A9 Mobile Blinds and Solar Protections

Topic

An external blind or solar shading device is far more effective at reducing solar heat gain through a window than an equivalent system installed on the interior side of the glass. The reason is purely positional: an external shade intercepts solar radiation before it passes through the glass, whereas an internal shade allows the radiation to enter first—contributing to the solar gain described in the window and passive solar design modules—and only acts later, once a significant portion of that energy is already trapped inside the room.

When the blind's slats are exposed to radiation, they heat up based on the amount of energy their surface absorbs: light-colored slats reflect more radiation and remain cooler, while dark ones absorb more and eventually radiate some of that heat into the interior. If there is a ventilated air gap between the blind and the glass, the air heated by the slats tends to rise and escape through the top of the cavity, carrying away much of the accumulated heat before it can be transmitted indoors—the same "chimney effect" found in other systems in this catalog.

The annual energy savings provided by a blind are estimated by comparing the solar gain that would occur without shading against the gain that actually occurs with the system installed, then translating that difference into avoided HVAC energy consumption.

For this reason, while this module begins by comparing external and internal shading, it concludes by demonstrating that the point at which solar radiation is intercepted—whether before or after passing through the glass—is just as important as any material property of the blind itself.

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