Solar control for the hot elevations
Solar control is about the sun that comes through the glass in the afternoon, and it is a different number from insulation: the U-factor holds heat in or out, the solar heat gain coefficient sets how much sun heat the glass lets through. On a hot or a westerly elevation the solar heat gain coefficient is the line that moves the cooling load, and the specification balances it against daylight — a very low coefficient lowers the heat and dims the room, and the schedule has to choose the point on that trade.
Two numbers, two jobs
The U-factor is the insulation: how fast heat crosses the unit when the air outside is hotter or colder. The solar heat gain coefficient is the sun: the fraction of the solar energy that reaches the inside as heat, from near 0 to near 1. On a shaded north wall, or in a cold climate, the U-factor is the number that matters and a higher coefficient brings free winter gain. On a hot, unshaded west wall the sun delivers more heat than the insulation ever saves, and the coefficient is the number that controls the cooling load.
What the coating does
- A solar-control coating reflects or absorbs part of the solar spectrum before it becomes heat inside; the selective coatings hold the heat rejection while keeping much of the visible light.
- Tinted glass cuts the sun by cutting the light: it lowers the coefficient but dims the room and reads as a colour from outside.
- Reflective and body-tinted builds cut more heat, at more daylight and a mirror look.
- The IGU combination of coating, tint and cavity sets the final coefficient, so it is configured per elevation, not per project.
Why it is per elevation
The sun hits different faces at different times of day and at different angles, so one glass build does not serve the whole house. The west elevation carries the low-coefficient build, because the afternoon sun is the load; a shaded or south face can carry a higher-coefficient build and keep the daylight. The drawing office marks the elevation on the schedule and configures each face, which is cheaper than over-specifying the whole house to the worst face.
What to put on the schedule
- The elevation and its orientation, because the sun load is set by the face.
- The target solar heat gain coefficient per elevation, and the U-factor the energy code sets.
- The visible transmittance the room needs, so the daylight is chosen, not left over.
- The configured build per face — coating, tint, cavity — because the coefficient follows the assembly.
Questions
It reduces daylight with the heat, so the balance is the specification. A selective coating holds more light at the same coefficient than a dark tint, which is why it is the common hot-elevation answer.