Low-e coating and glass optical & thermal performances
Low-e coated architectural glasses are extensively used nowadays for dramatically improved glass optical & thermal performances. This article aims to provide a detailed explanation on how low-e coating improves glass optical & thermal performances.
- What is low-e coating?
- How does low-e coating work?
- Low-e coating position
- What are double-silver and triple-silver low-e coatings?
- Can low-e coating be understood as “heat reflection mirror”?
What is low-e coating?
Low-e coating stands for low-emissivity coating. It is a thin layer of coating applied on glass surface for improved optical & thermal performances.
The name “low-emissivity” coating does not fully represent the optical characteristics of low-e coatings. It is more appropriate to understand low-e coating as “low-emissivity & optical-filtering” coating.
There are mainly two types of low-e coatings, namely hard low-e coating and soft low-e coating.
What is emissivity?
All surfaces above absolute zero (0 K or -273.15 °C) emit thermal radiation in the form of electromagnetic radiation. The spectral distribution of thermal radiation depends on the surface temperature.
- Sun surface is with very high surface temperature (around 5800 K). The thermal radiation by sun is mainly in the short wavelength range. It consists of ultraviolet (UV), visible light (VIS) and near infra-red (NIR) components, with peak in the visible green light (around 550 nm). The yellow color curve (5777 K) below represents the ideal solar radiation spectrum.
- Room temperature surfaces (e.g. glasses) are with low surface temperature (around 300 K). The thermal radiation by room temperature surfaces is in the long wavelength range. It consists of far infra-red component only and they are invisible, as there is no visible light component. The red color curve (300 K) below represents the ideal room temperature radiation spectrum.
In glass heat transfer analysis, the solar radiation part (i.e. the thermal radiation from sun surface, in the wavelength range of 300 nm – 2500 nm) and the far infra-red radiation part (i.e. the thermal radiation between glass surfaces and indoor/outdoor environments, in the wavelength range of 5 µm to 50 µm) are treated separately.
As described in the complete list of glass optical & thermal properties, most optical properties are related to solar radiation only (for example, visible light transmittance & reflectance, solar energy transmittance, reflectance & absorptance).
Emissivity is the glass optical property related to far infra-red radiation.
The thermal radiation emitted by a general surface is smaller than the ideal radiation shown in the figure above. Emissivity is the ratio of thermal radiation from a surface to the thermal radiation from an ideal surface:
- Emissivity = 1: an ideal surface with maximum thermal radiation (i.e. an ideal black body surface)
- Emissivity = 0: an ideal surface with zero thermal radiation (i.e. an ideal white body surface)
According to Kirchhoff’s law of thermal radiation, the emissivity of a surface is equal to its absorptivity:
Emissivity = Absorptivity
Absorptivity is the fraction of thermal radiation absorbed by a surface:
- Absorptivity = 1: an ideal surface absorbs all incident thermal radiation (i.e. an ideal black body surface)
- Absorptivity = 0: an ideal surface reflects all incident thermal radiation (i.e. an ideal white body surface)
If we compare two surfaces, one with high emissivity and one with low emissivity, we should have the following observations:
|High emissivity surface||Low emissivity surface|
|Thermal radiation to surrounding surfaces||Radiates more thermal radiation||Radiates less thermal radiation|
|Thermal radiation from surrounding surface||Absorbs more thermal radiation||Absorbs less thermal radiation|
|Overall radiative heat transfer with surround surfaces||Exchanges more radiative heat||Exchanges less radiative heat|
In summary, low-emissivity surfaces radiate less thermal radiation to surrounding surfaces, absorb less thermal radiation emitted by surrounding surfaces, and are with weaker overall radiative heat exchange with surrounding surfaces.
Most natural surfaces are high emissivity surfaces, with typical emissivity between 0.8 and 0.95, except polished metal surfaces, with typical emissivity between 0.02 and 0.1 (refer to this Wikipedia page for the emissivity of common surfaces).
The nominal emissivity of uncoated glass surfaces is 0.84. Therefore, uncoated glass surfaces are high emissivity surfaces.
Low-e coating reduces glass surface emissivity and reduces the radiative heat exchange between a glass surface and the surrounding surfaces.
