Window Performance – the Magic of Low-e Coatings

Low-e coatings reduce heat transfer through windows by limiting the amount of radiant energy they emit. Graphic: Marvin Windows. Click on image to enlarge.

Last week I wrote about the early strategies window manufacturers employed to improve energy performance: adding extra layers of glass and increasing the thickness of the airspace between the layers of glass. This week we'll look at a more revolutionary change to window design that appeared in the 1980s: low-emissivity coatings.

Make no mistake. The introduction of low-emissivity (low-e) coatings was indeed revolutionary. A lot of the initial work on low-e coatings was funded by the U.S. Department of Energy--demonstrating a role that the federal government should continue to play. A friend of mine, the late Blair Hamiliton (who later founded the Vermont Energy Investment Corporation and Efficiency Vermont), was involved in some of the earliest work on low-e coatings while at MIT.

In the mid-1970s Blair was working on technology in which a very thin, transparent coating of silver was vacuum-deposited onto a thin layer of mylar plastic that could then be suspended between two layers of glass. Properly fabricated, the plastic film was invisible, and it produced not only a triple-glazed configuration but also a way to magically reflect heat trying to escape back into the room. How low-e coatings work

Before the physicists reading this challenge that last point, let me clarify that low-e coatings don't really "reflect" heat. Rather, they slow the emission of radiant energy. Let's look at the energy flows through a window.

Once the solar energy shining through a window is absorbed by surfaces in a house--say a tile floor or plaster wall--the energy warms the surface, which in turn begins radiating its own energy. That energy being radiated by the floor or wall is long-wavelength electromagnetic radiation. The glass or suspended film with the low-e coating absorbs that heat radiation (rather then transmitting it), and the low-e coating greatly limits the re-radiation--or emission--of that energy. Thus the term low-emissivity.

Some of those low-e researchers at MIT loaded their lab materials into a panel van (the "Heavy Chevy") and moved from Cambridge, Massachusetts to Palo Alto, California where they founded Southwall Technologies in 1979. The company's product, Heat Mirror, was rolled out commercially in 1981--roughly doubling the R-value of a window from R-2 to R-4 and ushering in a new age of window technology.

Meanwhile, other companies were working actively on efforts to directly deposit low-e coatings onto glass. Both PPG (originally Pittsburgh Plate Glass) and Cardinal Glass introduced low-e glass in 1983, and other glass manufacturers soon followed suit. Over the next two decades, low-e coatings that were directly deposited onto the glass captured the vast majority of the market--and they continue to dominate today. While Southwall is still producing great product and a number of window manufacturers are incorporating its Heat Mirror film into windows, directly coated low-e glass is far more common today.

Where should the low-e coating be installed?

To be most effective in climates where you want to block heat loss but allow beneficial solar gain to enter, the low-e coating should be located on the outer surface of the inner pane of glass--in the window industry, this is known as the #3 surface (in denoting surfaces, you always start with the outermost surface). In warmer climates where you're more concerned with keeping unwanted heat out, the preferred location for the low-e coating is the #2 surface (the inner face of the outer pane of glass). I remember seeing a German window one time with a sash designed to be flipped seasonally to optimize the low-e coating location (those Germans!), illustrating that point.

While those low-e placements (#2 or #3 surface) are preferred, having the coating on the other surface isn't the end of the world. The difference between the overall energy performance of the window with the #2 vs. #3 surface in any climate is far less significant than the the difference between having a low-e coating and not having one. Some manufacturers only put the low-e on the #2 surface, citing concern about seal failure when the coating is on the #3 surface. That's not a huge problem even in a cold climate.

With low-e coatings directly deposited onto the glass there are two broad categories: soft-coat and hard-coat. With soft-coat low-e, a thin layer of silver is deposited onto the glass through a sputtering process after the glass has been manufactured. While the earliest soft-coat low-e had a single layer of silver, coatings with two layers (low-e squared) and three layers (low-e cubed) came along since that have even lower emissivity and lower heat loss. These sputtered coatings have been referred to as "soft-coat" because the coatings remain fairly delicate and have to be protected within the insulated glass unit (facing the air space)--though that might be changing, as described below.

With traditional hard-coat low-e, a low-emissivity layer of indium tin oxide is applied when the glass is still molten and just beginning to harden in the float-glass "lehr" where it is produced (see last week's blog for a description of float-glass manufacturing). Denoting the high-temperature production, these coatings are also referred to as "pyrolytic" low-e. The indium tin oxide becomes part of the glass and, as a result, the low-e coatings becomes more durable. That's why hard-coat low-e is the type of low-e preferred for storm windows where the coating has to withstand washing and other abrasive actions.

While hard-coat low-e is more durable than soft-coat, the emissivity isn't as low, so these glazings don't achieve as low a U-factor. On the other hand, they allow more sunlight to pass through, so they are usually better for houses that are relying on passive solar heating.

What's new with low-e?

The distinction between soft-coat and hard-coat low-e is getting muddier. Cardinal Glass recently introduced an ultra-clear sputtered coating for glass (LoE-i89) that can be installed on the #4 surface of an insulated glass unit--the surface of the glass facing the room. According to Jim Larsen of Cardinal, this glass provides a remarkably high visible transmittance of 89%--significantly higher than standard pyrolytic hard-coat low-e glass. He described this to me as a "sputtered hard-coat," with the "i" standing for indium. The coating is durable and it doesn't have the bluish tint that some people object to with pyrolytic hard-coat low-e.

Glass with this coating can be combined with a low-e-squared or low-e-cubed glass to achieve a center-of-glass U-factor as low as 0.20 (R-5) with a double-glazed window. Previously, we needed triple glazing to reach this level of energy performance.

Cardinal also recently introduced a new high-solar-transmission soft-coat LoE-180 (80% visible light transmission), which is a single-layer soft-coat low-e. This provides significantly greater solar gain than the original single-layer soft-coat low-e glazings.

Next week, we'll look at another significant innovation with windows in the past few decades: low-conductivity gas fills.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.