High-Solar-Gain Glazing

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High-Solar-Gain Glazing

Cold-climate glazing is surprisingly hard to find

Posted on Jul 31 2009 by Martin Holladay, GBA Advisor

Homeowners can now receive a federal tax credit for 30% of the cost of new energy-efficient windows. The credit was authorized by the American Recovery and Reinvestment Act (ARRA) signed by President Obama in February.

There’s just one problem with the new tax credit: the specifications for eligible windows were crafted by politicians, not window experts. The ARRA stipulates that eligible windows must have a maximum U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.30 and a maximum solar heat-gain coefficient (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) of 0.30 — requirements that have been dubbed “the 30-30 provision.”

Only low-solar-gain windows are eligible

There’s nothing wrong with the U-factor specification — except, perhaps, that it isn’t particularly stringent. The problematic provision is the SHGC spec.

By setting a maximum SHGC of 0.30, the ARRA actually excludes the best windows for cold climates. Cold-climate homes need windows with a SHGC in the range of 0.39 to 0.65; so if they comply with the tax-credit provisions, they’ll end up with windows that contribute to higher-than-necessary energy bills.

South-facing windows need a high SHGC

In Florida, solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. from windows increases a home’s air conditioning load, so low-solar-gain windows are usually the best choice. Even in hot climates, however, high-solar-gain windows usually save energy during the winter.


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Although some high-solar-gain windows — especially those facing east or west — can cause summer overheating, south-facing windows rarely cause overheating problems, especially if the windows are protected by a well designed roof overhang.

Since the average American family spends far more on space heating than on air conditioning, installing high-solar-gain windows on south walls makes sense for much of the country. Yet most U.S. window manufacturers have all but abandoned the market for high-solar-gain windows.

Low-solar-gain glazing is now the norm

During the 1980s, glazing manufacturers perfected spectrally selective coatings that made it possible to produce low-solar-gain insulated glazing. During the 1990s, as builders in hot climates learned how these coatings reduced cooling loads, low-solar-gain glazing took an increasing share of the U.S. market.

Most builders prefer to order just one type of glazing. Window manufacturers share the same interest, since they prefer to promote a limited number of glazing options. As a result, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. insulated glazing with a low SHGC is fast becoming the industry norm, from the Canadian border to the Gulf of Mexico.

“Everyone is marketing windows for air-conditioning climates,” explained Andy Shapiro, an energy consultant in Montpelier, Vermont. The problem is exacerbated by the fact that the Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. program doesn’t recognize the benefits of high-solar-gain glazing. “Low-solar-gain windows will meet Energy Star guidelines all over the country, so that’s the glazing that all the window manufacturers have gone to,” Shapiro told me.

Industry laziness

According to Christopher Barry, Director of Technical Services at Pilkington Glass, the ascendancy of low-solar-gain glazing is partly due to rapid population growth in the U.S. Sunbelt. “The bulk of recent residential construction has been in the southern states, with low-solar-gain low-e glass,” said Barry. “Laziness on the part of designers and window manufacturers has ledLight-emitting diode. Illumination technology that produces light by running electrical current through a semiconductor diode. LED lamps are much longer lasting and much more energy efficient than incandescent lamps; unlike fluorescent lamps, LED lamps do not contain mercury and can be readily dimmed. to forcing that one-size-fits-all glazing option on the whole market — north, central, and south — and sadly this is even beginning to happen in Canada.”

Soft-coat low-e glass — which tends to have a lower SHGC than hard-coat low-e — now accounts for most U.S. glazing. “They’re just selling soft-coat low-e,” says Ross DePaola, an engineer at Westlab, a Monona, Wisconsin, laboratory that performs NFRC testing for window manufacturers. “That hurts us in the winter time here in Wisconsin.”

Industry ignorance

Three years ago, when researching the availability of high-solar-gain glazing for an article in Energy Design Update, I quickly discovered that most U.S. window distributors don’t understand the advantages of high-solar-gain glazing.

Paul Kriescher, an energy consultant in Denver, agreed. “Window manufacturers don’t understand glazing,” Kriescher told me. “When you talk to the manufacturers’ sales representatives, they have no idea why you would want a high-solar-gain window on the south side of a house. It can be very frustrating. Sometimes they’ll say, ‘Well, we’ll have to research that.’ Then I have to tell them where they can get the glazing. It seems that the sales representatives have been given the mantra, ‘Go as low as possible with the solar heat-gain coefficient. Keep it simple.’ Windows represent one of the most profitable areas of building component manufacturing, but window manufacturers are not spending any time to educate their sales people on this issue. It’s a very big problem.”

More than one kind of low-e glazing

One of the few U.S. window representatives able to suggest high-solar-heat gain options was Jim Krahn, a technical expert at Marvin Windows. “We would recommend one of two products: LOF Energy Advantage from Pilkington, or Cardinal LoE-178,” said Krahn. (LoE-178 has subsequently been replaced by a new product, LoE-179.) He admitted, however, that Marvin doesn’t promote high-solar-gain glazing. “We offer it, but we don’t put many dollars behind promoting it,” admits Marvin Windows representative Jim Krahn. “We don’t get a request for high-solar-gain glazing very often.”

