A2 Windows: Light, Heat, Noise and Ventilation

Topic

A window consolidates into a single building element the four physical behaviors this catalog has addressed separately—thermal, acoustic, luminous, and ventilatory—meaning its design requires balancing objectives that sometimes compete within the same pane of glass.

From a thermal perspective, a window loses heat just as the heat transfer module described for any building envelope component; therefore, it is desirable to minimize heat transmission—a goal achieved far more effectively by double or triple glazing with special coatings than by single glazing. A second, independent thermal factor is the amount of solar radiation the window allows into the interior: a low solar heat gain coefficient is preferable when the aim is to prevent summer overheating, whereas a high coefficient is desirable when the goal is to harness solar heat in winter—meaning a window well-designed for one climate might be the worst choice for another.

Acoustically, window insulation generally follows the "mass law" established in the acoustics module, although laminated glass with damping interlayers performs better than symmetrical double glazing of the same total thickness. Regarding lighting, a larger window with clearer glass admits more natural light into the space—relative to the room's size and the light absorption of its interior surfaces—linking back to the concept of illuminance described in the natural light module.

Finally, when the window is opened, the airflow providing natural ventilation depends on the size of the opening and the wind pressure difference between opposite sides of the building; this makes cross-ventilation between facing windows by far the most effective method of natural ventilation. That is why this module—which opens by bringing together four behaviors in a single element—concludes by showing that no single window optimizes all four simultaneously: designing a window means deciding, for each façade and climate, which of those four behaviors to compromise slightly in order to gain in the others.

ID:4023