Solar heat gain (SHGC & SC) of opaque glasses

Related services Glass optical & thermal properties

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 glassOpaque white glass
Visible light transmittance0%0%
Visible light reflectance, front0%100%
Solar energy transmittance0%0%
Solar energy reflectance, front0%100%
Solar energy absorptance, front100%0%
Solar heat gain coefficient (SHGC)0.3230.000
Shading coefficient (SC)0.3710.000

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.

Online glass U-value, SHGC & shading coefficient calculator: V2.1.0

Related services Glass optical & thermal properties

We’ve just upgraded our online glass U-value, SHGC & shading coefficient calculator to V2.1.0, with the feature of argon gas concentration specification added.

It is now possible to vary the argon gas concentration to study its impact on glass U-value. The default argon concentration is 95%. Please click the screenshot below to access this online calculator and also read this article on the relationship between gas fill and double glazing unit (DGU) thermal performance.

Online glass U-value, SHGC & shading coefficient calculator: V2.0.0

Related services Glass optical & thermal properties

We are pleased to introduce our upgraded online glass U-value, SHGC & shading coefficient calculator. The current version is V2.0.0. Click the screenshot below to access this online calculator.

Online glass U-value, SHGC & shading coefficient calculator

Refer to this early post for the features of the V1.1.0 version.

New features added in V2.0.0

  • SHGC and shading coeffcient calculations
    • Note: limited to single glazing systems; this is due to the restriction of the mathematical model in ISO 9050/EN 410
  • More compact user interface
  • Tooltips on glass configuration schematics

Feedback and comments

We will improve this calculator regularly. If you have feebback and comments, please let us know.