Inert Gases (argon or krypton) between the layers of glass in multi-paned windows conduct heat less than air, making the window more efficient. All multi-pane windows leak, but usually not more than 10-15% over 25 years.

Polyester Films. treated with a low-e coating, thin transparent polyester films applied between layers of glass or just applied to the surface.

Window Specifications. Low-e windows are a must for passive solar homes in all climates; however, specifications vary, not only according to the climate, but also the location of the window regarding north, south, east, and west area of the house. This will be described in detail in the sections "Window Location" and "Glass to Mass."

Warm Edge, the non-conductive spacer between the 2 window panes, is an important part in a window that can prevent a lot of damage. Insulating the inside and outside frame prevents damaging condensation. It also improves the efficiency of a window by 10 %. Condensation is a main culprit in damage to a home. Moisture can do tremendous harm to a home (mold, rotting of material, etc.) as described in all chapters.

Energy-Efficient Window Sashes and Frames.

Weather Stripping results in more airtight and thus more energy-efficient windows. Hinged windows are tighter than sliding-sash windows. However, you may find that double-hung windows from some manufacturers are tighter than casement windows from others. It is best to check and compare.

Materials Used to Build Window Sashes.

Wood windows are (still) the number-one sellers in the U S. Wood is a renewable resource, and it has a "warm feel." It's a much poorer conductor of heat than metal and, therefore, fairly energy efficient. To prevent weather damage to wood windows, metal, vinyl, or plastic is applied to the outside of the window frame.

Metal sashes, being a good conductor, conduct the heat (or cold) out of a house; unless inserted with "warm edges" metal, window frames are a poor choice.

Fiberglass windows are extremely strong and have a thermal expansion coefficient close to glass. Fiberglass may become the window frame of the future. Fiberglass windows are not as widely available and are more expensive ; they must also be painted..

u- Value: Heat Loss is the opposite of R-value (U-value = 1/R). U-value is the heat transmission through a material.
U-values of smaller than 0.3 are required for passively conditioned homes.

Good window covers with insulated drapes or shades can easily add another R-3 to R-4 to a window. Also, install insulated window shades that seal around the edge of the window via magnetic strips to help block the flow of  air.

Exterior shutters can reduce heat loss, but must be opened and closed each day. To avoid this problem, consider rigid foam thermo-shutters on the inside, made from plywood glued to rigid foam, a lot more convenient than exterior shutters.

In warmer climates, reduced south-facing glazing cuts down on solar gain while additional north-facing windows release heat. The net result, if properly done, is a cool interior. The greater the need for heat, the greater the solar glazing requirements.
Excessive glazing can result in overheating, while at night, too much glazing can lead to much heat loss and uncomfortably cold temperatures. Over-glazing can also lead to overheating in the summer, especially if the house is not properly aligned to the sun.

As a general rule, solar glazing should fall somewhere between the 7 - 12 % range of the glass-to-mass ratio. In a warmer climate, you may stay between 8 - 10 % glazing. In a colder climate, 12 % glazing might be needed.  More in "Glass to Mass Ratio"

SUMMERTIME COOLING
Window shading is a necessity for most direct-gain passive solar homes - particularly in the late summer and early fall. The warmer the climate, the more important shading becomes. In order to keep the hot sun off the window, and to avoid ultimately heating up the interior, external shades are more effective than internal shades.
Canvas awnings, roll-down blinds, and vertical louvers all prevent sunlight from striking the window. External shades are more effective than internal shades but may require more maintenance and repair.

Glaze for Ventilation. Some of the solar glazing needs to be open-able for natural ventilation. Open-able windows should be on the north, east, and west sides of the house for cross-ventilation.

Create Sun-Free Zones. Although newer glazings that limit UV penetration help reduce this problem, ultraviolet light can damage carpets, window coverings, and furniture. Also, be aware of where you plan to watch TV or use the computer regarding glare.

Special Requirements for Protecting East and West-Facing Windows

Due to the low sun angle, overhangs are ineffective on east- and west-facing windows.

Trees planted on the east and west sides of a home block the early and late sun which may contribute to summertime overheating. Vine-covered arbors and trellises against the side of a house also provide shade for east- and west-facing walls and windows.

For passive solar heating unobstructed access to the sun for about 6 hours is essential. Trees on the east and west shade a home during the summer mornings and evenings.

Energy Ratings
To find out if the windows you're considering are appropriate for the climate you live in, look at the Energy-Star or National Fenestration Rating Council (NFRC) label. The NFRC label indicates the U-value and solar heat gain coefficient (SHGC) for that window. The Energy Star label shows how these ratings compare to the minimum energy-efficiency standards of the U.S. Department of Energy (DOE).

