BuildingGreen.com’s Alex Wilson has been publishing a series on the energy performance of windows. In Part 4, Wilson discusses the materials that seal and insulate the edges of the layers of glass in insulated glazing units, or multi-paned windows.
In the olden days, when windows were single-glazed and wood-framed, the window sashes insulated better than the glass. With the air films on both sides, an inch-thick wooden window sash provides about R-2, while a single layer of glass provides just half that. When we switched to double glazing, the glass and wooden sash insulated about equally.
With the advent of low-e coatings and low-conductivity gas fills, though, the glazing itself became better insulating than the frames and edges of the glass. All of a sudden, instead of the glass being the weak point, in terms of heat loss, the glass became better-insulating than the edges of the windows. A significant culprit of that window-edge heat loss is the heat-conducting glazing spacer that holds the two pieces of glass apart.