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 waves to enter. With all these options, it can be difficult to make the best choice.

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.

Materials and Terms
Types of Windows

Energy Efficient Window Design

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%.

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.

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.

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.

Additional Notes about Windows:

-Even high-quality multi-pane windows lose tremendous amounts of heat at night.
Cover south-facing (and other) windows at night, especially in houses that have huge glass "walls," which are nearly impossible to cover with window shades. Cover (especially southfacing) windows with insulated drapes or shades at night. Good window coverings can easily add another R-3 to R-4 to windows. Anything that helps block the flow of cold air from windows will be well worth the expense. Since shutters require being closed from the outside every night, more convenient are the (inside) rigid foam thermo-shutters, made from plywood and rigid foam covered with fabric.

-For clearstory windows, use the most energy-efficient windows. Consider motor-driven window shades to insulated clearstories at night.

-Payback for windows is usually between two and ten years. But also consider also smaller, less expensive heating and cooling systems.

-High-quality windows may last longer and require less maintenance, saving time, money, and hassle.