Jim Larsen, the director of technology marketing at Cardinal Glass, told me that LoE-178 has been around for years. “We introduced LoE-178 in 1982,” said Larsen. “Most of the window manufacturers offer it in their catalogs, but few people tend to stock it.” Cardinal’s LoE-179 #3 has a SHGC of 0.63; when installed in a typical window, it can result in a whole-window SHGC of 0.46.

High-solar-gain windows are so advantageous in northern climates that they make sense even for builders who can’t tell one orientation from another. “In Canada, even if windows are evenly distributed between the north and south, it’s still worth putting in high-solar-gain windows on all sides,” notes Morgan Hanam, manager for window services at Enermodal Engineering. “Our modeling using RESFEN proves it out, even considering the air-conditioning loads.” (RESFEN is a useful computer software program that evaluates the energy performance of residential windows to help designers optimize window specifications; it can be downloaded from the Web for free.)

Low-e doesn’t have to mean low SHGC

Stephen Thwaites, the technical director of Thermotech Fiberglass FenestrationTechnically, any transparent or translucent material plus any sash, frame, mullion, or divider attached to it, including windows, skylights, glass doors, and curtain walls., calculated that when Cardinal LoE2-170 glazing (low-solar-gain double glazing) is installed in a Thermotech frame, the whole-window SHGC is 0.28. The same window glazed with Pilkington Energy Advantage II (high-solar-gain double glazing) has a SHGC of 0.49. In other words, the window with high-solar-gain glazing admits 60% more solar heat than the low-solar-gain glazing. In spite of the significant difference in solar heat gain, the two windows have comparable U-factors (U-0.27 for the window with Cardinal LoE2-170, and U-0.30 for the window with Energy Advantage II).

Northern builders who settle for low-solar-gain glazing are “leaving a lot of BTUs on the table,” according to Thwaites. He compared the effect of double-pane Cardinal LoE2-170 with double-pane Pilkington Energy Advantage II; to simplify the comparison, Thwaites assumed that the same glazing was installed on all four orientations. For “a typical custom house with 200 square feet of windows” in a climate with a 212-day heating season, low-solar-gain windows result in the consumption of an additional 1,170 kWh (or 4 million BTUs) compared to high-solar-gain windows.

Paul Kriescher from Denver performed similar calculations. “For the home I’m in now — a production home where I specified the windows — assuming that natural gas costs 93 cents a therm, there is a net annual savings of $172 using windows with a SHGC of 0.55 on the south orientation instead of standard 0.32 SHGC windows,” Kriescher told me in 2006. “The modeling includes air-conditioning impacts.”

It’s available, until you try to buy it

Scott Pigg, a senior project manager at the Energy Center of Wisconsin, was so frustrated by the difficulties he encountered trying to buy high-solar-gain windows for his own home that he finally gave up.

“I had done a RESFEN analysis,” said Pigg. “I ran the simulations with hard-coat low-e and soft-coat low-e, and it made a big difference. We had decided to go with Marvin windows. When you call Marvin, they just ask, ‘Do you want a low-e coatingVery thin metallic coating on glass or plastic window glazing that reduces heat loss through the window; the coating emits less radiant energy (heat radiation), which makes it, in effect, reflective to that heat; boosts a window’s R-value and reduces its U-factor. ?’ That’s it for glazing options. I liked everything else about the Marvin windows, but I wanted a different low-e coating — I wanted a hard-coat low-e for better solar gain. I asked my contractor, and my contractor had no idea what I wanted. Then he came back to me, and said it would take three extra weeks. Then he said it couldn’t be done. I didn’t believe it, so I called Marvin directly. They said it was possible. Then my contractor said it would cost an extra $200 per window. I asked Marvin about that, and they wouldn’t give me any pricing information.”

Pigg ended up settling for low-solar-gain glazing, noting, “I will just have to suffer with windows that are somewhat better suited for southern cooling climates, as this seems to be where the entire industry is headed.”

Cold-climate builders should shop north of the border

Builders who have tried to buy high-solar-gain windows agree on one point: Most Canadian window manufacturers have a better understanding of high-solar-gain glazing than their U.S. counterparts. “All of the Canadian fiberglass window manufacturers get the concept of orientation-specific glazing,” said Andy Shapiro. “They are great.”

According to energy experts, the window manufacturers best able to handle requests for high-solar-gain glazing include Accurate Dorwin of Winnipeg, Manitoba; Fibertec Windows of Concord, Ontario; and Thermotech Fiberglass of Ottawa, Ontario.

Once you find the perfect windows, however, you’ll encounter one final frustration: the most energy-efficient windows aren’t eligible for the new tax credit. For reasons that remain unclear, the tax credit is reserved for windows that perform somewhat worse than those best suited for northern climates.

Last week’s blog: “Green Homes Don’t Have To Be Durable.”

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Image Credits:

  1. Bruce Richey