Components of heat transfer through a window.

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.

Determining Insulating Value
The U-factor (also referred to as U-value) is the standard way to quantify overall heat flow. For windows, it expresses the total heat transfer coefficient of the system (in Btu/hr-sf-°F), and includes conductive, convective, and radiative heat transfer. It represents the heat flow per hour (in Btus per hour or watts) through each square foot of window for a 1 degree Fahrenheit temperature difference between the indoor and outdoor air temperature. The insulating value or R-value (resistance to heat transfer) is the reciprocal of the total U-factor (R=1/U). The higher the R-value of a material, the higher the insulating value; the smaller the U-factor, the lower the rate of heat flow.

Given that the thermal properties and the various materials within a window unit, the U-factor is commonly expressed in two ways:

  • The U-factor of the total window assembly combines the insulating value of the glazing proper, the edge effects in the IGU, and the window frame and sash.
  • The center-of-glass U-factor assumes that heat flows perpendicular to the window plane, without addressing the impact of the frame edge effects and material.

The U-factor of the glazing portion of the window unit is affected primarily by the total number of glazing layers (panes), their dimension, the type of gas within their cavity, and the characteristic of coatings on the various glazing surfaces. As windows are complex three-dimensional assemblies, in which materials and cross sections change in a relatively short distance, it is limiting, however, to simply consider glazing. For example, metal spacers at the edge of an IGU have a much higher heat flow than the center of the insulating glass, which causes increased heat loss along the outer edge of the glass.

Overall U-factor
The relative impact of these "edge effects" becomes more important as the insulating value of the entire assembly increases, and in small units where the ratio of edge to center-of-glass area is high. Since the U-factors vary for the glass, edge-of-glass zone, and frame, it can be misleading to compare the U-factors of windows from different manufacturers if they are not carefully and consistently described. Calculation methods developed by the National Fenestration Rating Council (NFRC) address this concern.

In addition to the thermal properties of window assembly materials, weather conditions, such as interior/exterior temperature differential and wind speed, also impact U-factor. Window manufacturers typically list a winter U-factor for determined under relatively harsh conditions: 15 mph wind, 70 degrees Fahrenheit indoors, 0 degrees Fahrenheit outdoors. A specific set of assumptions and procedures must be followed to calculate the overall U-factor of a window unit using the NFRC method. For instance, the NFRC values are for a standard window size-the actual U-factor of a specific unit varies with size. Originally developed for manufactured window units, new methods are available to determine the U-factor of site-built assemblies.

The U-factor of a window unit is rated based on a vertical position. A change in mounting angle affects a window's U-factor. The same unit installed on a sloped roof at 20° from horizontal would have a U-factor 10–20% higher than in the vertical position (under winter conditions).


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