the annual thermal comfort simulation below, where the Predicted Percentage of Dissatisfaction (PPD) is shown for 10 am in the morning:
Image 4 : Thermal comfort dissatisfaction reduction with low-E glass
When comparing a low-E window to a normal window, the low-E window significantly outperforms in terms of occupant thermal comfort. The Predicted Percentage of Dissatisfied (PPD) metric reveals that the low-E window results in nearly 7% less dissatisfaction compared than a normal window for a person placed in the middle of the room, or 2.15 meters from the glazing.
Image 5 : PMV reduction from low-E glass
The above graph show the thermal sensation of people in the room, expressed in terms of the Predicted Mean Vote (PMV). The annual simulations show values between 0 (Neutral) and 1 (Slightly Warm). The analysis shows that there is no significant difference between the low-E window and the standard window during the morning and evening. However, for the rest of the daylight hours, when the outdoor temperature is warmer than the indoor temperature, the low-E window consistently has a lower PMV value than the normal glass, hence, helping people in the room to feel less hot.
This suggests that the low-E window helps to moderate the midday temperature spikes, effectively softening the impact of intense sunlight and contributing to a more stable and comfortable indoor environment throughout the day.
Low-E glazing: How it works
First, in the glass context emissivity refers to the ability of the glass to emit heat radiation, when the glass gets hot. The low-E coating also helps ensure that the glass becomes spectrally selective, allowing the visible light to pass through, while blocking the ultraviolet (UV) light and infrared (IR) light, both of which are not visible to the human eye.
Image 6 : Solar spectrum with ideal spectrally selective glass for the tropics
As illustrated in the image above, the ideal glass allows only visible light to pass through, blocking harmful ultraviolet (UV) rays that cause fading in interior materials like fabrics and wall coverings, as well as infrared (IR) rays that transmit heat into a building. Low-E coatings are engineered to minimize the transmission of UV and IR light through the glass while still allowing visible light to enter.
Solar heat absorbed by the glass, is dissipated from the surface of the glass by convection, conduction and radiation. This ability to radiate energy off from the glass surface is known as emissivity, a value ranging from 0 to 1. Most materials have an emissivity value between 0.8 and 0.9, which means they are 80-90% effective in radiating heat, emitted as long-wave infrared energy. Since radiant energy significantly impacts how heat transfers through windows, reducing the emissivity of the glass surface improves the window's insulating properties.
Low-E coating of glass effectively lowers the surface emissivity from 0.84 (normal glass) down to 0.3 (hard coat low-E for SGU) and all the way down to 0.02 (soft coat low-E for the best performing DGU). The low-E coating is placed on the glass surface facing indoors, hence, helping to reflecting air-con cooling back into the room and preventing outside heat from radiating into the room.
Low-E glazing: How it is made
There are two main methods for applying low-E coatings to glass, hard coat and soft coat, each offering distinct advantages in terms of performance and durability.
The difference between soft coat and hard coat low-E glass primarily lies in their application methods and durability. Soft coat is applied to glass after it has been manufactured, using a vacuum chamber and sputtering technique. This coating, while highly effective in reducing emissivity and reflecting infrared light, is relatively delicate and requires protection, typically being used in double glass units (DGU) where it is sandwiched in the air gap between two layers of glass.
Table 2 : Glass emissivity depending on coating
For example, soft coat low-E glass is commonly used in high-performance windows for office buildings, where optimal spectral selectivity, thermal insulation and energy efficiency are required.
In contrast, a hard coat is applied during the glass production process while the glass is still hot, using pyrolytic methods. This results in a more durable and robust coating that adheres directly to the glass surface, making it resistant to damage from physical impacts and scratches, hence, suitable for use on single pane windows, also called single glazed units (SGU).
Hard coat low-E glass is often used in buildings that are unwilling to spend the extra money for double-glazing, but still wants some thermal performance from the cheaper single or laminated glazing. SGU low-E glass is used in office buildings, shopping malls and residential buildings. Although hard coat low-E may not offer the same level of energy efficiency as soft coat low-E, its greater durability makes it suitable for installation on single glazing.
Thermal discomfort from Radiant Asymmetry
This article has described how low-E windows can help to improve thermal comfort by reduce heat radiation. In extreme cases, where people face a big hot surface, for example a large facade window or a big skylight, thermal discomfort can be caused by the radiant asymmetry experienced in the space.
Image 8 : Radiant asymmetry for warm wall
For example, in hot climates, such as Malaysia, imagine a person sitting in a cool 20°C room next to a big window with a surface temperature of 35°C. While the person might feel thermally balanced with hot window off-set by the cool room temperature, the radiant asymmetry of 15°C (35°C minus 20°C) will cause a 2.5% thermal dissatisfaction rate.
In cold climates, the importance of low-E glass is even more pronounced, as a radiant asymmetry of 15°C from a cold wall would result in a thermal dissatisfaction rate of 20%.
Interestingly, people are most sensitive to vertical radiant asymmetry from hot ceilings. This is relevant in hot climates with hot overhead sun. For example, imagine a person sitting in a cool 20°C air-conditioned atrium space below a skylight with a surface temperature of 35°C, the radiant asymmetry (15°C) would cause thermal dissatisfaction for about 40% of the people.
Conclusion
The use of low-emissivity (low-E) coatings on windows is effective for reducing solar heat gain, reducing heat radiation to the indoors, improving energy efficiency and thermal comfort of buildings by maintaining a more stable and comfortable indoor environment. Thus, Low-E glass is essential for enhancing comfort and reducing thermal dissatisfaction in both hot and cold climates.
References and acknowledgements:
Thanks to Stephanie Bacon for letting us undertake the thermal investigations in her apartment at The Estate condo, Kuala Lumpur, Malaysia
Gregers Reimann, article contributor
ASHRAE 55 Standard (radiant asymmetry graph)
OTM Solutions (glazing emissivity values, link)
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