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The Revolution in Window Performance — Part 1

Early gains in window energy performance were made by adding additional layers of glass and deepening the air space

Posted on Mar 22 2012 by Alex Wilson

I've been working for the past couple weeks on a report on windows — the latest in BuildingGreen's series of special reports on green building (the last one covering insulation). This focus has reminded me just how much we expect of our windows and what an amazing job they do.

First, let's look at everything that windows are expected to do: They provide us with views to the outdoors — whether of drifting snow on a frigid winter morning or kids playing in the backyard in mid-summer. They block the escape of heat and prevent condensation on the glass. They keep out wind-driven rain.

On the south side of our homes, especially in colder climates, windows are often expected to transmit beneficial solar heat, helping keep us warm. Yet, in warmer climates — and often on the west and east sides of our houses in more northern climates — we want windows to block the entry of unwanted solar gain.

Windows are usually expected to provide ventilation by allowing us to open them to channel fresh air into our homes — yet, we also want the same windows to be airtight when closed to keep out those cold winter drafts.

Windows often have to accommodate screens to keep out insects and other attachments to block sunlight or further reduce heat loss. Some windows also have to allow egress (escape from the house) in the event of fire.

All this, while being durable to the elements, long-lasting, low-maintenance, attractive, and — yes — affordable.

How are windows supposed to do all this?

They do it through some ingenious technologies and design features. I'll highlight a few of the most important features below. I'll follow with others next week:

Transparency and optical clarity

While the small-lite (twelve-over-twelve) windows in our 1780s home are certainly attractive — and treasured — they don't provide a great view to the outdoors. The hand-blown and rolled glass distorts the view. Polished plate glass, developed perhaps 100 years ago, provided better visibility, but was very expensive.

The real revolution came in the 1950s with the development of float glass, in which molten glass is poured out on a bed of molten tin. I had the good fortune 15 years ago to visit a float glass factory in Toledo and watch this process in person. The molasses-like molten glass flowed in slow-motion from a 500-ton furnace heated by massive jets of natural gas onto the molten tin and then move along a quarter-mile-long lehr where the 12-foot-wide sheet of glass slowly cools and is cut into manageable pieces. The molten tin provides a mirror-smooth surface and produces highly transparent, distortion-free glass; no polishing is required.

Various coatings on the glass and additives in the glass affect the visible light transmittance. In hot climates, it is common for glass to be either tinted or have sun-blocking low-emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. coatings. These sun-control windows can significantly impair visibility through them — as well as changing the appearance of the outside of a house. In commercial buildings where tinted glass has long dominated, it has become increasingly popular to use ultra-clear low-iron glass to provide glass that's even clearer than standard glass. (I'll say more about coatings next week).

Multiple layers of glass

The first strategy for reducing heat loss through windows was to add another layer of glass. This was first done with storm windows more than 200 years ago. The most basic storm windows — like those on my house — can be installed in the fall and removed in spring; more common today are triple-track storms with operable sash and screens. But starting in the 1940s and '50s, manufacturers began producing insulated-glass units (IGUs in industry parlance) with two layers of glass separated by an air. Sometimes referred to as Thermopane glass, which was the trademarked name Libby Owens Ford (LOF) used for such glass when they introduced it to the U.S. market in the 1950s.

While one layer of eighth-inch-thick, clear glass insulates to about R-1, two layers separated by an air space insulate to about R-2 — doubling the insulation. Nearly all of that insulating value is provided by the air spaces, not the glass itself. Even with single glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., the R-1 is provided almost entirely from the still air on each side of the glass. LOF's early Thermopane glass had welded-glass edges, so the seal was extremely good. Andersen was an early adopter of this glass, and many of their early Thermopane windows are still working just fine more than 50 years later.

Other glass manufacturers used spacers and organic sealants to achieve the air space between layers of glass, and this approach eventually won out. Today, virtually all IGUs are made using spacers — usually aluminum channel, but sometimes stainless steel, butyl rubber, or silicone — that are held in place and sealed with highly durable sealants.

