Windows cost building owners billions of dollars per year in energy, maintenance, and repair. But you should look at your glazing with a glass half full mentality.
“Windows have the technical potential to be net energy suppliers to virtually all buildings in all climates,” says Stephen Selkowitz, group leader of the windows and envelope materials group in the Building Technology and Urban Systems Derpartment of Lawrence Berkeley National Laboratory. “But that requires matching the right technology with the right design.”
Consider the following factors to make sure your windows are working for you.
Clues for Replacement
A full-scale retrofit of the glazing system is rarely justified by energy savings alone. Window replacement usually follows certain red flags.
“If the glass is fogging up or the seals around it are broken, then it’s time to replace your system. The windows are reaching the end of their useful life if they haven’t already,” says John Carmody, director of University of Minnesota’s Center for Sustainable Building Research. “Once you decide to make the investment, that’s when you should look carefully at the most efficient windows.”
Other meaningful warning signs come from occupant complaints.
“The two major concerns that people report are glare and thermal comfort,” explains Colin Blackford, regional technical advisor with manufacturer Guardian Industries. “Eventually there will be an opportunity to dramatically increase occupant satisfaction and energy performance by upgrading glazing.”
In extreme cases, spikes on your energy bill may reveal the problem, adds Erich Klawuhn, vice president of product management at manufacturer View.
“At an office building in downtown Jackson, MS, they planned a major HVAC retrofit because of the high energy usage,” he says. “But at the last minute, they investigated the facade and upgraded it with high-performance glass. Because that project lowered the load, they discovered the existing HVAC system was adequate.”
Characteristics of Success
Specific solutions will vary based on building location, size, and use, but the main goal is the same.
“The core approach is to specify high-performance glazing that acts as a selective and active filter for energy flows,” Selkowitz says. “Generally, in cold climates, you want to minimize thermal losses and optimize collection of sunlight to offset other losses. In hot climates, the strategy is to minimize solar gain in cooling periods.”
However, picking the right products to achieve these goals is complicated, Carmody adds.
“Every building is a balance. There are tradeoffs, because a lot of its energy use is HVAC and lighting,” he says. “That’s where it gets tricky. If you have glazing with really great thermal resistance, but it also blocks out daylight, then you’re losing out on the lighting side.”
Low-e coatings were a breakthrough in the glazing industry because they’re less visible than past coatings, so they can let in light while keeping out heat, Carmody adds. “They’re really powerful because of that win-win,” he says.
Electrochromic technology is another recent development, Selkowitz says. It works by wiring the window and electrifying it in either a positive or negative direction to change the optical tinting, Klawuhn adds. The tinting can be adjusted automatically based on weather conditions or manually for occupant concerns.
Important ratings to consider include thermal resistance (U-factor), solar heat gain coefficient, and visible light transmittance.
However, determining the right values might require a little homework.
Tools and Tips
The Center for Sustainable Building Research offers a free facade design tool that allows you to input custom circumstances to learn about your options.
“Instead of going by generalized rules of thumb, jump online and put in window area, climate, and orientation,” Carmody says. “Then the tool gives you a rank order of 20 choices from least to most efficient. It’s a quick and dirty way to compare.”
Berkeley Lab also offers a free tool called COMFEN that is slightly more sophisticated, adds Carmody. It allows you to consider different zones side by side based on glazing, shading, and other criteria.
There are additional considerations that might impact your decision.
“If you have a renovation where you’re gutting the whole building, or a situation where you’re also updating other systems, there’s an interesting opportunity because of the impact glazing has on everything else,” Carmody says. “If better performing windows allow you to downsize the chiller or boiler, that’s huge. There might be other consequent benefits that make the windows make sense.”
Chris Curtland firstname.lastname@example.org is assistant editor of