As long as there have been windows in buildings it has been recognized that they are a source of draftiness, heat loss or gain, migration of air contamination as well as their original purpose, to provide light and ventilation. Originally, windows were simply openings in the building envelope that allowed in light. They were closed with wooden shutters and covered with draperies to help keep out the cold. The advent of glass allowed for the development of window sashes that permitted light to enter, but kept out (some of) the cold when closed.
The first attempts to provide better energy efficiency and eliminate some of the draftiness were storm windows. Originally, these were simply wooden frames, glazed with a single sheet of glass, that were mounted over the exterior of the existing window. They effectively provided "double glazing" but no ventilation, so it
was an annual ritual in the Fall to install the storm windows and in the Spring, to remove them. Later developments provided operating sash storm windows in which the meeting rail, or where the sashes came together, closely aligned with that of the prime window. These were quite effective, but still allowed some air convection through loose-fitting sashes and poor weatherstripping.
Modern insulated glass came into being in the early 1970s. Two pieces of glass, a separator and a sealant were formed into a single unit "sandwich" that was glazed into sashes designed to accept the thicker pane. The dead-air space in the sandwich reduced the heat transference through the glass (conduction), but windows were still drafty at the meeting rails and around the edges of the sashes (convection). Wool pile weatherstripping with Mylar fins helped eliminate much of the convection and the modern, energy efficient window came into being.
The insulated glass units were originally about 1/2 to 5/8 inches thick. Testing showed that the greater the dead-air space in the sandwich, the more energy efficient the unit, so windows were redesigned to accept thicker panes of insulated glass, up to 1 inch in depth.
It was reasoned that if two lites (panes) of glass reduced conduction, three lites would be even more efficient. . . and triple-glazing was born. Because of frame web and sash weight limitations, this was usually applied to commercial structures with fixed glass. Residential buildings were usually were fitted or retrofitted with "double-glazing" in the windows.
In and effort to improve the energy efficiency of window glass a clear film was suspended between the lites of glass in an insulated unit, to further reduce conduction. This improved the energy efficiency markedly. Then came "low emmissivity" glass. It was discovered that glass could be coated with a microscopic film of metal that would reflect heat without reducing the visibility very much. Depending upon the climatic conditions where the windows were to be installed, the Low-E glass would have the coating on either the number two or three surface in the glass unit, i. e. in a warm climate, the surface coated would be the inside of the inside lite of glass in the insulated sandwich, thus reflecting the heat to the outside. In a cooler climate, the inside of the outside lite of glass would be coated to reflect the heat to the inside.
It has been estimated by the window industry that modern insulating glass has reduced the energy loss through windows by as much as 80% or more. The fringe benefit is the reduction of outside noise by more than half.
The first attempts to provide better energy efficiency and eliminate some of the draftiness were storm windows. Originally, these were simply wooden frames, glazed with a single sheet of glass, that were mounted over the exterior of the existing window. They effectively provided "double glazing" but no ventilation, so it
was an annual ritual in the Fall to install the storm windows and in the Spring, to remove them. Later developments provided operating sash storm windows in which the meeting rail, or where the sashes came together, closely aligned with that of the prime window. These were quite effective, but still allowed some air convection through loose-fitting sashes and poor weatherstripping.
Modern insulated glass came into being in the early 1970s. Two pieces of glass, a separator and a sealant were formed into a single unit "sandwich" that was glazed into sashes designed to accept the thicker pane. The dead-air space in the sandwich reduced the heat transference through the glass (conduction), but windows were still drafty at the meeting rails and around the edges of the sashes (convection). Wool pile weatherstripping with Mylar fins helped eliminate much of the convection and the modern, energy efficient window came into being.
The insulated glass units were originally about 1/2 to 5/8 inches thick. Testing showed that the greater the dead-air space in the sandwich, the more energy efficient the unit, so windows were redesigned to accept thicker panes of insulated glass, up to 1 inch in depth.
It was reasoned that if two lites (panes) of glass reduced conduction, three lites would be even more efficient. . . and triple-glazing was born. Because of frame web and sash weight limitations, this was usually applied to commercial structures with fixed glass. Residential buildings were usually were fitted or retrofitted with "double-glazing" in the windows.
In and effort to improve the energy efficiency of window glass a clear film was suspended between the lites of glass in an insulated unit, to further reduce conduction. This improved the energy efficiency markedly. Then came "low emmissivity" glass. It was discovered that glass could be coated with a microscopic film of metal that would reflect heat without reducing the visibility very much. Depending upon the climatic conditions where the windows were to be installed, the Low-E glass would have the coating on either the number two or three surface in the glass unit, i. e. in a warm climate, the surface coated would be the inside of the inside lite of glass in the insulated sandwich, thus reflecting the heat to the outside. In a cooler climate, the inside of the outside lite of glass would be coated to reflect the heat to the inside.
It has been estimated by the window industry that modern insulating glass has reduced the energy loss through windows by as much as 80% or more. The fringe benefit is the reduction of outside noise by more than half.
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