Brooklynite is considering the purchase of a home in New York City that comes with a second story full of single-pane windows. If the deal goes through, one of his first projects will be to replace the windows with something more energy efficient.
“I’ve read about low-e glazing and how it reflects outdoor infrared while keeping heat inside,” Brooklynite writes in a recent Q&A post. “But what about in the winter, when I want the sunlight coming in to warm the house?”
Even in summer, the amount of light falling on the windows won’t change significantly. Exterior window treatments would not be permitted because the house is covered by historical landmark rules. Finally, New York City is a noisy place, so windows that provide some sound attenuation would be helpful.
“I’d love a flexible window solution that allows me to take advantage of the sun when I want it, and banishes it when I don’t,” Brooklynite says. “Does such a thing exist? Or should I get over it and just be satisfied with windows that effectively keep the boiler-produced heat inside the winter?”
That’s our starting point for this Q&A Spotlight.
Consider self-tinting glass
Eric Whetzel, who writes a blog about building a high-performance house, suggests Brooklynite consider a type of glass called Suntuitive, whose opacity changes depending on light conditions.
Whetzel explains that the exterior surface temperature of the glass determines when its tint changes. On a cold, cloudy day, the darkness of the glass doesn’t change so the glass maximizes the daylight potential of the windows. On a cold, sunny day the glass may or may not tint up, he adds, but if it does it won’t completely block sunlight.
“It’s more like sunglasses for your house,” Whetzel writes. “It tints when you need it but it stays clear when the sun isn’t an issue.”
Whetzel chose the glass for his home because he and his wife wanted to minimize window treatments as much as possible. Installing Suntuitive in west-facing windows allowed them to do that, giving them an unrestricted view of their backyard in the dead of winter or on the hottest afternoons of summer. You can read Eric’s post about choosing windows here.
What about shades?
Tom May says, yes, there is a solution to Brooklynite’s predicament. “They go by names such as blinds, shades, or curtains,” he writes.
Jamie B. agrees: “Motorized shades on an annual sun schedule, I personally think is an important part of an energy plan,” he says.
The problem with shades, he adds, is from a design perspective. “If you or your occupants are in the house most of the time, i.e. work from home, etc., it really blocks out the sunlight that [gives] a house [a] fun and open feeling. There are semi-blocking shades but they’re still not great.”
Jamie B. adds that he’s waiting for electronically shaded glass to become mainstream. “I think that’s our future of fenestration glazing, where we can easily set schedules and [opacity], not rely on loud mechanical parts, get rid altogether of the interior design of curtains and blinds and really customize a plan that suits privacy, feel of the space, and solar gain/energy efficiency.”
You’re missing the point
Peter Engle thinks the conversation has taken a wrong turn.
“Everyone is missing the point of how low-e windows function,” Engle writes. “Low-e coatings are ‘selective’ surfaces, as the properties change with the wavelength of the light.”
According to Engle, infrared light is long wavelength and is reflected from either side of the window. Sunlight has a shorter wavelength, so it passes through the window.
“So, when it is hot outside, the glass reflects the heat back to the exterior. When it is hot (warm) inside, the glass reflects the heat back to the interior. Sunlight always passes through and warms the interior when it is absorbed by the interior surfaces. Those surfaces then warm up and re-emit the energy as heat, which is kept inside by the glass.”
Close but not exactly, replies Zephyr7.
Sunlight includes infrared light, Zephyr7 says. All low-e coatings act as “wavelength selective filters” that allow only a relatively narrow range of wavelengths to pass. Light that is not passed through is reflected.
“In the case of a bright, sunny day, a big chunk (but not quite all, which is why aggressive low-e coatings look tinted) of the visible spectrum is passed through to the indoors, but wavelengths outside of the pass range are reflected back out,” Zephyr7 says. “Much of the ‘feels warm in the sun’ energy is in the IR range, probably some UV, and that is what is reflected and the primary contributor to the ‘solar gain’ SHGC (Solar Heat Gain Coefficient).”
Visible light can still warm things up, Zephyr7 adds, but if 30% of the incoming energy has been blocked there will be 30% less heat coming through the window, regardless of which specific wavelength is blocked.
“By reflecting wavelengths we don’t need for vision, it’s possible to block energy without impacting the visible spectrum,” Zephyr7 says. “The visual tint of the more aggressive Low-E coatings is because the filters aren’t entirely perfect.”
Choose a hard coat low-e
Dana Dorsett says, most of the energy in the solar spectrum is in the visible light and near infrared while heat radiating from the landscape or from objects inside the house is in the deep infrared range.
