Most people don’t know that windows started out as just holes in the walls of the primitive houses of nomadic tribes. The word window comes from an Icelandic word that means the wind’s eye. Windows were first designed as a way to let in the flow of wind and light. Later, as people became less nomadic and spent more time in their houses, windows evolved from a hole-in-the-wall structure to a structure that allowed the inhabitants of a house to be protected from outside elements. Windows made out of paper, cloth, and animal bones were widely used in ancient Asia and Europe. The Romans were the first to use glass for windows, which later became a symbol of wealth all over the world.
Glass windows can be effective in meeting the dual function of protecting the inhabitants of a house from outside elements without sacrificing the inflow of wind and light. There are generally five classes of glass windows: sash, casement, jalousie, stained, and skylights.
- A sash window is constructed with one or more movable panels or “sashes” that form a frame to hold panes of glass which are often separated from other panes by narrow strips of wood or metal. This is the traditional type of window used in most homes in the United States.
- The casement window is a type of window that is attached to its frame by one or more hinges. Casement windows were the most common type of windows in the United States before the sash windows were introduced.
- The jalousie window consists of parallel glass louvers set in a frame. These louvers are usually thin and flat by design. Jalousie windows are best-suited for porches that are not climate-controlled or houses that are located in mild-winter climates. These window types are common in homes in Florida, southern California, and Texas. They can remain open when it’s raining because they keep most of the rain from entering in based on the thin, flat, and outward protrusion of the louvers.
- The stained glass window is a popular architectural feature of churches and cathedrals and less common in residential buildings.
- Skylights indicate a glass structure built into the roof area of a house to allow the natural light coming in from outside to provide the internal lighting of the house.
Window Performance Criteria
Today, the best types of glass windows for homes are those that perform well to block the undesirable outside elements such as extreme heat without blocking the desirable outside elements such as natural lighting. Windows typically take up 10 to 25 percent of the exterior wall area of new homes. In essence, windows are critical to how today’s homeowner consumes energy. Research shows that windows in heating-dominated climates account for up to 25 percent of a typical house’s heating burden; and windows in cooling-dominated climates account for up to 50 percent of the house’s cooling burden.
The good news for homeowners is that recent technological advances have enhanced the thermal performance of windows. These advances include improvements in window framing and low conductance materials, low emission and solar control coatings, thermal break and edge spacers, and forward-thinking sealing techniques.
- Low conductance materials, such as wood, vinyl, and fiberglass, perform better than high conductance materials such as aluminum. But if aluminum frames must be used as is sometimes the case in heating-dominated climates, then look for thermal breaks in the frame design to avoid condensation. And finally, insulated frames perform better than those that are not insulated.
- Low emission, solar control coatings, and sealing techniques help to minimize heat loss in heating-dominated climates and in turn help to minimize solar heat gain in cooling-dominated climates. So windows with lower U-factors perform better in heating-dominated climates whereas windows with lower Solar Heat Gain Coefficients (SHGC) perform better in cooling-dominated climates. SHGC is a measure of the amount of solar energy that a window allows to pass through it. And U-Factor is a measure of how well a window prevents heat from escaping i.e. the rate of heat loss.
Tips on Purchasing
Energy efficient windows, specifically ENERGY STAR rated windows, can help to reduce a homeowner’s energy bills by up to 15 percent. For these savings in energy bills to be realized, windows and skylights must meet the appropriate U-Factor or SHGC requirements based on climate zone.
Here in the United States, there are four climate zones for qualified windows and skylights:
- Northern: colder climates, demanding mostly heating windows
- North/Central: temperate mixed climates
- South/Central: temperate mixed climates
- Southern: warmer climates, demanding mostly cooling windows
- Northern climate: U-factor: <0,35; SHGC: any
- North-Central climate: U-factor: <0,40; SHGC: <0,55
- South-Central climate: U-factor: <0,40; SHGC: <0,40
- Northern climate: U-factor: <0,65; SHGC: <0,40
- Northern climate: U-factor: <0,60; SHGC: any
- North-Central climate: U-factor: <0,60; SHGC: <0,40
- South-Central climate: U-factor: <0,60; SHGC: <0,40
- Northern climate: U-factor: <0,75; SHGC: <0,40
Benefits of Purchasing and Installing
In addition to the savings on energy bills, many people fail to realize that some of the benefits of high-performance, energy-efficient windows go to the very heart of what it means to be a homeowner.Â In other words, what does it mean to have more than just a hole in the wall?
- Peace and quiet. The multiple-layered designs in the insulated and glazed frames of high performance windows block unwanted noise from outside.
- Better air quality. The sealing techniques employed by high-performance windows reduce air leakage into a house which in turn reduces the dust or dirt particles that invade many homes today.
- Less wear on home furnishings. Low emission coatings can block up to 98 percent of the ultraviolet rays emitted by the sun.Â This radiation can cause curtains, carpeting, and furniture to fade and wear faster.
- Higher house resale value. The advantage of a more energy-efficient house can mean higher resale value.
Purchase and installation is the first and perhaps most difficult step in accessing some of these benefits.Â Thankfully, there are resources available to help homeowners such as the federal tax credit.Â Windows and skylights installed by December 31, 2010 can qualify for the federal tax credit of up to $1500.Â We advise homeowners to seek a mean green contractor that can assist with the purchase and installation of these high-performance, energy-efficient windows and skylights.