The Revolution in Window Performance - Part One

This Marvin Tri-pane window has three panes of glass, creating two dead air spaces. Photo: Marvin Windows. Click on image to enlarge.

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 clarityWhile 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-emissivity 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).

This chart shows the dramatic impact of the spacing between panes of glass in a window. With air in the cavity, the optimal spacing is about a half-inch. Graph: Randi Ernst. Click on image to enlarge.

Multiple layers of glassThe 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 glazing, 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.

A triple-glazed window in Växjö, Sweden; triple glazing has been standard in the country since 1976. Photo: Alex Wilson. Click on image to enlarge.

In the 1970s, manufacturers tried adding a third layer of glass to further increase the insulating value. This increased the R-value (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 spacesAs 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 an even 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.