Solar control window film and glass optical & thermal performances
Solar control window film is a popular and effective window retrofitting technology for improving the optical & thermal performances of existing glasses in the summer. This article aims to explain how solar control window films improve glass optical & thermal performances, as well as the laboratory testing methods.
- Solar control window film and solar heat gain
- Solar control window film and light-to-solar-gain (LSG) ratio
- Solar control window film and U-value
- Position of solar control window film
- Laboratory testing of solar control window films
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Solar control window film and solar heat gain
Solar control window films can be applied to the indoor side or outdoor side of a glass to reduce the solar heat gain through the glass. A comprehensive introduction of window film technology may be found here.
Solar control window films serve as an optical filtering layer, reducing the direct solar energy transmission to the indoor space.
In terms of the solar heat gain control mechanism, there are three types of solar control window films:
- Tinted solar control window film
- Spectrally selective solar control window film
- Spectrally selective & near infra-red (NIR) reflective solar control window film
Tinted solar control window film
Shown below is a comparison of the optical & thermal properties of a clear glass without and with a tinted solar control window film. For the comparison, expand the block below.
- Glass: 4mm clear glass
- Film: tinted solar control window film
| Glass only | Glass with film | |
|---|---|---|
| Visible light transmittance | 0.883 | 0.324 |
| Visible light reflectance, front | 0.081 | 0.055 |
| Visible light reflectance, back | 0.081 | 0.053 |
| Solar energy transmittance | 0.828 | 0.439 |
| Solar energy reflectance, front | 0.075 | 0.057 |
| Solar energy reflectance, back | 0.074 | 0.056 |
| Emissivity, front | 0.846 | 0.846 |
| Emissivity, back | 0.847 | 0.894 |
| Solar heat gain coefficient (SHGC) | 0.856 | 0.599 |
| Shading coefficient (SC) | 0.984 | 0.688 |
| Summer condition U-value [W/(m2K)] | 5.32 | 5.46 |
| Winter condition U-value [W/(m2K)] | 5.89 | 6.03 |
Spectral transmittance and reflectance curves:
In the above example, the solar control window film considerably reduces the solar energy transmission, while just slightly altering the solar energy reflection. It signifies that the solar control window film darkens the clear glass, absorbing more solar radiation.
However, in this example, the solar energy transmission is rather high in the near infra-red range (NIR, 780 nm – 2500 nm). This can be improved with spectrally selective solar control window films.
Spectrally selective solar control window film
Shown below is a comparison of the optical & thermal properties of a clear glass without and with a spectrally selective solar control window film. For the comparison, expand the block below.
- Glass: 4mm clear glass
- Film: spectrally selective solar control window film
| Glass only | Glass with film | |
|---|---|---|
| Visible light transmittance | 0.883 | 0.493 |
| Visible light reflectance, front | 0.081 | 0.065 |
| Visible light reflectance, back | 0.081 | 0.061 |
| Solar energy transmittance | 0.828 | 0.245 |
| Solar energy reflectance, front | 0.075 | 0.054 |
| Solar energy reflectance, back | 0.074 | 0.051 |
| Emissivity, front | 0.846 | 0.846 |
| Emissivity, back | 0.847 | 0.899 |
| Solar heat gain coefficient (SHGC) | 0.856 | 0.472 |
| Shading coefficient (SC) | 0.984 | 0.542 |
| Summer condition U-value [W/(m2K)] | 5.32 | 5.45 |
| Winter condition U-value [W/(m2K)] | 5.89 | 6.01 |
Spectral transmittance and reflectance curves:
In the example, the solar control window film permits high transmission of solar energy in the visible light spectrum (VIS, 380 nm -780 nm) and low transmission of solar energy in the ultra-violet (UV, 300 nm – 380 nm) and near infra-red (NIR, 780 nm – 2500 nm) ranges. It is a type of solar control window film that is spectrally selective. Since a spectrally selective film enables less NIR transmission while maintaining the same level of visible light transmission, it is more energy efficient than the tinted solar control window film in the first example.
