A little history
Low-e coatings have had a dramatic impact on improving the energy efficiency of windows, doors (with lites) and skylights since the wide-spread commercial use of them for began in the 1980’s. Prior to that the best energy efficient windows, doors and skylights were primarily products that combined two panes of glass into a single sealed unit, now commonly known as an insulating glass unit or IGU, and then installed in a frame or door ( three panes were also sometimes used in windows but that was uncommon). As many of you may recall and for those of you not yet born, those products were generically referred to and marketed as “thermopane.”
While the use of double-pane windows in the U.S. generally dates back to the 1930’s, and the term “thermopane” to the mid 1940’s when it was originated by the Libbey-Owens-Ford Glass Company as a brand name for the double-pane products they manufactured, it really wasn’t until the late 1960’s and early 1970’s when the use of multi-pane products started to become more common in U.S. residential construction.
At the time, the glass used in those products was clear and if additional solar control was desired it could be tinted. While double- and triple-pane glass units resulted in a significant improvement in energy efficiency, even over single-pane products combined with storm windows and doors, there’s only so much efficiency that can be gained by clear glass. Using windows for example, single-pane windows generally have a U-factor of around 1.0. Make that a double-pane window and you bring that U-factor down to around 0.5. That can be further reduced to an extent depending on the window framing and design, but that’s about it on the whole. Add another pane to make it a triple, and the U-factor of the window can be brought into the range of around a 0.31 or 0.35, but that is still a hard stop and a triple IGU is essentially two double-pane units sharing a common middle pane of glass, making triple-pane windows heavier and bulkier, more difficult to manufacture and install, and more expensive so they weren’t commonly used. R&D is underway to address those issues but that’s a topic for another article.
Whether double- or triple-pane, the other major drawback of multiple clear pane IGU’s in a window, door or skylight is that you still get very little or solar control.
Enter low-e coatings
While the use of multiple panes of clear glass was a significant leap forward in energy efficiency it didn’t go far enough in response to the more pronounced recognition of the need for greater efficiency in homes and buildings that occurred in the early 1970’s. That was driven largely at that time by the OPEC oil embargo of 1973. That in turn prompted the federal government to initiate more R&D of energy efficient building technologies, including for glass used in windows, doors and skylights and attention turned to low-e coatings.
According to Lawrence Berkley National Laboratory (LBNL) the concept of low-emissivity or low-e coatings on glass dates back to the World Ward II era but it was not commercialized in the building industry and never really went anywhere. Fast forward to the mid 1970’s and R&D attention by the federal Energy Research and Development Administration (ERDA), now the Department of Energy (DOE), turned to advancing technologies for improving the energy efficiency of glass based on the use of low-e coatings and making the technology commercially viable. So, beginning in the mid 1970’s, LBNL with ERDA funding, led the way on R&D of low-e glass technologies and engaged private industry to develop prototypes. That was also with an eye to the future as the energy efficiency of residential and commercial construction was entering a new era of regulation with the development of the first national building energy codes.
Fast forward again to the mid 1980’s. By then, energy codes were gaining a stronger foothold across the country and those R&D efforts had demonstrated the commercial viability of low-e technology and the great value of its contribution to significantly improving the energy efficiency of windows, doors and skylights. That was critical to gaining the confidence of manufacturers which it did and most major glass manufactures we’re by that time producing low-e products. In turn, so were window, door and skylight manufacturers using them.
Low-e and energy efficiency
“Low-e” is now the generic term when talking about energy efficient windows, doors and skylights. In simplest of terms a low-e (low emissivity) coating is a microscopic metallic coating applied directly to the glass surface that reduces the thermal emittance or heat transfer through the glass. That helps keep heat in during cold months and heat out during warmer months and in warm climates.
Low-e coatings work like this. They are highly transparent and virtually invisible so they still allow visible light on the solar light spectrum to enter into a home or daylighting, but limit the amount solar infrared (heat) light entering the home from the outside by reflecting it back. The coating also serves as a radiant barrier to keep heat inside the home from escaping. By using low-e coatings in different ways to maximize these factors, windows for example with a U-factor of 0.20 and even below are now available for markets in northern colder climates, and windows that can prevent 80% or more solar heat light from entering the home are now available for southern hot climates.