Energy Star breaks the country into 3 zones. To meet the standards, windows used in areas like Florida and Southern Texas must have a SHGC rating of .40 or lower and a U-factor of .75 or lower. In states like Maine and Montana, the DOE says windows should have a U-factor of .35 or less but has no requirement concerning solar heat gain. That's because in the far North heat loss is a big issue but solar heat gain isn't.

The NFRC label also contains information on visible light transmission (VLT), which can be reduced-by some coatings. The VLT rating indicates the percent of visible light able to pass through the glazing. Since the whole idea of using glass windows is to admit light, look for a VLT rating of .60 to .80 (60 to 80 percent). On the plus side, low-e coatings do block some of the UV light that can fade and damage furniture. These ratings don't show up on the NFRC label, but can often be obtained from the window manufacturer.

In Cold Climates
Low-e coatings were introduced in the 1980's, and were devised to save energy in cold climates. Early low-e windows were designed to let light and solar heat pass through the glazing into the building, and to reflect heat back into the building when it tried to escape through the glass. By lowering the U-value or heat loss rating of the glazing, a low-e coating reduces the amount of energy needed to heat the building. Sitting next to a window when it's cold becomes much more comfortable because the heat that radiates from your body is reflected back into the house. What's more, the glass stays warmer, so moisture is a lot less likely to condense on the inside of the window.

In Hot Climates
The problem with windows designed for cold climates is that they keep heat in but they don't keep it out. As a result, early low-e windows weren't much help in the South, where it's harder to stay cool than it is to stay warm. In the 1990's, manufacturers introduced windows with spectrally selective coatings. Designed for use in hot climates, windows with these coatings admit up to 40 percent less solar heat than ones with clear glass panes. As a result, less heat enters through the glass, so the house is easier to cool.

Convection is the transfer of heat between a surface and a moving fluid, such as air or water. The pas­sage of air along a surface encour­ages heat transfer from the warm material to the cooler air. In your house, this is most evident at the windows, where you can feel a draft along the surface in the winter as the inside air cools, becomes dense, and falls when it hits the glass. The movement draws warmer air from ceiling level, which is cooled by the poorly insulated glass at the window and sinks in turn, creating a convec­tive cycle. Convection patterns are important because they occur not just around windows but also around other thermal sources such as woodstoves and interior chimneys.

Radiation is energy transferred by waves and is the means by which the sun delivers its heat. There Must be a line of sight between` the energy source and the target in order for radiation to be effective. This kind of heat transfer can be manipulated to some extent by increasing reflectivity and diffusion by using tighter colors, or incorporating shad­ing where needed.

Payback for windows is usually between two and ten years. But consider the fact that a smaller, less expensive heating and cooling systems is now needed.

There have been many recent improvements and developments when it comes to windows, making it challenging to keep up with all the newest terms and technology. There are not only multi-pane windows, but also windows with coatings in between. Depending on climate, consumers can now choose windows that allow more or less heat or light

It is important to choose windows carefully and keep open-able windows to a minimum. Even when closed, open-able windows are more likely to have air leaks. Use only as many open-able windows as you really need for ventilation and comfort.

Air Infiltration. (leakage) between the sash and the frame, depends on the type of window and the quality of construction. Air leakage should be less then 0.30 cfm/ft². It is important to seal between the window and rough opening. It would be counterproductive to have an expensive leak-free window installed that leaks around the outside. Use caulk or foam before installing window trim.

Size of Windows and Installation. Windows generally lose more heat around the edges. In other words, everywhere the glass is fitted against a material (a sash), there is a potential for leaking. Therefore, the objective is to reduce the number of edges. This is most easily done by installing one large window instead of a number of small windows. If you want the divided light look, your best bet is to purchase windows with an applied grill. (More on window size in the chapter "Glass-to-Mass")

Solar Heat Gain Coefficient (SHGC) is an important figure for windows in a solar home. It measures how well a window blocks heat from sunlight. It is the fraction of the heat from the sun that enters through a window, expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits.

Blocking Ultraviolet Radiation indicates how well the window protects carpets, drapes, and furniture (although visible light also 'fades fabric). Most windows reduce UV penetration by 75%.

Low-e coating. (low -emissivity) glass has coatings of clear silver or tin oxide on the glass. It allows light to pass through a window but blocks infrared radiation (heat). However, it reduces the transmission of sunlight and, therefore, decreases solar gain. There are 2 types of coatings: Soft Coating and Hard Coating
The soft coating is layers of silver oxide and anti-reflective coatings applied to the inside surface of one of the panes of glass. In hot climates, low-e coatings should be applied to the inner surface of the outer pane, blocking the warm outside air. In cold climates, low-e coatings should be applied to the interior surface of the inner pane, to keep the heat inside a home. Emissivity of a window can increases the R-value nearly 60 percent.
The hard-coating is a thin layer of tin oxide incorporated into the surface of a pane of glass during manufacturing, which makes these coatings more durable than soft coats. However, they are not as effective as soft coats. There are frequently new types of coating available.

Window Essentials