U-factor (Insulating Value)

U-factor (Insulating Value)

For windows, a principle energy concern is their ability to control heat loss. Heat flows from warmer to cooler bodies, thus from the inside face of a window to the outside in winter, reversing direction in summer. Overall heat flow from the warmer to cooler side of a window unit is a complex interaction of all three basic heat transfer mechanisms—conduction, convection, and long-wave radiation (see figure to the right). A window assembly's capacity to resist this heat transfer is referred to as its insulating value, or u-factor.

Conduction occurs directly through glass, and the air cavity within double-glazed IGUs, as well as through a window's spacers and frames. Some frame materials, like wood, have relatively low conduction rates. The higher conduction rates of other materials, like metals, have to be mitigated with discontinuities, or thermal breaks, in the frame to avoid energy loss.

Convection within a window unit occurs in three places: the interior and exterior glazing surfaces, and within the air cavity between glazing layers. On the interior, a cold interior glazing surface chills the adjacent air. This denser cold air then falls, starting a convection current. People often perceive this air flow as a draft caused by leaky windows, instead of recognizing that the remedy correctly lies with a window that provides a warmer glass surface (see figure to the right). On the exterior, the air film against the glazing contributes to the window's insulating value. As wind blows (convection), the effectiveness of this air film is diminished, contributing to a higher heat rate loss. Within the air cavity, temperature-induced convection currents facilitate heat transfer. By adjusting the cavity width, adding more cavities, or choosing a gas fill that insulates better than air, windows can be designed to reduce this effect.

All objects emit invisible thermal radiation, with warmer objects emitting more than colder ones. Through radiant exchange, the objects in the room, and especially the people (who are often the warmest objects), radiate their heat to the colder window. People often feel the chill from this radiant heat loss, especially on the exposed skin of their hands and faces, but they attribute the chill to cool room air rather than to a cold window surface. Similarly, if the glass temperature is higher than skin temperature, which occurs when the sun shines on heat-absorbing glass, heat will be radiated from the glass to the body, potentially producing thermal discomfort.

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