©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
UPDATED on May 5, 2016
Everybody has an opinion on windows, and there’s a lot to talk about. Which frame material do you prefer: wood or fiberglass? Do you like double-hungs, sliders, or casements? Who provides better warranty service, Marvin or Pella?
Window selection is a complicated topic, so I'll approach the issue in small bites. In this article I’ll focus on 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..
Windows are crucial to a home's thermal performance and the comfort of occupants. In a cold climate, the wrong windows will act like holes in a home's thermal envelope, leaking tremendous amounts of heat. In contrast, the best performing windows can actually collect more heat than they lose  during the winter months, turning your walls' weakest link into an asset.
In a hot climate, windows with the wrong type of glazing are often the leading cause of summer overheating — they're probably the main reason that your air conditioner struggles to keep your home cool on summer afternoons. That's why the right type of glazing can transform an unlivable room into a pleasant oasis.
Passivhaus Windows 
If you are building a new home, the cost to upgrade from run-of-the-mill windows to high-performance windows is relatively small, and the incremental cost can easily be justified by future energy savings. Upgrading to better windows will never be cheaper than during new construction.
If your existing home has bad windows, however, the cost to replace every window in your home with new high-performance windows is often prohibitive. After all, the cost to replace an existing window will always be significantly more than the incremental cost to upgrade to better windows when the house is being built.
Old-fashioned single-glazed windows have been relegated to garages and barns. These days, the vast majority of new residential windows come with either double or triple glazing.
Most double- and triple-glazed windows are fitted with insulated glazing units (IGUs). The typical IGU consists of two panes of glass connected by a spacer at the perimeter. The spacer establishes the distance between the panes and seals the gap; it needs to be gas-tight so that the gas between the panes (often argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. or kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing.) doesn’t escape and so moisture can’t enter. Warm-edge spacers — for example, the Super Spacer, the Inex spacer, the Intercept spacer, or the Swiggle Seal spacer — are better at reducing heat flow through the perimeter of the window than old-fashioned aluminum spacers.
IGUs can also be produced from three (or, very rarely, four) layers of glass.
There are two important numbers you need to know when choosing glazing: the U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. and the solar heat-gain coefficient (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.).
The U-factor measures the rate of heat flow; it's the inverse of R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . In all climates, a low U-factor is better than a high U-factor.
The SHGC tells you how much heat enters a building through the window when the sun is shining. When it comes to SHGC, there is no simple rule to guide you. If you live in a sunny climate with high air conditioning bills, you probably want windows with a low SHGC, especially on the east and west sides of your house. (North windows don't see much sun, and south windows are relatively easy to shade, at least during the summer when the the sun is high in the sky between 10:00 a.m. and 3:00 p.m.)
If you live in a cold climate with high heating bills, you probably want windows with a high SHGC, especially on the south side of your house.
If you live in a mixed climate, you probably want to choose different types of glazing for different orientations of your house. South-facing windows should probably have high-SHGC glazing, especially if the windows are adequately shaded during the summer. West-facing windows should probably have low-SHGC glazing. Selecting the best glazing for each orientation is best done with an energy modeling program like RESFEN  or the Passive House Planning Package .
[Author's note: Since this article was written, an increasing number of energy experts have begun questioning traditional passive solar design principles that call for south-facing windows to have high-solar-gain glazing. For more information on this topic, see Reassessing Passive Solar Design Principles .]
A window’s U-factor and SHGC numbers appear on the familiar NFRC label found on most new windows. Note that the NFRC reporting requirements require U-factors and SHGCs to be whole-window measurements, not glazing-only measurements. You can’t compare a whole-window U-factor with a glazing-only U-factor. If a window salesperson brags about a certain U-factor or SHGC, be sure to ask whether the advertised number measures the performance of the whole window or only the glazing.
In most of the U.S. and Canada, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. windows are now standard for new homes. A low-e coating is an almost invisible layer of metal that is applied to one side of the glass by the glass manufacturer. The effect of a low-e coating is to lower the U-factor of the glazing; that is, it slows heat transfer.
Most low-e windows have double-pane IGUs with a low-e coating on one side of one pane. In almost all cases, the low-e coating faces the air space between the panes, so that the coating is protected from abuse. (It should be noted, however, that hard-coat or pyrolytic low-e coatings are tough enough to be used on single-pane storm windows.)
Low-e coatings don't just reflect heat in one direction; they work in both directions. Since the addition of a 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. coating adjacent to an air space has the effect of lowering the U-factor (that is, raising the R-value) of the air space, a low-e coating helps slow heat transfer from the interior to the exterior during the winter, and also helps slow heat transfer from the exterior to the interior during the summer.