“Hard coat low-e coatings reflect primarily the deep infrared and almost nothing in the visible spectrum,” Dorsett says. “Some of the softer low-e coatings are broader spectrum than that, and most low-e coatings that reflect solar heat well also reduce the visible light transmittance.”
Even so, any low-e coating will lower direct solar gains somewhat while improving heat retention when the indoors is warmer than the outdoors, he adds. In New York City, the idea would be to go with a higher gain/lower heat rejection hard coat, not a soft low-e coating.
Brooklynite also could consider reglazing the windows with hard coat low-e glass, with the coating facing the indoors, Dorsett suggests. Adding interior storm windows could bring the overall performance of the windows up to nearly the code minimum. To minimize wintertime condensation between the two panes, put the low-e glass on the original sash, not the interior storm.
If Brooklynite wants to spent more money, Dorsett recommends a Connecticut company that will rework the sash to accommodate thin, double-pane glass.
Engle quibbles with some of the details, but he adds this:
“The point is that low-e windows let most of the visible light in, and they keep most of the heat out from whichever side is warmer. They do also stop the part of sunlight that is in the long range IR (heat) spectrum, but the remainder is available to warm the inside of the house.”
Look for very low SHGC
Greg Smith thinks it would be helpful to reread an article posted by Martin Holladay several years ago in which he reassessed passive solar design principles. In it, Holladay quotes building scientist Joseph Lstiburek, who had this to say:
“Don’t bother with the passive solar. Your house will overheat in the winter. Yes, you heard that right. Even in Chicago. … You should go with very, very low SHGCs, around 0.2, in your glazing. If this sounds familiar to those of you who are as old as me, it should.
“We were here in the late 1970s when ‘mass and glass’ took on ‘superinsulated.’ Superinsulated won,” Lstiburek continued. “And superinsulated won with lousy windows compared to what we have today. What are you folks thinking? Today’s ‘ultra-efficient’ crushes the old ‘superinsulated,’ and you want to collect solar energy? Leave that to the PV.”
Whetzel hesitates to tangle with the widely respected building scientist, but all he can do is recall his own experience. Windows on the south side of Whetzel’s Passive House have a SHGC of 0.5. With the thermostat set at 69°F, the temperature in the kitchen and family room on the south side of the house may rise to 73°F, but that just makes the space feel cozy.
“In effect,” Whetzel says, “I think it’s possible to balance high solar heat gain in the winter while avoiding overheating as long as the total amount of glass and its placement [are] carefully considered. One of our favorite outcomes regarding the building of our home is this passive solar effect in winter, along with little sun getting in during the summer. It does improve your mood, or your quality of light if you like.”
Our expert’s opinion
Here’s what GBA Technical Director Peter Yost makes of the conversation:
Getting all the performance attributes you want in your glazing will be tough. Sometimes I think we ask too much of insulated glass units, especially in terms of solar shading. By the time short-wave infrared from the sun hits your glass, in my mind you have already lost the battle with solar heat gain.
The problem is that the solution—operable external shades—pose their own challenges. Some folks don’t like the way they look, they are not cheap, and how they are activated—sensors, remote, by hand—all figure into the pros and cons for each of us.
Having said that, here are great resources to make decisions about glazing and window coverings:
- Dynamic glazing—thermo, photo, electro, even gas: Windows for High Performance Commercial Buildings.
- A discussion of full spectrum wavelengths: Window Technologies Properties Primer.
- For more on the wavelength profile of different low-e coatings: Window Technologies: Low-e Coatings.
- A discussion of color rendering (unequal color band transmission of different low-e coating): Singing the blues in the key of low-e.
- Good guidance on residential window selection: Efficient Windows Collaborative Selection Tool.
- Good guidance on residential window coverings selection: Efficient Window Coverings Collaborative. This last website has been taken down, at least for now, but efforts are underway to revive it soon.
One caution: Be careful when you combine high-performance interior window coverings with high-performance glazing. In my own home, on the second story facing west, we installed black-out double-cell shades with sidetracks and low-e double glazed windows (sputter-coat high solar heat gain low-e for Vermont, Climate Zone 6). On a sunny summer day in the afternoon if the shade is fully deployed, I have measured temperatures in the space between the glass and shade of as much as 170°F! The seals in the insulated glass units are only tested according to ASTM standards for around 140°F. As Joe Lstiburek is wont to say: no good thermodynamic deed goes unpunished.
–Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.