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Windows and Thermal Performance
It’s a common misconception that if windows allow more light into a facility, they will automatically permit more heat to enter the space, too. With new spectrally selective glazings or glass (low-e is one example), it’s possible to control the amount of heat without sacrificing light transmittance. It’s even possible now to select windows with different VT ratings within the window itself, says Lowinski. Multiglazed windows can have different properties from top to bottom, and as a result, change the glare pattern — or the angle and intensity at which sunlight hits occupants — for those closer to or farther away from the windows.
When it comes to balancing HVAC loads with how much heat windows transmit, facility executives have a few things to consider. From a cooling performance standpoint, the most important property is solar heat gain coefficient (SHGC). SHGC refers to a window’s ability to block solar heat from transferring through the glass and into a space. If a window has an SHGC of .40, that means it is blocking 60 percent of the solar heat.
SHGC, then, is critical to determining how windows will affect cooling loads. Clearly, the glass used in the atrium at the State of Illinois Center didn’t have the requisite SHGC to match its HVAC system’s capacity.
A category of windows known as dynamic glazing products can actually change visible transmittance and heat gain properties depending on how much sunlight is available. There are two kinds, says Lowinski. One kind only changes from clear to opaque, while another has various shades and is as sensitive as the system’s sensors and controllers (sometimes powered by sunlight themselves) can make it.
For many commercial buildings, cooling is the greatest piece of the electricity spend, says Benney. So if facility executives are considering a window replacement, consulting a mechanical engineer on how window choices may affect loads is an important step. Energy modeling software, such as eQuest or DOE-2, both offered free of charge by the Department of Energy, is a useful tool in determining the relationship between windows and the HVAC system. Building information modeling software, though still slow to catch on in the facility management world, could also be a valuable tool for identifying how HVAC, lighting and window properties all affect each other.
Another window property that influences HVAC energy use is U-factor. The U-factor of a window measures the rate of heat loss through the window. It’s the inverse of R-value, which measures an assembly’s insulating properties. The U-factor of a window is particularly important during winter heating season in cold climates. Therefore, in heating-dominant climates, a low U-factor of around .20 or .30 is desirable.
In recent energy codes — and in what’s expected to be included in the 2010 version of ASHRAE 90.1 — U-factor requirements are becoming much more stringent, says Benney. “You’re probably not going to see much more hollow aluminum anymore,” he says. “You’re going to have to use frames with thermal breaks, which are becoming more and more common.”
Windows and Thermal Performance