In the 1970s, manufacturers tried adding a third layer of glass to further increase the insulating value. This increased the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. (at the center of the glass) to about R-3. American manufacturers generally had difficulties with this approach, because as they increased the weight of the windows they didn't beef up the frames and hardware adequately. Problems ensued, and the industry shied away from triple-glazing.

In Europe, however, triple became very common. I was surprised when visiting Sweden a few years ago to learn that triple glazing has effectively been required by code since 1976. You simply don't see new windows in Sweden that aren't triple-glazed.

Thicker air spaces

As the pressure to provide better-insulating windows grew in the 1970s, glass manufacturers increased the thickness of the air space from a quarter-inch to about a half-inch. This change can yield a dramatic improvement in energy performance — up to 40% in some cases — without much additional cost. The benefit is achieved because less heat flow occurs via gas-phase conductivity — one of the modes of heat flow through windows.

With air spaces, the optimal thickness is about a half-inch. If the space is thicker than that, another mode of heat transfer — convection — begins increasing heat flow. This occurs because convective loops form in the air space. Finding the optimal air space thickness means finding a balance between gas-phase conductivity and convection.

The demand for thicker air spaces, unfortunately, spelled the doom for those welded-glass-edge windows that LOF produced. With thicker spaces, the stress on the edges of the glass increases, and thermal expansion and contraction causes breakage.

Next week we'll look at a more significant innovation in window performance: low-emissivity coatings.

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. The BuildingGreen Guide to Insulation Products and Practices is available from BuildingGreen.

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Image Credits:

  1. Marvin Windows
  2. Randi Ernst
  3. Alex Wilson

Thu, 03/22/2012 - 11:57

Alex, very informative artticle, but question on spacers
by William Rau

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Aren't metal spacers much less effective than warm edge or super spacers made from silicon or other non-metallic materials?

Thu, 03/22/2012 - 17:01

Impact of spacers
by Alex Wilson

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Yes, indeed. The worst glazing spacers--aluminum channel--are still the most common. Stainless steel spacers with a thin cross-section are a lot better than aluminum (because of the lower conductivity of stainless steel as well as the thinner section), but as you note, the butyl rubber and silicone foam warm-edge spacers are the best at blocking heat loss.

As we make windows with glass that insulates better, the edges of that glass become more important.

Tue, 03/27/2012 - 10:06

Edited Tue, 03/27/2012 - 14:43.

by Greg Smith

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Good blog, I enjoyed reading it and I am looking forward to the next installment on coatings and other options.

There are a number of innovations in the works for windows and IGU's (technically Insulating Glass Units rather than Insulated...but that's really picking nits since even in the industry most people refer to Insulated).

Vacuum glazing is a very interesting technology that I bring up only because you talked a little about the original thermopane windows with welded glass edges and there is a very common misconception that the original welded glass IG units were vacuum filled (or not filled - which makes more sense I suppose).

That was never true of course, at least not intentionally, but I hear or read now and then that they were vacuum glazed and that going to inserted spacers between the glass eliminated vacuum glazing.

You also mention that most IGU's are manufactured using aluminum spacers. Did you mean currently or did you mean since the beginning of IG fabrication using spacers? If you meant currently then I am curious where that information came from since I would be rather surprised if that were the case.

Considering the much maligned aluminum spacer it's interesting that the picture of the Swedish triple included at the end of the blog includes a couple of aluminum spacers....

The graph is also interesting because it compares air versus gas infill in a LowE coated IGU rather than in a clear glass IGU. I won't comment further since I suspect that may be part of the next blog.

Have a great day!


Tue, 03/27/2012 - 11:06

Aluminum spacers
by Alex Wilson

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Thanks for your note.

If I'm mistaken and most IGUs today aren't made with aluminum spacers, I apologize; I thought aluminum still dominated the market and that the other types of spacers (stainless steel, butyl rubber, silicone) were less common. I'll try to find that out.

As for the illustration, I looked for a graph showing the effect of the air space thickness with clear glass, rather than low-e, but wasn't able to find that, so I used the low-e glazing configuration--but was hoping that readers would focus mostly on the impact of the spacing.

I've been following vacuum glass and wrote about it in 2009 ( I'll continue to follow and report as warranted.

