Energy-efficient windows: no overnight success story

Technology and consumer demand have always been the driving force for innovation in residential windows. Over the years they have helped advance the aesthetics, energy efficiency and more recently the eco-friendliness of the glass and frame materials. Windows evolved first from uncovered holes in the wall to openings covered with tightly stretched animal skins, and now to the energy-efficient, low-maintenance, glass units of today.

Glass windows evolve

The single most important component of any window is glass, the material around which all modern windows are built. The Romans were the first to use it as a window material. It wasn't until the 1600s -- when English window makers began putting small panes of glass together with strips of lead -- that windows got bigger. Plate glass was introduced in the latter half of the nineteenth century.

However, it was not until 1945 that window technology began to really take off. PPG, formerly Pittsburgh Plate Glass, produced the Twindow®, so-named because it consisted of two pieces of glass that sandwiched dead air space between them. Thus, the first insulated glass window unit was born. Soon, glass manufacturers and fabricators began making a glass unit with gas inside the air space to further improve thermal effectiveness. Andersen windows used that technology in the early '70s -- a Freon-type gas -- and later, a carbon dioxide mix. The industry eventually settled on argon gas as the standard, though more exotic choices like krypton gas are available.

In 1989, a low-emissivity -- low-e -- reflective coating came into use. It filters out the radiant spectrum of light to keep windows from conducting too much heat from the sun. Double and triple coatings are now in use to further improve reflectivity and thermal performance while maintaining transparency.

Frame and sash materials

When double hung windows were first introduced into Georgian-style homes in England, they were built on site and the frames were made of wood. Completely factory-made windows were not available until 1937 when Andersen sold the first factory-glazed and assembled wood casement window. Factory-made wood windows and World War II technology came together soon after and paved the way for improved window frame materials.

1. Clad wood. Wood has been around as a window frame material for centuries, but it needs a finish coating or paint to help prevent deterioration. Innovative manufacturers found other, more durable materials with which to clad the wood and protect it. Today Weather-Shield and Marvin use an extruded aluminum that covers the outside section of the wood frame and sash members. Several others such as Pella and Pozzi use roll-formed aluminum; however, it's a little thinner and less dent-resistant than extruded cladding. Two manufacturers, Andersen and Ply-Gem, offer vinyl cladding.

2. Aluminum. Aluminum window production began after World War II as an inexpensive alternative to wood. It did not require painting -- or if it was painted, did not require repainting. However, aluminum conducts heat and cold 1,100 times more than wood, causing frost and condensation on the windows in cold, winter climates. The aluminum industry introduced window frames with a thermal break to solve that problem -- a section of an inert, non-conductive material divides the frame into an inside and an outside section. Aluminum windows today are still used for many commercial applications but less for residential windows.

3. Vinyl. Vinyl windows were introduced in 1954 post-war Germany. Thermal Industries made the first vinyl windows in the U.S. ten years later. Today, vinyl is the second most-produced window material. Vinyl is much more thermally efficient than aluminum, is less expensive than wood to manufacture, can be extruded into many shapes, requires little maintenance and can be recycled, both pre-consumer and post-consumer.

4. Fiberglass. Designed with various hollows that trap air like vinyl window frames, fiberglass windows are able to provide nearly as much insulation as wood frames. Their manufacture consumes much less energy than that of vinyl lineals. In a process called pultrusion, fiberglass lineals are pulled through a die, and at the same time, encased in a heat-set resin. As a result, fiberglass is currently the strongest window frame material available.

The rise of energy-efficient windows

Energy Star-rated windows for new homes were first offered in 1995. Energy Star guidelines focused on improvements in several key areas of the home, including high-performance windows.

Energy-efficient windows now combine advancements in frame and glass in one window designed to replace single-glazed aluminum windows and storm windows. Vinyl, aluminum-clad wood, extruded vinyl, and now even fiberglass are the preferred materials for these replacement windows that are meant to lower energy use and utility bills. Painting is all but eliminated, and frost caused by condensation and cold intrusion is kept outside by thermally improved sash and frame members.

Glass has gone from single to double panes; then to double panes with gas infill, and in some cases, triple glazed windows -- three panes in the same sash.

And insulating values have gone from R-1 to R-5 -- even in some cases R-9.

In an effort to encourage more energy-efficient windows, the Department of Energy launched in 2010 the "Highly Insulating (R-5) Windows and Low-E Storm Windows Volume Purchase Program." This voluntary program requires windows meet minimum insulating and air infiltration values -- both well below what is considered the current U-value norm of .22 for operable windows and .20 for fixed windows.

Environmentally conscious window manufacturing

Energy savings are inherent in buildings outfitted with the best windows available today, but window evolution has not stopped there. Manufacturing practices are more environmentally conscious with many window companies sourcing materials locally to lessen energy consumption for transportation. Recycled vinyl is used by most vinyl lineal extruders. Some wood window manufacturers use their sawdust by-product to heat their facilities. Glass often contains a portion of both pre- and post-consumer recycled product.

Window development is constantly being driven by demand for easier-maintenance products that deliver better and better energy savings. New, cutting-edge materials like electrochromic glass -- glass that darkens as the outside light increases -- hold promise of further decreasing energy consumption. Government programs, too, have encouraged development of products such as the current generation of low-e3 glass coatings.

From animal skins to single glazing; to low-e coated gas-filled, insulated glass units; from wood to aluminum; to thermally efficient vinyl and foam-filled fiberglass -- windows just continue getting better.