Current Efficient Window Technologies

Currently, the best windows available include: double- and triple-paned, insulated, glazed, smart, and thin-film electric windows. Benefits gained from utilizing these technologies include reduced fading of interior decorations, less unwanted energy loss out of buildings, reduced levels of captured heat within a building, and the reduction of glare on surfaces. Ultimately, by improving window technology, benefits can be maximized while energy costs are lowered.

Image courtesy of Los Angeles Department of Water and Power

Window Efficiencies: U-Values And R-Values

The window industry measures window efficiency in terms of U-values and R-values. U-values measure insulating efficiency and conducted heat gain or loss through a window. They are measured in BTUs (British Thermal Units) per hour per square foot per degree Fahrenheit (btu's/hour/square foot/�F). An R-value is the reciprocal of a U-value (U = 1/R). The lower the U-value, the better the insulating performance against conducted heat gain or loss. A window system with a U-value of 0.35 outperforms a system with a U-value of 1.1.

Example: "Consider a 6 sq.ft.window in Minneapolis, winter night, outdoor temperature 5�F, indoor temperature 70�F. The fenestration system has an overall U-value (including frame, edge of glass, and center of glass) of 0.40."

Types of Windows

Single Paned

In the past, most homes only had single paned windows. They are the simplest, and in most cases, the most inefficient windows available. Consisting of a sole piece of glass inserted into a frame, single paned windows have poor insulation and little resistance to radiant heat flow. However, some single paned windows implement metal frames which conduct heat; while more efficient windows have thermal breaks which prevent heat flow.

Insulated Glass Units (IGU's)

Air Flow

An air flow window is constructed of a double glazed window with an exterior single paned window so that air may flow in between. Air flow construction not only improves thermal performance, but it also allows for controlled air changes within a building. There are two types of air flow windows: an exhaust air window and supply air window.

Glazings & Coatings

Low-E

Low-e glazings are thin, clear coatings of undetectable metal oxide or semiconductor films that are placed right onto the glass. They are called low-e for low-emissivity, because they help to increase the effeciency of a window. Emissivity is a measure of a material’s ability to radiate energy, and the lower the emissivity of a coating the better the glass performs in the reduction of heat transfer.The application of low-e glazings is done in either soft or hard coats. Soft coats are delicate and their energy performance is better than hard coats, but they degrade when exposed to air and moisture. Hard coatings are less easily damaged and are utilized where add-ons are used.

Low-e coatings usually face air spaces within windows and lessen the heat flow between panes. The coating also absorbs and reflects radiant heat and ultraviolet rays. Basically, low-E coatings work by controlling specific portions of the sun’s energy that enter through a window: ultraviolet light, visible light, and infrared energy. Because high levels of ultraviolet light can be harmful to humans and damage and fade furnishings, it is beneficial to exclude this type of light, but desirable to keep visible light, which provides natural daylight- the cheapest source of lighting available. Infrared energy that enters may be desirable or undesirable. It may be wanted for sunshine and warmth for personal comfort or to naturally heat a room’s interior (passive solar heat gain/heating), but may be undesirable when it becomes discomforting, such as when a room becomes hot and strains are placed on air-conditioning systems.

Windows which include low-e films can reduce energy loss by as much as 30% to 50%, and cost only 10% to 15% more than windows without these films. Low-e glass is designed to be extremely effective during the summer, by reflecting much of the sun's heat away from the inside of the home. But when applied to the outside of a window, it can reflect between 70%-75% of the heat that would normally be lost to the outdoors back into the home. During the cooling season, low-e glass reflects about 25% more heat back inside than a single-paned window and about 11% more than a standard double-paned window. The U-value for a double-paned window with low-e glass starts at about 0.4. Often low-e glass is combined with gas filling, for U-values as low as 0.15.

Utilizing low-e glazings is inexpensive compared to replacing a window. There are even films that you can apply yourself.These films last 10 to 15 years, can conserve energy and lead to a more comfortable life. Advantages of low-e Windows include:

• substantial reduction in ultra-violet radiation
• reduced fading of carpets
• drapes and furniture
• reduced mildew and deterioration of the window frame
• increased comfort near windows
• reduced summer heat gain and winter heat loss (lower energy bills)

All of these benefits lead to a greater control of the internal environment, greater occupant comfort and lower heating and cooling costs.

Spectrally Selective Coatings

Spectrally selective, or optical coatings filter out 40% to 70% of the heat normally transmitted through clear glass. This type of coating completely allows light to be transmitted. It can be applied in a manner so as to either increase or decrease the amount of solar heat that passes through a window. These coatings can also "reduce the electric space cooling requirements of new homes in hot climates by more than 40%. Because of the energy-saving potential of spectrally selective glass, some utilities now offer rebates to encourage its use. "

Heat Absorbing Glazings

Heat absorbing glazings utilize tinted coatings to absorb solar heat gain, but they still allow some heat to pass through by conduction and radiation. Furthermore, heat-absorbing glass only reflects a small percentage of light.

