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 and ), 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 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 (originally Pittsburgh Plate Glass) and 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 European window a few years ago — it must have been German or Austrian — with a sash designed to be flipped seasonally to optimize the low-e coating placement, 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”; the “i” in “LoE-i89” stands 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 (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 and executive editor of . To keep up with his latest articles and musings, you can .