What is Window Shading Coefficient?
The Window Shading Coefficient describes the ability of a window to transmit solar heat. The window shading coefficient of window blinds or shades is directly proportional to the solar heat of the shade it transmits. The shading coefficient for window shades is expressed as a number between 0 and 1. The lower the window shading coefficient, the less solar heat it transmits and the greater shading ability it has. A window covering that reduces heat by 75% has a shading factor of 0.25.
Windows – Energy Holes in Our Homes
While designing the interiors of our homes, windows demand a lot of our attention. They have so much to offer to us in terms of aesthetics, privacy, light management, and insulation. An appropriately dressed window forms the focal point of a room while beautifully blending with the rest of the decor. Yet, when it comes to their energy efficiency, they are referred to as “energy holes”.
The reason for them being labeled thus is the fact that they offer outside heat energy easy access into our homes during the summer, and they cause the loss of heat energy from our homes in winter when we most need it. This has a direct impact on our energy bills, burning a hole into our wallets if not dealt with thoughtfully. Making them energy efficient is the prime goal of every homeowner. In other words, we want to make optimum use of our window blinds and shades, resulting in the least consumption of energy.
Using suitable window treatments has an impact on heating and cooling bills, and contributes to the energy efficiency of a window. Treatments designed for this purpose are referred to as energy efficient window treatments. All window treatments have energy saving properties, but some are inherently more efficient than others.
Energy Efficient Window Coverings
While considering the energy efficiency of window treatments, a number of factors are taken into consideration. Their Shading Coefficient, R-value, E-value, percentage of UV Blockage, Solar Heat Gain Coefficient and a whole lot of other mind-boggling terms help describe their effectiveness in keeping your room insulated and energy efficient.
To evaluate the effectiveness of a special approach to shading or to compare a range of solutions, it is necessary to have a common standard. This brings us back to the Shading Coefficient. As previously mentioned, a Shading Coefficient, in simple terms, is a number between 0 and 1 that tells us how much shade a window along with its treatment is offering to us. If considered more seriously, it is a factor that defines the performance of a window and associated window treatment to control total solar energy by a measurement system based on clear, 3mm thick glass. A Shading Coefficient is a function of the entire window system – including the window treatment.
It is the measure of a window treatment’s ability to reduce solar heat gain by reflecting heat back into the outside. The lesser the number, the better the product is at preventing heat. This means that lesser solar heat will enter our homes when we use it. For example, a window with a Shading Coefficient of 0.40 will prevent 60% of Solar Heat Gain while a Shading Coefficient of 0.3 will block out 70%. A window treatment that reduces heat by 75% – 85% translates into a summer coefficient of .25 to .15.
In hot weather, a lower Shading Coefficient translates into less heat gain, more reflected heat, and lower cooling costs. In colder climates, a window treatment with a higher Shading coefficient allows the sun to better heat the room.
Shading From Solar Heat Gain
Solar radiation is the primary source of heat. When these radiations strike a window, some heat energy is reflected, the rest is either absorbed by the glass pane of the window or transmitted to the interior.
Reflected solar radiation bounces off the surface and does not penetrate the glass. Absorbed solar radiation is soaked up by the glass and eventually is dissipated by convection or conduction to the interior. Transmitted heat energy penetrates the glass and heats the building’s interior.
These three Solar Heat Gain factors determine a Window’s Shading Coefficient.
Difference Between Shading Coefficient & Solar Heat Gain Coefficient
Window Shading Coefficient is the measure of how much heat the glass pane and the fabric together let into a room.
Solar heat gain coefficient is how much heat is making it into the room through all parts of the complete window opening system – glass and frame assembly plus fabric. It is also an account of the heat that is absorbed and either re-radiated into the room or sent by convection away to the outside.
For both cases the lower the number, the more heat that fabric with the window itself blocks heat from entering the room. The (approximate) mathematical relationship between the two is
SOLAR HEAT GAIN COEFFICIENT= 0.87 x SC (shading coefficient)
Typical Values for Shading Coefficient of Window Blinds & Shades
The Shading Coefficient concept can be used to rate various types of window shading techniques, the values being used in various thermal evaluation methods or simply to compare the worth of different window treatments with regard to the reduction of solar gain. Here are some examples:
Window Shading Coefficient System:
Interior mounted, dark color Roman shades fully drawn 0.80
Interior mounted, dark color Venetian Blinds fully drawn 0.75
Interior mounted, medium color Venetian Blinds fully drawn 0.65
Interior mounted, medium color Roller shades fully drawn 0.62
Interior mounted, dark color Drapes fully drawn 0.58
Interior mounted, white Venetian Blinds fully drawn 0.56
Interior mounted, white Roman Shades fully drawn 0.41
Interior mounted, light color Drapes fully drawn 0.40
Exterior mounted Canvas awning 0.25
Exterior mounted Venetian blinds 0.15
Exterior mounted Shutters / Adjustable Louvers 0.13-0.10
(Light colors have a lower Shading Coefficient than dark colors)
The highest rated Shading Coefficient window coverings include Drapery, Cellular Shades, Room Darkening and Blackout shades.
Shading Coefficient of Shades
The United States Department of Electricity (D.O.E) consider window shades the most effective way to save energy with window treatments.
#1. Cellular Shades or Honeycomb shades offer the lowest Shading Coefficient among window coverings and are the best energy efficient shades. A typical Cellular shade has an SC of 0.25 – 0.45. It varies by product. Cellular Shades vary by pleat size, number of cell layers, as well as energy efficiency. Each of these factors controls the SC of the cellular shade.
#2. Solar Shades have an SC between 0.3 AND 0.6. The amount of solar heat blocked depends on the tightness of the screen. Off all window treatments, a correctly mounted roller shade made of the non-permeable material is the simplest, most efficient and cost-effective window treatment.
#3. Roman shades have an SC of 0.3. They are another great way to insulate your windows.
Solar Sheer shades are another good shade to block the sun. They have a typical SC of 0.35. Even when the vanes are raised or tilted like a Horizontal blind, there is still sheer material filtering the light and blocking the sun.
Shading Coefficient of Wood Blinds
Wood blinds are not known for being particularly energy efficient. If used correctly they do have some energy efficiency properties. They can be motorized and set to open and close the tilt of the blind automatically when sunlight hits the window. This will decrease the amount of light coming through the window, increase the shading co-efficiency that reduces the amount of energy passed into the home and keeps the home cooler.
The ideal window covering saves at least 50% of a home’s heating and cooling energy. Researching for exact Shading Coefficient of Window Blinds or Shades will help in selecting the best energy efficient shades.