Blog Post

Window Performance – the Magic of Low-e Coatings

March 27, 2012

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.

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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.

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Comments

March 27, 2012 - 2:14 pm

A slight correction to the energy physics of glass: glass is relatively opaque to far infra-red light (heat), so the first pane from the incident direction will absorb radiated heat (not necessarily the pane or film with the coating), and either re-radiate that heat from its other surface or not, depending on the emissivity of that surface.

Clear glass has an emissivity of about 0.84, meaning it will radiate about 84% of the heat and reflect 16%. The primary mode of heat transport through center-of-glass in a multiple-glazed window (ignoring the conductivity of the frame and edge spacers) is radiant (the air or noble gas fill has very low conductivity - air is 1/40 that of glass). With a low-E coating, the emissivity can be reduced to as little as 0.04, thereby eliminating most of the radiant heat transfer between panes.

Emissivity and reflectivity are complementary, so if the lowE coating is on the next surface (the other pane) it will reflect most of the heat back to the incident pane, which will remain at a higher temperature and reduce the likelihood of condensation in winter or keep the heat outside in summer.

LowE glass is also relatively opaque to skin- and furnishing-damaging UV radiation.

April 2, 2012 - 1:09 pm

In your paragraph "What's New with Low-e?" I did not see mention of a new low-e film that was introduced nearly two years ago, EnerLogic window film. This is a revolutionary patent-pending window film product, developed in part through a grant from the U.S. Department of Energy.
EnerLogic window film, with an emissivity of 0.07, offers a retrofit solution for people wanting not only the solar-heat and glare control, and UV protection that window films offer, but also the insulating benefits of a low-e coating. EnerLogic window film does this at a substantially lower cost than replacement windows and with much less impact on the environment than sending existing windows to landfills when replaced. The insulating performance of this film is actually so efficient that when installed on single-pane windows, the windows will have the insulating performance of dual-pane windows on an annual basis (average of winter and summer U-factors). The same is true with regard to EnerLogic giving dual-pane windows the annual-insulating performance of triple-pane windows. The emissivity and insulating performance of EnerLogic film has been confirmed by multiple test labs, including independent testing for NFRC Certification.

April 2, 2012 - 6:27 pm

It's helpful when readers point to new products that have been overlooked in your reports, but I'm surprised you would publish what amounts to an unvetted sales pitch from EnerLogic. It may be a product of interest, but their website is mostly hype with very little independent test data.

April 2, 2012 - 6:47 pm

Robert, you can get NFRC data on EnerLogic films here:

http://search.nfrc.org/search/apd_film/film_search_productline.aspx?mfr=...

The numbers are quite impressive and have led us to list EnerLogic in GreenSpec among our approved window films:

http://www.buildinggreen.com/auth/productsByCsiSection.cfm?csiMF2004ID=1813

I believe that one downside of EnerLogic, which I hope Steve could talk about, is the very low Visible Transmittance of some of their higher-performing films.

April 3, 2012 - 3:40 pm

I'm confused do you mean LoE i89 or LoEi81? I can't find LoE i89 on Cardnial's website. My curiosity was peaked because a VLT of 89% is fantastically high. Single pane clear glass, as listed on Cardinal's own website comes out at 90% (probably really the classic 89% number). Dual pane clear glass comes out at 81% (.9x.9). Even Starphire glass dual pane IGUs have a VLT of around 86% to 87%.

Do you really mean LoE i81? 80% VLT with those performance characteristics is certainly nothing to sneeze at. It is very possible I am missing something here. Help me understand!

April 3, 2012 - 4:38 pm

Sam,
This is a very good question. Cardinal is in the process of replacing their LoE-i81 with the new glass, LoE-i89. I think the company is reluctant to publicize it too widely until the inventory has turned over. I was told last Friday that the website should be updated in about two weeks with this new glass--which has remarkable properties--89% visible light transmittance!

If you are using LBNL's Window 6 software for analyzing glazing performance, this glass has already been incorporated into the database (Glazing ID #2159). Those of us not using the Window software will have to wait until the new information is available on Cardinal's website. I believe that's all that's being shipped to window manufacturers, though.

April 4, 2012 - 1:26 pm

I realized my mistake. The 89% VLT must refer to a single pane of 1/8" glass. This would make the resulting performance reasonable. This is still fantastic, the VLT of a single pane of clear glass with these performance characteristics is great.

August 21, 2016 - 4:08 pm

How would 

Triple pane double low e 366 (surface 2 and surface 5)  

compare against

Triple pane double low e 366, i89 (surface 2 and surface 6)

I can only seem to find data on i89 w e366 on double pane windows and can't seem to find how the 2x e366 would perform.  Generally I don't care about SHG and just want to keep heat from coming in or going out.  If I also don't really care about VLT should I stick with e366 on #2 and #5 or do the e366 w #2 and i89 on #6?