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This post was originally published on Draper's blog site.

Solar control solutions for inside the window seem to make sense. But one question we often hear is, “Why would you use exterior or outside solutions?” It’s actually a good question, and the answer is in the way energy from the sun is turned into heat.

Solar control is about managing the sun’s energy. It may seem obvious, but when questioned, most people can’t explain how energy from the sun turns into heat. This is a very important issue to grasp in order to understand why it makes sense to use an exterior shading system rather than an interior one.

The solar spectrum
The first thing to understand is that sunlight is made up of energy with a range of different wavelengths. This solar spectrum can be split into three main sections: ultra violet, visible light, and infrared. All of these have a short wavelength. Even though one part of the spectrum is known as infrared, it carries very little heat.

When sunlight hits a surface, it is absorbed or reflected. The surface might be the ground, vegetation, water, or a person. The absorbed part of the solar energy is emitted as long wave energy. This is what actually produces heat.

Hot cars
A basic example of this process is a car on a bright, sunny winter’s day. It’s cold outside but the vehicle’s interior is very hot, thanks to absorbed solar energy.

Since glass is nearly transparent to short wave energy, solar energy comes through the windshield and is absorbed by the surfaces inside the car—seats, steering wheel, dashboard, gearshift, etc. That absorbed short wave energy is then emitted as long wave energy, i.e. heat. The long wave energy doesn’t pass through glass very easily, so the hot air is trapped inside the car.

To deal with this issue, many people put reflective screens in the windshield on sunny days. This reflects the solar energy back out of the glazing before it gets absorbed and converted into heat.

Hot buildings
As with the example of the car, solar energy that gets into a building will be absorbed by the surfaces it hits and will be turned into heat. An interior shading system will, to a certain extent, act like the silvered screen in the windshield of a car – it will reflect the solar energy back through the glazing to the outside. Unlike a car screen, most shading systems are not completely opaque. A shading system is typically designed to allow some light into the building and to afford views to the exterior. Because of this, not all solar energy will be reflected. Some will be absorbed by the system, and some will pass through it. If the shading system has a light color the amount of energy reflected will be increased and the amount of energy absorbed will be reduced. Dark fabrics will absorb more of the solar energy. To try and address this, some screen fabrics have an aluminized finish applied to the side that faces the glazing to improve the amount of reflectivity.

Any solar energy that is absorbed by the shading system will be then be radiated into the building. Similarly, any solar energy that is transmitted through the shading system and is absorbed by surfaces inside the building will also be radiated as heat. As with the example of the car, this energy is trapped inside the building and not much of it escapes to the exterior, particularly if a low E finish is applied to the glazing.

The energy absorbed by the shading system is radiated into the room. What passes through will be absorbed by surfaces inside the room and convert into long wave energy. Glass used in buildings, particularly with a low E coating is not transparent to long wave radiation. The absorbed heat is therefore trapped inside the building.

Take it outside
An exterior shading system performs similar to an interior one. It reflects some solar energy, absorbs some, and allows some to pass through to the interior of the building. Solar energy absorbed by the system, is radiated to the outside of the building. In this case the lack of transparency of glazing to long wave radiation means that very little of it gets inside the building. As a result, the shading performance of the system increases noticeably when compared with an interior system.

Not all exterior systems provide the same level of solar control. Some are better for heat control, some do better with glare, and others mitigate a range of issues. There are even times when the best solution is provided by a combination of exterior and interior systems.

To get an idea of the various ways Draper can keep long wave solar energy from making buildings too hot, click here.

You can also click here to contact one of our solar control solutions specialists.

If you’re an architect, we offer an AIA course on the subject. “Managing heat and light with exterior shading systems” is worth one learning unit, and can be taken online or as part of a lunch-and-learn session. Details can be found here.

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