A deeper dive
There are two overarching forms of low-e coatings – hard-coat and soft-coat. Both are metallic formulations. Hard-coat low-e coatings are generally formulations relying upon tin oxide and are applied as part of the glass manufacturing process while the glass is still in a semi-molten state. It is applied as a single layer only and the thickness of the layer generally doesn’t vary greatly.
Soft coat low-e coatings are formulations relying upon metallic silver that are applied to the glass after it is manufactured in a process known as sputter coating using a vacuum chamber. The thickness of those coatings are more varied and multiple coating layers can be applied depending on the thermal performance and solar control desired.
Varying the coating thickness and the number of coating layers then allows for energy flows for solar heating, daylighting, and cooling to be optimized for the climate they are used in. To briefly illustrate, back in the day a “thermopane” window installed in Atlanta, Georgia was likely to be pretty much the same as a “thermopane” window installed in say Minneapolis, Minnesota. Clear glass is clear glass so the energy efficiency of them was therefore essentially the same. Today on the other hand, a low-e window installed in Atlanta will have significantly different efficiency values than a low-e window installed in Minneapolis making each of those windows better suited to the respective climate conditions because of how low-e coatings are used. That also makes for improved occupant comfort as well.
Because of the flexibility afforded by using soft-coat low-e coatings, they are used in the vast majority of IGU’s in residential windows, doors and skylights so we’ll focus on them.
Soft-coat low-e IGU’s have from one to three layers of a low-e coating on one of the glass surfaces that is on the inside the sealed IGU. That is to protect the coatings from environmental elements. As a general rule of thumb, while there is a significant improvement in the U-factor of the IGU by applying the first low-e coating, adding a second or third layer doesn’t improve upon that much further. However by applying additional low-e coating layers there is a significant gain in the amount of infrared heat light that is reflected and prevented from entering a home (the Solar Heat Gain Coefficient (SHGC)), so adding second and third layers are more about solar heat gain control.
Accordingly, there are three general types of soft-coat low-e coatings that are selected based on the level of solar heat gain control that is required or desired. The National Renewable Energy Laboratory (NREL) characterizes them as follows:
High-Solar-Gain Low-Emittance Coatings
High-solar-gain low-e coatings typically have an SHGC value greater than 0.40 and are designed to reduce heat loss but admit solar gain. High-solar-gain products are best suited for heating-dominated climates and particularly to south-facing windows in passive solar designs. Unless properly shaded, high-solar-gain windows may result in overheating from excess solar gain in swing seasons.
Moderate-Solar-Gain Low-Emittance Coatings
Moderate-solar-gain low-e coatings typically have an SHGC value of 0.25–0.40. Such coatings reduce heat loss, maintain high light transmittance, allow a reasonable amount of solar gain, and are suitable for climates with heating and cooling concerns.
Low-Solar-Gain Low-Emittance Coatings
Low-solar-gain low-e coatings typically have an SHGC value less than 0.25. This type of low-e product, using a highly spectrally selective low-e glass, reduces heat loss in winter and reduces heat gain in summer. They are most suitable for cooling-dominated climate.
Solar heat gain, both wanted and unwanted, can be further controlled by the low-e coating depending on which glass surface on the inside of the IGU it is located on. In heating dominated climates like Chicago, placing a low-e coating on the outside surface of the inside pane is recommended to maximize winter passive solar gain but at the expense of a slight reduction in the ability to control solar heat gain in the summer. In cooling climates like Dallas, a coating on the inside surface of the outside pane is generally best to reduce solar heat gain and maximize energy efficiency. Multiple low-e coatings can also be placed on two surfaces within a triple-pane IGU to further improve overall U-factor of the window, door or skylight assembly it is installed in.
Not the only star
Low-e coatings have been an indisputable gateway to making highly efficient windows, doors and skylights commonly available for construction. But it’s important to remember low-e coatings are not the only critical component for making windows, doors and skylights energy efficient. It is the complex combination of low-e coatings together with glass, IGU spacer and gas fill technologies, framing material type, style, manufacturing processes, and other attributes that are engineered by manufacturers to deliver the spectrum of products that are available today.
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