Low-e triple glazing can have either one or two low-e coatings. (If a triple-glazed IGU has two low-e coatings, it's sometimes referred to as “double-low-e” glass.) It's also possible to order triple glazing without any low-e coatings; such windows may allow more solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. on sunny days, but they will lose more heat at night than a triple-glazed window with a low-e coating.
Knowing that a window has low-e glazing tells you almost nothing about its solar heat-gain coefficient (SHGC). A low-e window might have a low SHGC, making it a good choice for a house in Florida, or a high SHGC, making it a good choice for a house in Minnesota. Just because a window is low-e, doesn’t mean it’s good at reducing solar gain.
Filling the space between the panes of an IGU with argon gas instead of air improves the performance of the glazing by lowering the unit's U-factor. Most low-e glazing units are filled with argon gas.
The optimal thickness of the space between panes of argon-filled glazing units is 1/2 inch. Increasing or decreasing the thickness of this space degrades performance. For krypton, the optimal space is thinner — only 3/8 inch — so krypton, a much more expensive gas than argon, is usually reserved for applications where the total glazing unit thickness must be minimized.
Don't be tempted to buy glazing without argon or krypton gas. Like a low-e coating, argon gas is a cost-effective upgrade. In almost any location in the U.S., argon gas and low-e coatings will quickly yield energy savings exceeding their cost.
The vast majority of low-e windows sold in the U.S. have low-SHGC glazing. These windows perform well in Texas and Florida, but they make little sense up north, where homes would benefit from high-SHGC glazing. Every time a cold-climate builder installs a low-SHGC window instead of a high-SHGC window, the homeowner is condemned to higher energy bills for the life of the window.
There are two main reasons that low-SHGC windows dominate the market:
If you live in a sunny climate, low-solar-gain double glazing makes sense. Popular options include:
If you want to buy a high-SHGC window, you'll probably need to do some research before placing your order. Many window reps are ignorant about glazing options, so be prepared to educate the sales staff (or go over their heads) to get what you want.
Several glass manufacturers make high-SHGC double glazing. Good bets include:
When ordering your windows, you can mention these products by name. At this time, Pilkington's Energy Advantage is probably the best-performing high-solar-gain double glazing available.
Until recently, few U.S. window manufacturers were able to supply high-solar-gain windows. Even those who were able to supply them often erected hurdles that made special orders difficult. (There hurdles included technical ignorance, high prices, and in some cases an outright refusal to supply special-order glazing.) That's why U.S. builders looking for high-SHGC windows often seek out Canadian window manufacturers, many of whom have an excellent understanding of the technical advantages of high-solar-gain windows for cold climates.
During the past few years, however, some U.S. manufacturers have begun to market high-solar-gain glass. For example, Pella now offers a glazing option called “NaturalSun”; it's a high-solar-gain double glazing from Cardinal. The center-of-glass specs for NaturalSun glazing are pretty good: a SHGC of 0.70 and a U-factor of 0.27. This glazing results in a whole-window SHGC of 0.51 and a whole-window U-factor of 0.32.
When it comes to triple glazing, a surprising number of U.S. window manufacturers are still confused. For inexplicable reasons, the triple glazing usually offered to U.S. buyers is almost always low-solar-gain triple glazing. These window manufacturers fail to understand that no one in Florida or Texas is buying triple glazing. The buyers are all up north, in Montana and Minnesota and Maine, where only high-solar-gain glazing makes sense. So wake up, window manufacturers! If you're going to offer triple glazing, make sure it is high-solar-gain triple glazing.
Although I’ve focused my attention on glazing U-factor and SHGC, there’s another number that matters: visible transmittance, or VT.
A high VT is better than a low VT, unless there is reason to believe that glare will be a problem. Windows with a low VT are less likely to cause glare than windows with a high VT.
Every time you add another layer of glazing to a window, the VT goes down. When the VT drops below 0.40, everything seen through the window begins to look a little gray.
In general, neither glass manufacturers nor window manufacturers have “branded” any particular type of triple glazing. As a result, those interested in triple-glazed windows are forced to compare U-factors and SHGC numbers.
For more guidance on buying triple-glazed windows, see “Choosing Triple-Glazed Windows.” 
In you are in a hot climate, you probably want a double-glazed low-solar-gain window. Look for these specs:
If you are in a cold climate, you probably want a double- or triple-glazed high-solar-gain window. If you settle for a double-glazed window, look for these specs:
If you can spring for a triple-glazed window, look for these specs:
Last week’s blog: “Fastening Furring Strips to a Foam-Sheathed Wall.”