If you’re on the lookout for the best replacement windows or want to equip your existing ones with window film, you may have heard about something called shading coefficient (SC).
In short, the shading coefficient indicates the total amount of heat passing through a glazing system as opposed to the total amount of heat passing through a single-pane clear glass.
Although the shading coefficient is now rarely used in the fenestration industry, you may still have to consider it, especially if you want to install window film, for example. As such, this article explains what the shading coefficient is, why it matters, and how it affects your home’s energy use.
Key Takeaways
- The shading coefficient (SC) measures the effectiveness of window treatments or glazing materials in reducing solar heat gain compared to the solar heat gain of clear glass. Lower SC values indicate better performance and lead to reduced cooling costs.
- Understanding the shading coefficient when choosing windows can help you make an informed decision in certain situations because a good SC can improve the energy efficiency of a house. However, this metric isn’t widely used by window manufacturers.
- The shading coefficient (SC) differs from the Solar Heat Gain Coefficient (SHGC) because the latter is used in regard to the whole window system (including the frame), while the former is only used for the glass.
What Is the Shading Coefficient?
The shading coefficient (SC) assesses how well window treatments or glazing materials can block solar heat. It tells us the total solar heat transmittance of a window system compared to how much heat passes through a standard type of glass, usually a clear glass measuring 1/8 in or 3 mm.
In simple terms, the SC gives us a number that indicates how effective a certain glass type is at keeping heat out compared to standard glass. So, if you plan to install window film, for example, you’ll likely need to calculate the SC to understand how the final glass differs from standard glass with no window film. It’s also worth mentioning that the shading coefficient definitions and calculations do not take into account the frame, mullion, and edge effects.
What Is a Good Shading Coefficient?
Shading coefficients – are higher or lower values better? Lower shading coefficients are definitely better. Let’s see why.
Shading coefficient values go from 0 to 1. Lower values indicate that the glass is better at reducing solar gain. For example, an SC of 0.10 means that the window allows only 10% of the solar heat gain compared to the standard glass. On the other hand, an SC of 0.98 lets in almost almost all the solar heat. As such, a lower SC means less heat comes through a window, which helps keep buildings cool and reduces the need for air conditioning.
This can be quite confusing, and we don’t blame you. Considering the name of this metric – shading coefficient – one would expect that higher numbers are better. This is one of the reasons why the SC isn’t as widely used nowadays.
Why Is Shading Coefficient Important?
Shading coefficients are important, particularly for houses in hot climates or for windows constantly exposed to sunlight. The significance of SC may vary depending on the building’s design and location. In scenarios where direct sunlight is not a major factor, such as windows shaded by overhangs, the shading coefficient becomes less critical.
Choosing glass with a lower SC means less solar heat is transmitted through the window. This equals less money spent on cooling, which improves energy efficiency and contributes to sustainability efforts.
Nonetheless, the SC isn’t the only metric to consider when choosing glazing systems. You should always pay attention to a window’s Solar Heat Gain Coefficient, U-value, Visible Transmittance, and Air Leakage, which are added to NFRC and Energy Star labels.
How to Calculate the Shading Coefficient of a Glazing System?
As mentioned, the shading coefficient (SC) is determined by comparing the solar heat gain of a glazing system to that of a single layer of clear, 1/8-inch-thick glass under the same conditions. This comparison helps assess the performance of the window in controlling heat gain.
To calculate the SC of a glazing system, we need to turn to the Solar Heat Gain Coefficient (SHGC) of the standard clear glass, which is 0.87. You’ll also need the SHGC of the whole window system. As such, here’s the generally accepted shading coefficient formula:
- SHGC = 0.87 x SC
- SC = SHGC / 0.87
For example, here’s how to calculate the SC of the glass that is part of a window system with a SHGC of 0.5:
- SC = 0.5 / 0.87 = 0.57
Shading Coefficient vs. Solar Heat Gain Coefficient (SHGC)
Both the shading coefficient (SC) and the Solar Heat Gain Coefficient (SHGC) are used to measure the amount of solar heat that passes through a window. The difference between them is that shading coefficients take into account only the glass. On the other hand, the solar heat gain coefficients take into account the entire window, including the frame, the sash, and the mullion. The SHGC measures the fraction of incident solar radiation passing through a window. In simpler terms, it measures how well the whole window assembly blocks solar heat. As such, the SHGC is a crucial factor in determining a building’s energy efficiency.
