Engineered for Performance
A few years ago, I coined the term Engineered Shading Solutions. This is defined as "Sustainable systems for interior and exterior window coverings, which engage the latest technologies to control heat, glare, and light”.
While a great deal of the technologies are concerned with the hardware and control systems, the third leg of these engineered solutions are the fabrics. For it is the fabrics that actually control the heat, glare and light.
Engineered for Performance
Solar shade fabrics are a good place to start with the challenge of controlling solar heat gain into a room. In winter, we want the shades raised to allow the solar radiation to heat up the cold spaces. In warmer months, the solar heat gain needs to be controlled.
Solar shade fabric fights solar heat gain on several fronts: openness of weave, color of the fabric, and innovative reflective material. Let’s start with openness. Solar shade fabrics are manufactured with a variety of openness, such as 3, 5, and 10 percent open. A 5 percent openness means that 95 percent of the cloth is closed. As you can imagine, the tighter the weave, the more solar heat gain protection the fabric provides.
In choosing openness, the decision is influenced by the orientation of the windows. Northern-facing windows need the least protection (10 percent openness as suitable). For exposures with direct sunlight, 3 or 5 percent is required to effectively reduce solar heat gain.
Beyond Openness
The best way to measure performance is by measuring the effects of solar energy upon the fabric. Penetration of the effects of the sun is divided into 3 parts: absorption (AS), reflection (RS) and transference (TS). AS is how much solar energy is absorbed by the fabric. RS refers to the amount of energy reflected by the fabric. TS measure how much solar energy is transmitted through the fabric. The total of all 3 equals 100% of the solar energy at that window.
Visual Transmittance
Selecting an openness alone based on orientation may lead to disappointment. The color of the fabric plays a role in the amount of light entering a room. This is called visual transmittance. To compare, take the same fabric with the same openness (one in white and one in black). The white color brightens the room more then the black because the darker colors absorb more of the light. Therefore, choosing a fabric based on openness alone may not provide the desired solution.
The color can significantly affect the performance of a fabric in reducing solar heat gain. As we all know, lighter colors reflect light and darker colors absorb light, which converts into solar heat gain. A white color 5 percent open fabric allows 49 percent solar heat gain with a one-inch clear glass window. Comparing this with a black fabric shows 56 percent solar heat gain. Clearly, the lighter the color the fabric, the better solar heat gain reduction.
Glare Control
Glare is caused by a significant ratio of luminance between the visual task and the glare source. A solar shade will reduce the disparity in light levels as in between a computer screen and a window.
To understand the cause of glare, let’s take a look at the 1-3-10 Luminance Ratio Rule, which states that the ratio of light levels between the visual task and an adjacent surface needs to be less then 1 to 3, and that the light level between a visual task and a non-adjacent surface should fall within a 1-to-10 ratio.
In the photo below, the room without an interior shading solution, the light levels fall way beyond the 1-3-10 Luminance Ratio Rule, and the result is glare. However, with glare-reducing fabrics, the levels are corrected, which reduces the glare by as much as 95 percent.
Let’s compare the same fabric in the same colors as our previous example on solar heat gain control. A white 5 percent open fabric will reduce glare by 57 percent, and a black fabric reduces glare by 90 percent. Clearly, the darker color has a significant advantage in reducing glare over light colors because the darker color absorbs the light.
Transparency
One of the benefits of solar shade fabrics is the transparency or the ability to see through the fabric itself. Of course, this transparency is regulated by the openness, but it is also determined by the color. In the picture below, the top row is white fabric in a series of openness of 1, 3, 5, and 10 percent. The bottom row is the same arrangement of openness. Clearly the black fabric provides better transparency than the white color.
Transparency comparison of white versus black fabric
The ability to see through the fabric is an advantage of solar shade fabrics. It is important to remember that there is a tradeoff between seeing through the fabric with a larger openness in exchange for allowing too much solar heat gain.
Exterior VS Interior Fabrics
Solar shade fabrics have both similar and quite dissimilar performance qualities depending on whether it’s an interior or an exterior installation. Let’s use the chart below to examine the differences.
The best way to make any type of analysis is to compare and contrast the extremes. In this case the extremes would be color: ebony verses white. The ebony color allows 4% of the light to be transmitted (Ts), 4% of the light to be reflected (Rs) and with a quick calculation of deducting these amounts from 100% of solar energy (100%- 4% - 4%) leaves 92% of the light that is absorbed by the dark color fabric. The white color absorbs only 29% after the same calculations. The explanations for these results have already been discussed above.
Now let’s compare these two fabrics installed as either an interior VS an exterior shading solution. Looking under the “gtot” internal column for Glazing C (non-reflective double glass) we see that the ebony color allows 50% of the solar heat gain to enter through the shade cloth. However, when it’s installed as an exterior shade that number sinks to only 8%. This is due to the fact that the interior shade absorbs the light and radiates its heat into the room, while the exterior shade radiates the same heat into the atmosphere. The white color also shows significant improvement dropping from 37% to 14%, but not nearly as good as the ebony color.
Metalized Fabrics
An innovative, highly effective solution that reduces solar heat gain and glare regardless of interior color is metalized fabric also known as Solar Reflective Coating (SRC). The technique is to apply a highly reflective, ultra-fine layer of aluminum onto the screen fabric to dramatically improve the solar energy performance. A light color reduces glare by 82 percent and solar heat gain by 45 percent. The dark color reduces glare by 95 percent and solar heat gain by 42 percent.
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