What is an energy-efficient window?
For windows, two qualities broadly define energy efficiency: solar heat gain coefficient (SHGC) and thermal transmittance (U-factor). The first of these is defined as “the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation” by ASHRAE 90.1-2013. Essentially, SHGC is a measure of how much of the sun’s heat transmits through the windows into the building interior.
In the past, maximum reduction of SHGC beyond that dictated by code was not considered optimal for buildings in cold climates because solar energy could help heat the building during the winter. However, due to inherent inefficiencies in building enclosures, the industry has largely revised its thinking on this issue, and recommendations now favor a reduction in SHGC across climate zones. There is, of course, always a trade-off—as SHGC is reduced, so is visible light transmission (VLT) or glass transparency.
The other major determinant for energy efficiency in windows, U-factor, is defined by ASHRAE 90.1-2013 as “heat transmission in unit time through unit area of material or construction… induced by a unit temperature difference between the environments on each side.”
A measure of a material or assembly’s propensity to transmit energy, U-factor is the inverse of R-value, which measures ability to resist energy transfer. Window manufacturers’ data should provide whole-assembly U-factor values, including both frame and glass, rather than the center-of-glass U-factor values that tend to make the window seem more efficient than it is.
In terms of the energy code, defining what constitutes an energy-efficient window often demands calculations based on fenestration area and the performance of other building envelope elements. Both IECC and ASHRAE 90.1 provide two compliance paths:
- the Prescriptive Building Envelope Option, for buildings in which vertical fenestration is no more than 30 percent of gross wall area; and
- the Building Envelope Trade-off Option.
The number required for the prescriptive path can be increased to 40 percent when there are daylighting controls (i.e. methods to automatically regulate artificial lighting within daylight zones, defined by ASHRAE 90.1-2013 as “the floor area substantially illuminated by daylight”).
The Prescriptive path assumes windows are less energy-efficient than opaque wall assemblies, and it provides maximum values for U-factor and SHGC. Most current codes also permit adjustments in the SHGC and U-factor for a given set of conditions. For instance, the use of dynamic glazing (discussed later in this article) and projections on the building (such as eaves and cornices), in combination with window orientation, allows for an increase in the maximum allowable SHGC. For certain fenestration categories (i.e. fixed, operable, doors, and skylights), an area-weighted average that estimates the efficiency of a whole building section can be employed to calculate the maximum U-factor.
The Building Envelope Trade-off path is intended to demonstrate buildings with more than 40 percent vertical fenestration can function as efficiently as those with less window area. It offsets thermal transfer across the fenestration with efficiencies in wall and roof assemblies. However, for a window replacement project, the Building Envelope Trade-off route may not be an option, because it can be difficult (or even impossible) to identify efficiencies elsewhere in the building that could compensate for excess window area.