Low-e coating or low-emissivity & optical-filtering coating?
In addition to the low-emissivity effect in the far infra-red range (wavelength range: 5 µm – 25 µm), low-e coating also alters glass optical property in the solar radiation range (wavelength range: 300 nm – 2500 nm).
Shown below are the spectral transmittance/reflectance curves of an uncoated glass and a low-e coated glass.
It is evident that the low-e coating changes the optical property of a glass significantly. We can call this effect as “optical-filtering” effect.
The optical-filtering effect of low-e coating is in most cases more important than the low-emissivity effect, because the optical-filtering effect affects the visual appearance (e.g. color) and daylight performance (e.g. visible light transmittance) of a glass, whereas the low-emissivity effect does affect them.
Therefore, it is more appropriate to understand low-e coating as a type of low-emissivity & optical-filtering coating, particularly in the summer daytime condition (to be discussed below).
Low-e coating types
There are mainly two types of low-e coatings, namely hard low-e coating/soft low-e coating, as listed in the table below:
|Hard low-e coating||Soft low-e coating|
|Coating process||Pyrolytic coating: online coating while float glass is being produced and hot||Magnetron sputter vacuum deposition (MSVD) coating: offline coating after glass is produced and at room temperature|
|Durability||Highly durable, resistant to scratches and moisture; can be used on glass external surfaces||Not resistant to scratches and moisture; cannot be used on glass external surfaces; need to be sealed in an insulated glass unit (IGU) or a laminated glass|
|Typical emissivity||0.1 – 0.3||0.02 – 0.1|
It is out of the scope of this article to discuss more details of low-e coating process and other physical characteristics. Interested readers may read this article.
How does low-e coating work?
The heat transfer mechanism of glasses with low-e coating can be explained in two typical conditions:
- Winter nighttime condition
- Summer daytime condition
The winter nighttime condition is a special scenario with far infra-red radiation only and without solar radiation. The summer daytime condition is a generic scenario with both solar radiation and far infra-red radiation.
Low-e coating in winter nighttime condition
In the winter nighttime condition, the indoor temperature is warmer than the outdoor temperature. The heat transfer direction is from the indoor side to the outdoor side. Listed below are the heat transfer paths:
- Convection and radiation (far infra-red) from indoor environment to glass indoor side surface
- Conduction from glass indoor side surface to outdoor side surface
- Convection and radiation (far infra-red) from glass outdoor side surface to outdoor environment
Low-e coating does not affect all heat transfer paths, except the far infra-red radiative heat transfer path involving the low-e coated surface (the indoor side surface in the figure above), due to the low-emissivity effect of low-e coating.
It results in lower glass U-value and hence better glass energy efficiency in the winter nighttime condition (Note: we have an online glass U-value calculator for you to try out the influence of low-e coating emissivity on glass U-value).
Low-e coating in summer daytime condition
In the summer daytime condition, the outdoor temperature is warmer than the indoor temperature and, in addition, there is also solar radiation. The heat transfer direction is from the outdoor side to the indoor side.
In addition to the heat transfer paths in the winter nighttime condition, there is also:
- Radiation (Solar) from sun to indoor environment, with transmission, reflection, and absorption of solar radiation by the glass
In the summer daytime condition, low-e coating reduces radiative heat transfer of both far infra-red radiation (due to the low-emissivity effect) and solar radiation (due to the optical-filtering effect).
It results in both lower glass U-value and solar heat gain coefficient (SHGC) and hence better glass energy efficiency in the summer daytime condition.
Low-e coating position
Low-e coating is applied onto a glass surface. There are a few possible positions of low-e coating for a specific glass configuration. In this section, the preferred low-e coating positions of 3 typical glass configurations are discussed.
Single pane glasses
For single pane glasses, the preferred low-e coating position is surface #2. As this surface is exposed to moisture and external environment, only hard low-e coating can be used.
As outdoor wind speed is higher than indoor airflow speed, the convective heat transfer on the outdoor side is stronger than that on the indoor side. Reducing the radiative heat transfer on the indoor side is more effective than on the outdoor side.
With low-e coating on surface #2, the U-value reduction is significant, whereas, with low-e coating on surface #1, the U-value reduction is negligible.