Tue, 03/27/2012 - 20:33

Thanks Alex, great
by Greg Smith

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Thanks Alex, great info.

Looking forward to the next installment.

Thu, 03/29/2012 - 14:51

Keck and Keck Solar Homes
by kfiUcdc7Xj

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It might be a good follow up topic to discuss the original solar homes that Keck and Keck designed in the Chicago area in the late 1930's/early 1940's. These were demonstration homes that they designed for LOF to showcase their new double insulated glass. Pretty amazing for back then. LOF published 'how to' books in 1944 and 1947.

The homes were purported to be the first solar homes in 'modern' America and a subdivision that was designed by Keck shortly thereafter in Glenview, IL has been described as the 'first solar subdivision in America'.

There are some fairly amazing correspondence letters between Keck, LOF and Keck's developer client. The letters are from the State Historical Society of Wisconsin.

Thu, 03/29/2012 - 15:08

Tue, 04/03/2012 - 14:32

Films as the interior glazing
by Peter Talmage

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As you note, clarity is very important when it comes to windows. I have recently installed Serious Windows in a project and I noted that when the sun is shining through the glazing at a shallow angle the internal window film forward scatters the light making the window look noticeably fogged. Have you or anyone else seen this ? Thanks.

Wed, 04/04/2012 - 21:46

Glass spacing
by Bronwyn Barry

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Hi Alex,

I'm enjoying this series on windows. Thank you!

You've raised a question I've puzzled over for a while: Does the chart showing optimal spacing between glass panes apply to IGU's using warm edge spacers? It does not specify which spacers were used to generate the data and I've wondered about this. Does it make sense that the internal convective currents between the panes could be lower in units with warm edge spacers because the temperature differentials between center and edge are lower?

The reason I ask is that 'those Germans' tend to build a much thicker IGU, even for the 2-pane windows. The typical 2-pane ones I've seen are just shy of one inch overall (24mm), including the glass thickness. The u-values they claim for the 2-pane glass are 0.19 BTU/hrft2F - typically better than our local c-o-g values. This may be due to a testing protocol difference in obtaining those u-values, but perhaps you can shed some more light? I'd love to know.

Wed, 04/04/2012 - 23:48

Spacing between the glass
by Alex Wilson

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See my blog that will go live tomorrow--on gas fills. I address why the German spacing is wider. It's all about the temperature at which windows are tested. The Germans assume a lower delta-T across the window. At a lower delta-T there's less convection that happens, so a thicker spacing is justified. For the vast majority of time (and places) in the U.S. the same argument holds true. A thicker spacing makes sense.

Sat, 05/05/2012 - 14:18

Insultated Glass optimal spacer size
by Thomas Nedelsky

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If the delta-T is 30 deg F. ,inside air to out side air, what is the optimal air space?

Tue, 10/22/2013 - 15:10

Convective current breaks
by Jeff Thies

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I'm upgrading my solar collectors by adding double glazed 2 mil mylar "storms". The frames are wood. In the house, these are 1/2" thick and mounted inside. Works well.

Outside I have a vertical 200 SF of solar hot air, and a tilted 80 SF of solar hot water.

My question then is, should I add crossbraces, like the firestops in walls? What distance?

Would not the optimum spacing be greater for slanted glazing?

Note that polyester (mylar) needs UV protection, here it is supplied by the UV barrier on the Sun Tuf outer glazing

Wed, 10/23/2013 - 06:56

Response to Jeff Thies
by Martin Holladay, GBA Advisor

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Most solar collectors use single glazing, not double glazing. While a second layer of glazing helps retain heat at night and during cloudy weather, it reduces solar gain. Most energy calculations show that single glazing results in more heat collection and better performance than double glazing.

However, your system is unusual, and is unlikely to be cost-effective in any case. With such a large area of solar collectors, you may have reasons other than thermal performance to be pursuing the idea of double glazing.

Whether or not your home-made wood-framed solar collectors need cross braces is more of a structural question than a thermal performance question, it seems to me. There are many variables that lead me to believe that your Mylar layer will reduce rather than improve the thermal performance of your system, so the question of convective loops between the two layers of glazing seems to me to be a minor issue compared to more fundamental design issues.

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