By considering both the glass and the frame, SHGC provides a more accurate representation of a window’s performance in real-world conditions. Therefore, when comparing SC and SHGC, it’s essential to understand that while both metrics aim to measure solar heat gain, SHGC offers a more detailed perspective.
According to the National Fenestration Rating Council, the SHGC values range between 0 and 1. Higher numbers indicate higher solar heat gain. Higher numbers are recommended for houses that need passive solar heating to reduce heating costs. On the other hand, windows with a low SHGC reduce cooling costs. In other words, if you live in Northern Canada, you’ll need windows with a higher SHGC. If you live in a region with a warmer climate, you’ll benefit from windows with a lower SHGC.
Factors Affecting Shading Coefficient
The most important factors influencing the shading coefficient are the glazing materials and colors and window treatment types. For example, darker glass films equal lower shading coefficients. Darker or more reflective glass has a lower SC, as it can block more solar heat.
From a more scientific perspective, the shading coefficient is influenced by a window’s radiation properties, which include:
- transmissivity, which is the ability of the surface of a material to transmit radiant energy;
- absorptivity, which is the ability of the surface of a material to absorb radiant energy;
- emissivity, which is the ability of the surface of a material to emit energy as thermal radiation;
- reflectivity, which is the ability of the surface of a material to reflect radiant energy.
All these properties are, in turn, affected by glass color, tint, and reflective coatings.
The angle at which sunlight hits a window can also affect how much heat passes through the glass. When sunlight hits the glass at a sharper angle, less heat passes through. This can also affect solar transmittance and the SC. This means that the SC also depends on window design and direction.
Shading Coefficient in Different Glazing Systems
Various glazing systems have different shading coefficients. Here are some examples that can help you get a bigger picture in terms of SC:
- Single pane, heat-absorbing glass: SC – 0.58
- Single pane, reflective glass: SC – 0.51
- Double pane, clear glass: SC – 0.74
- Double pane, Low-E coating: the shading coefficient of Low-E glass ranges from 0.38 to 0.64, depending on where the coating is applied
- Double pane, heat-absorbing glass: SC – 0.49
- Double pane, reflective glass: SC – 0.38
- Triple pane, clear glass: SC – 0.68
- Triple pane, heat-absorbing glass: SC – 0.34
- Triple pane, reflective glass surface: SC – 0.34
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Frequently Asked Questions
What is the shading coefficient (SC)?
The shading coefficient (SC) measures the total amount of heat that passes through the glazing system (without taking into account the frame, sash, and other window parts) compared to the amount of heat that passes through a single clear glass.
Is a higher shading coefficient better?
A higher shading coefficient means a glazing system lets more solar heat in. Therefore, you should look for a lower shading coefficient.
How to calculate the shading coefficient?
To calculate the shading coefficient, you need the window’s SHGC. Then, divide the SHGC by 0.87; the result is the SC.
What factors affect the shading coefficient?
The shading coefficient is affected by factors such as the color and reflectivity of the glass, the angle of sunlight, and window treatments like coatings and films that can influence the amount of solar heat gain.
What is the difference between SHGC and shading coefficient?
The shading coefficient only considers the glass portion, while the Solar Heat Gain Coefficient (SHGC) incorporates both the glass and frame in its measurement. SHGC values range from 0 to 1, with lower values indicating better resistance to solar heat gain.
Is the shading coefficient important in choosing windows?
The shading coefficient is important, particularly for houses in hot climates. Windows with a lower SC can improve energy efficiency and reduce cooling costs.