For laminated glasses, a commonly used low-e coating position is surface #4, like single pane glasses.
Another commonly used position is surface #2. In this case, the low-emissivity effect of low-e coating is not used, as glass is opaque to far infra-red radiation, and the low-e coated surface does not have far infra-red radiative heat exchange with other surfaces. Only the optical-filtering effect is used in this configuration.
Additionally, the low-e coating is adjacent to the lamination interlayer and is not exposed to moisture or external environment. Soft low-e coating is often used here, for its better optical-filtering performance.
For laminated glasses, it is also possible to apply low-e coatings on both surfaces #2 and #4 for better performance.
Between surfaces #2 and #3, surface #2 is better, as surface #2 is in front of surface #3. The mechanism is explained below for double glazing units.
Double glazing units (DGUs)
For DGUs, there are two possible low-e coating positions (surface #2 or surface #3), as shown above.
The thermal insulation performances between the two configurations are identical, i.e. the U-values are the same.
The solar heat gain performances between the two configurations are different. Typically, the solar absorptance of a low-e coated glass is higher than that of an uncoated glass. If the low-e coating is on surface #2, it is more difficult for the absorbed solar radiation to be transferred to the indoor space than to the outdoor space, as the thermal resistance on the outer glass pane indoor side (3 layers: gas gap, inner glass pane and indoor air) is greater than that on the outdoor side (1 layer only: outdoor air). In contrast, if the low-e coating on surface #3, it is more difficult for the absorbed solar radiatin to be transfered to the outdoor space, i.e. more solar heat gain. The SHGC of a glass system with low-e coating on surface #2 is lower than that of a glass system with low-e coating on surface #3.
Therefore, in summer conditions, low-e coating on surface #2 is preferred, as low SHGC is preferred in summer; in winter conditions, low-e coating on surface #3 is preferred, as high SHGC is preferred in winter.
Additionally, for better performance, it is possible to apply hard low-e coating on surface #4 (in addition to the low-e coating on surface #2 or #3).
Though it is also possible to use low-e coatings on both surface #2 and surface #3, it is not cost-effective to do so and there are no commercial glass products with this configuration to our knowledge.
What are double-silver and triple-silver low-e coatings?
A commercial soft low-e coating consists of multiple silver layers and dielectric layers (ceramic material) organized in a stack.
Soft low-e coatings with more layers of silver are with better spectral selectivity, i.e. allow more visible light to transmit through and block more ultra-violet (UV) and near infra-red (NIR) radiation.
For example, the example low-e coated glass shown above is with 3 layers of silver (triple-silver) in its low-e coating. The transmittance in the visible light range (380 nm – 780 nm) is in general greater than 0.7, whereas the transmittance in the near infra-red range (780 nm – 2500 nm) is low (nearly zero between 1000 nm – 2500 nm). This glass is with superior spectral selectivity and is very energy efficient (maximizing daylight utilization while minimizing near infra-red heat transmission).
Soft low-e coatings with more layers of silver are with better spectral selectivity and lower emissivity.
Double-silver and triple-silver low-e coatings are extensively used nowadays. Triple-silver low-e coatings are with better performance than double-silver low-e coatings.
In addition to double-silver and triple-silver low-e coatings, single-silver and quad-silver low-e coatings are also commercially available, but they are not widely used at this moment.
For more detailed information on low-e coating and number of silver layers, please read this article.
Can low-e coating be understood as “heat reflection mirror”?
The “heat reflection mirror” model is suitable for the winter nighttime condition. In winter nighttime, low-e coating reflects the far infra-red radiation from the indoor side back and keeps the warmth indoors.
However, the “heat reflection mirror” model does not explain the optical-filtering effect in the summer daytime condition.
Additionally, the “heat reflection mirror” model may imply that, in order to reflect the heat back, the low-e coating needs to face the warm side (indoor side in the winter condition). In the summer daylight time condition, low-e coating on the indoor side also reflect the “cool” back. Essentially, the low-emssivity effect of low-e coating acts like a radiative heat exchange barrier and it does not need to face the warm or cool side particularly.
Last update: 24/07/2021