Window, Lighting and HVAC System Interactions
Part 1: Value Engineering Windows? Here's One Mistake to Avoid
Part 2: Windows and Thermal Performance
Part 3: Operable Windows and IAQ
Part 4: Showcase: Windows
Windows and Thermal Performance
By Greg Zimmerman, Executive Editor - August 2009
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.”
Comments
linquist wrote re: Windows and Thermal Performance
on 2/18/2010 6:04:43 PM
Michael,
You can have your windows/glazing measured for thermal performance by a testing lab but it may cost more than it's worth. Thermal imaging can also show energy loss but can't easily quantify it. Single glazed 1/4" glass in a window will achieve about an R-1 and commercial aluminum curtain walls or windows can achieve an R-3 to R-5 but are expensive. The highest performing window on the market is a fiberglass window with high performance glazing involving window films and it achieves about an R-11. Unfortunately glass is not very efficient even with the low-E coatings available. High performance window films can achieve impressive performance ratings that dramatically outperform the highest performing glass available on the market. In addition, your building can now be modeled using DOE software with your existing glass with and without window films to show not only a KW reduction but also an ROI, which would be about 1.2 years with the highest performing films. Unfortunately if you buy by name brand only, you can easily get an underperforming film that can make your solar gain problem worse than before. I would be glad to discuss this further and I can be reached at phil@clear-wall.com or by calling 619-993-7595. Best of luck in your research.
MichaelP wrote re: Windows and Thermal Performance
on 2/11/2010 6:29:59 PM
I would like to know how to determine the R Value of an existing window, in order to perform a cost analysis for either replacement or looking into Film application, such as the 3M clear tint. I cannot recall the exact name of the product right now, I have a building that was built in 1959, single pane glass windows, according to my glass repair vendor its not even tempered. I could use some guideance.
thank you
melvinhjacobs wrote re: Windows and Thermal Performance
on 2/9/2010 11:20:19 AM
Are you familiar with the Traco window?? If so, how does it compare with those that you list under the "showcase" catagory???
