Double glazed windows are made from two panes of glass that are separated by a layer of air or gas and then sealed. They are designed to provide a better barrier against outside temperatures than single paned windows because the two layers of glass and the buffer layer act as insulators. The glass used features a specially-coated surface that re-radiates heat in cold months and prevents heat from entering during hot weather. Originally created for extreme climates, double glazing is now widely used in nearly all locations, both for new construction and as replacement windows.
Many kinds of glass can be used to create double glazed windows, from regular to decorative, but the most common selection is high performance Low Emissivity glass. Low E glass contains a virtually invisible metallic layer in the pane of glass, along with a special coating that can be selected in different glazes to accommodate a range of climates. High-Solar-Gain glass is preferred for areas with cool summers and very cold winters because it reduces heat loss and increases heat, or solar gain. Moderate-Solar-Gain glass lets in a smaller amount of heat without losing any light and is often chosen in areas with mild summers and winters. Low-Solar-Gain glass offers the highest protection from UV rays and is selected in areas with extremely hot summers and mild to moderate winters.
Double glazed windows with an inert gas — most often argon — between them are considered a better insulator than those with just a sealed pocket of air. Gases like argon allow less heat to escape and less cold to enter because they have a higher density, therefore offering a better buffer than air. Double glazed windows created in factories are sealed airtight to further enhance insulation and to eliminate the potential for condensation to form between the panes.
There are many of benefits to double glazed windows, and their presence or addition is generally considered to increase the value of a home. The most significant benefit is their ability to conserve energy and reduce costly energy bills, since they keep homes warmer in the winter and cooler in the summer. They are also relatively low maintenance, offer better insulation against noise, and provide a measure of increased security, since each layer of glass is another barrier against intruders. Windows with high UV coatings also protect people and prevent furniture, floors, rugs, and photographs from fading and sun damage.