However, both tinted and spectrally selective solar control window films are with considerable solar energy absorption. The glass surface temperature in the sun with film is higher than with glass only. The higher the glass surface temperature, the higher the secondary heat gain, which is a component of the solar heat gain. For better performance, the spectrally selective solar control window film can be further enhanced with high reflection in the NIR region.
Spectrally selective & near infra-red (NIR) reflective solar control window film
Shown below is a comparison of the optical & thermal properties of a clear glass without and with a spectrally selective & NIR reflective solar control window film. For the comparison, expand the block below.
- Glass: 4mm clear glass
- Film: spectrally selective & NIR reflective solar control window film
| Glass only | Glass with film | |
|---|---|---|
| Visible light transmittance | 0.883 | 0.690 |
| Visible light reflectance, front | 0.081 | 0.083 |
| Visible light reflectance, back | 0.081 | 0.081 |
| Solar energy transmittance | 0.828 | 0.373 |
| Solar energy reflectance, front | 0.075 | 0.198 |
| Solar energy reflectance, back | 0.074 | 0.121 |
| Emissivity, front | 0.846 | 0.846 |
| Emissivity, back | 0.847 | 0.841 |
| Solar heat gain coefficient (SHGC) | 0.856 | 0.505 |
| Shading coefficient (SC) | 0.984 | 0.580 |
| Summer condition U-value [W/(m2K)] | 5.32 | 5.29 |
| Winter condition U-value [W/(m2K)] | 5.89 | 5.86 |
Spectral transmittance and reflectance curves:
The reflection in the NIR range (780 nm – 2500 nm) is high in the example above. As a result, less solar energy in the NIR spectrum is absorbed by the glass and the secondary solar heat gain is reduced. Because of this, spectrally selective & NIR reflective solar control window film provides optimum solar heat gain control performance.
Solar control window film and light-to-solar-gain (LSG) ratio
Glasses with high visible light transmission and minimal solar heat gain are preferable in general. The two requirements conflict with one another. The light-to-solar-gain (LSG) ratio is then used to characterize the performance of a glass in balancing daylight transmission and solar heat gain.
The LSG ratio is defined as the ratio of a glass’ visible light transmittance to its solar heat gain coefficient (SHGC). Glasses with high LSG ratio are preferred because they can transmit more daylight while admitting the same amount of solar heat (or less solar heat gain with the same daylight transmission level).
Expand the block below to see the LSG ratios calculated from the above-mentioned examples.
It is obvious that the solar control window film that is spectrally selective and NIR reflective achieves the optimum LSG ratio.
Solar control window film and U-value
Solar control window films, as the name suggests, significantly limit solar heat gain through glasses. Solar control window films, on the other hand, do not improve glass U-value.
As seen in the examples (expand the block below for the U-values), solar control window films may even slightly raise the glass U-value, implying that the thermal insulation performance of the glass may be poorer as a result of their use.
The cause is that solar control window films are typically made of polymer materials with high emissivity. A typical window film has an emissivity of roughly 0.9, which is greater than an uncoated glass (nominal emissivity 0.84) or a low-e coated glass (typical emissivity 0.2). Higher emissivity results in a higher radiative heat transfer rate between the glass and its surroundings.
Solar control window films essentially eliminate the low emissivity effect of low-e coating (i.e., the glass U-value increases significantly) for glasses with hard low-e coating on the indoor side surface, though the optical filtering effect is still intact (i.e., the glass SHGC still decreases substantially). It is preferable to use solar control window film on the outdoor side of such low-e coated glasses instead.
Nonetheless, solar control window films do not cover up the soft low-e coating in a double glazing unit (DGU).
Position of solar control window film
In most cases, solar control window films are applied on the indoor side of glasses, but there are also outdoor solar control window films.
