As discussed in the article solar control window film and glass optical & thermal performances, tinted solar control window films are often used. Such window films darken the glasses and it results in lower solar heat gain (i.e. smaller SHGC & SC; SHGC: solar heat gain coefficient; SC: shading coefficient).
Some tinted solar control window films are with very low visible light transmittance (e.g. less than 5%). The glass looks almost opaque. However, the SHGC and SC of such glasses are not close to 0 (indeed they are still quite high). This post aims to explain why the SHGC & SC of opaque glasses are still very high.
Two examples: opaque black glass and opaque white glass
Listed in the table below are the optical & thermal properties of two opaque glasses:
- Opaque black glass: a perfect black glass with 0% transmittance and 0% reflectance
- Opaque white glass: a perfect white glass with 0% transmittance and 100% reflectance
Bear in mind the relationship: transmittance + reflectance + absorptance = 100%
0% transmittance means that both glasses are opaque.
0% reflectance means the glass absorbs 100% incident radiation and reflects 0% back. The glass looks black.
100% reflectance means the glass absorbs 0% incident radiation and reflects 100% back. The glass looks white.
| Opaque black glass | Opaque white glass | |
|---|---|---|
| Visible light transmittance | 0% | 0% |
| Visible light reflectance, front | 0% | 100% |
| Solar energy transmittance | 0% | 0% |
| Solar energy reflectance, front | 0% | 100% |
| Solar energy absorptance, front | 100% | 0% |
| Solar heat gain coefficient (SHGC) | 0.323 | 0.000 |
| Shading coefficient (SC) | 0.371 | 0.000 |
As presented above, the SHGC & SC of the opaque black glass are still very high, whereas the SHGC & SC of the opaque white glass are 0.
The reason is that the opaque black glass absorbs 100% of solar radiation. This portion of solar heat can still be transferred to the indoor space and it is counted as part of the SHGC & SC.
In practice, most tinted solar control window films are in dark colors (close to the opaque black glass). Therefore, their SHGC & SC are still high, despite of the small transmittance.
The theory: secondary solar heat gain
As shown in the sketch below, there are two components in the glass solar heat gain:
- Primary solar heat gain
- Secondary solar heat gain
For more details, please refer to our glass optical & thermal property testing page.
For the opaque black glass, its primary solar heat gain is 0, but the secondary solar heat gain is still high. Actually, the secondary solar heat gain of the opaque black glass is the highest, as its solar energy absorptance is 100%.
For the opaque whie glass, both its primary solar heat gain and secondary solar heat gain are 0. It is possible to archieve zero solar heat gain with the opaque white glass.