To reduce the amount of light and heat that enters into buildings, gray and bronze are the most common tint colors used. Compared to other colors of glass, blue- and green-tinted windows allow more visible light, while slightly reducing heat transfer. A major drawback to transmitting less than 70% of visible light is that plants inside could die or they may not grow as fast. Because black-tinted glass absorbs more light than heat, it should not be used in very hot climates.

Reflective Coatings

Reflective coatings significantly reduce the amount of daylight that passes through clear glass. Generally, reflective coatings block more light than heat, but can also slow the transmission of heat when applied to tinted or clear glass. In very hot climates, reflective glazings are frequently used because of the great need of solar control. The downside to this is that the reduced energy demands for cooling can be offset by the resulting need for additional electrical lighting.

Superwindows

Superwindows combine multiple low-e coatings, low-conductance gas fills, barriers between panes (which reduce convective circulation of the gas fill), and insulating frames and edge spacers in order to attain high thermal resistance. These windows allow optical properties such as solar transmittance to be customized for specific climate zones. A small amount of heat from diffused winter sunlight can allow superwindows to be net suppliers of energy. Superwindows that are now available have a center-of-glass R-value of 8 or 9, but have an overall window R-value of only about 4 or 5 because of edge and frame losses.

Smart Windows & Thin Films

"Smart windows" utilize chromogenic (optical switching) glazings that change in color depending on the amount of light that passes through them. They darken in bright light, and lighten in dim light. The unique technology of smart windows is characterized by the ability to vary radiant energy (in the forms of visible light as well as solar energy) by electronically changing the window from being transparent to opaque. There are two catergories of smart windows: those with passive glazings and those that are active.

Passive glazings can vary the amount of light transmitted through a window by altering the window's characteristics according to changes in sunlight (photochromic). These glazings regulate heat transmittance according to ambient temperature swings (thermochromic).

Some electrochromic devices (which are similar to thin transparent batteries) utilize thin films that are placed between two sheets of glass. The film optically dsiplays its state of change as the electrical field within it is altered, switching the window's tint between transparent and opaque. These films are approximately 100 times thinner than a human hair.

By implementing smart windows into buildings or cars, an occupant is able to control interior light levels and the flow of infrared heat. This would reduce the energy necessary for lighting, heating, and cooling. Additionally, glare could be controlled, and windows could provide privacy without the use of blinds or drapes. "The first electrochromic products, rear-view auto mirrors that automatically dim bright lights at night, are now on the market, and switchable auto sunroofs may be available soon."

For the last decade, Sustainable Technologies Australia (STA) has been working on prototypes of smart windows, which change color with the intensity of the sun. The company's performance testing has shown that the glazing eliminates 95% of near-infrared in the dark blue/green state. This is an achievement which has not been equaled by any other type of glazing currently available. Before STA's SMART window, "the biggest drawback (was) the cost of providing a power source and controls for the windows." "NREL researchers worked on various ways of self-powering electrochromic windows with solar cells or photovoltaic (PV) films." 

Image used with the permissions of
Sustainable Technology Australia, Ltd.

"Other switchable glazings need a constant flow of electricity to maintain their visible state, but the STA SMART Window is highly energy efficient. An 'EC layer' has a memory that retains the switched state without continuous applied power. Therefore, electrical voltage applied for a short time will activate the darkened 'heat reject state' (in which the window transmits sufficient clear light for human comfort and visibility but rejects virtually all hot infra red radiation) and the return to the colorless 'low-e state' (in which the window transmits most of the light but rejects haze heat while maintaining comfortable room conditions) when required. The product is SMART because it can respond to changes in the climate by switching between the two states. STA SMART Windows use low voltage DC to operate and consume virtually no power compared to other switchable glazing."

In the near future, STA SMART Windows can lead to energy savings in excess of 30% in a wide range of latitudes, with savings from cooling, lighting and heating applications. The main use of smart windows will be to balance needs for energy-wasteful air conditioning in buildings in places where overheating may be a problem during summer, especially in office buildings with large glass facades, and during winter, when it is necessary to decrease the heat loss from the interior of buildings through windows.

A second applicable thin-film product in the area of building integrated photovoltaics (BIPV) and windows is the PowerView^™ PV laminate products made by Solarex which is now part of BP Solar.  They have been described as "the perfect building materials" by architects since they blend the functionality of glazing products as they pass desired levels of natural light into a building's interior while blocking the remainder and silently converting it to electricity for use in your building. This is particulaly important since getting the right ambiance can improve peoples quality of life and productivity in working, shopping and living environments.