Position on uncoated single glazing
For uncoated single glazing glasses, the preferred solar control window film installation position is the indoor side surface.
There is no apparent benefit of installing solar control window film on the outdoor side surface.
Position on hard low-e coated single glazing
When a solar control window film is applied to the indoor side of a single glazing glass with hard low-e coating, the low-e coating is blocked, dramatically raising the glass’ U-value.
For optimum thermal performance, it is preferable to apply outdoor type solar control window films for such glasses.
Position on double glazing units (DGUs)
For DGUs, applying solar control window films on the indoor side does not interfere with the low-e coating. However, applying films on the outdoor side is still favorable in terms of thermal performance.
When a solar control window film is attached to the indoor side of a DGU, the additional heat absorbed by the film is close to the indoor air and can be easily transferred to the indoor space. Due to the larger secondary heat gain, the overall solar heat gain is higher.
When a solar control window film is attached to the outdoor side of a DGU, the absorbed heat is close to the outdoor air and can be easily transferred to the outdoor environment. Because of the deceased secondary heat gain, the overall solar heat gain is lower.
Laboratory testing of solar control window films
Please visit our glass testing page for more information on glass optical & property testing.
Can solar control window films be tested without glass substrate?
It is technically possible to test a solar control window film without a glass substrate, despite certain practical challenges (for example, the material is soft and there is glue on one side).
However, in practice, all manufacturers test solar control window films on clear glass substrates. We tested a solar control window film product both with and without a clear glass substate. Expand the block below to get a comparison of the results.
| Glass with film | Film only | |
|---|---|---|
| Visible light transmittance | 0.324 | 0.339 |
| Visible light reflectance, front | 0.055 | 0.048 |
| Visible light reflectance, back | 0.053 | 0.052 |
| Solar energy transmittance | 0.439 | 0.495 |
| Solar energy reflectance, front | 0.057 | 0.054 |
| Solar energy reflectance, back | 0.056 | 0.056 |
| Emissivity, front | 0.846 | 0.928 |
| Emissivity, back | 0.894 | 0.881 |
| Solar heat gain coefficient (SHGC) | 0.599 | 0.631 |
| Shading coefficient (SC) | 0.688 | 0.725 |
| Summer condition U-value [W/(m2K)] | 5.46 | 5.60 |
| Winter condition U-value [W/(m2K)] | 6.03 | 6.16 |
The data presented above show that the results are considerably different. The “film-only” practice is inconsistent with the prevailing industry norm. We advise our customers to test solar control window films with a glass substrate.
Generic performance of a solar control window film
To evaluate the generic performance of their solar control window film products, most glass manufacturers use 3 – 6 mm clear glasses as the glass substrate.
The same procedures used to test single glazing glasses are used to test glasses with solar control window film.
There currently is no consensus standard on the type and thickness of glass substate used for solar control window film generic performance testing. The generic performance data of a film are influenced by the glass substrate used.
Performance of a solar control window film on a specific glass
There are two options available when it comes to testing the performance of a solar control window film on a particular type of glass.
One approach is to directly test the solar control window film on the glass to be used. The testing procedures are identical to those used for regular glasses.
The alternative technique involves using the LBNL Optics/WINDOW program to calculate glass performance with applied film. NFRC 304 defines the specifics. On this topic, we have a blog entry as well.
Extended optical & thermal properties of solar control window films
Listed below are two commonly used extended optical properties relevant to solar control window films (main article: Complete list of glass optical & thermal properties):
The UV rejection and IR rejection are often seen in the marketing brochures of solar control window films in Singapore.
Additionally, as discussed above, light-to-solar-gain (LSG) ratio is also often used in technical contexts.
In our test report of solar control window film generic performance, the following three additional results are reported by default:
- UV transmittance
- UV rejection
- LSG ratio
It is possible to report the other extended properties on request.
Last update: 23/07/2022
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