Double glazing or insulated glass in the fenestration (doors, windows, skylights) of any building will save money on energy usage. Tests of building structures using heat imaging have shown that the greatest energy loss is through the roof, the doors and the windows. Most buildings are now constructed with some type of insulation in the attics and in the exterior walls, or this can be readily added. Energy efficient window and doors may be installed at the time of construction, or they may be added on older structures.
Heat is transferred, either into or out of a building, though openings (convection) in the building or through conductive materials used in the exterior building envelope (conduction). Insulation in the attic and outer walls takes care of some of this transfer, energy efficient doors and window takes care of a lot of the remaining energy loss.
The energy efficiency of double glazing or insulated glass comes from the dead-air space between two panes of glass. The larger the dead-air space, the more energy efficient the glass, as the heat is transferred less readily through the dead-air than it is through a single pane of glass. The terms “double glazing” and “insulated glass” are interchangeable in laymen’s terms, but to window and glass manufacturing professionals, they are two different things.
Double glazing – two separate window units (think prime window and storm window) that provide a large dead-air space. These are quite energy efficient, but there may be heat transfer through the sash materials and around weather stripping.
Insulated glass – two pieces of glass, a separator filled with a moisture absorbent desiccant and a sealant, combined into a “sandwich” that is glazed into a frame or sash built to accommodate the thicker glass unit. Energy efficient window frames – The most energy efficient window framing material is that of a non-conductive nature; wood, vinyl (PVC) or thermally-broken aluminum. The most cost effective is PVC, due in part to the ease of assembly of the frames and marine or drop-in glazing of the glass units. The framing material is virtually non-conductive and modern weather stripping assures a minimum of air transference around the operating sashes.
With the rise in energy costs and concern about the economy in general, there is renewed interest in making homes and other buildings as energy efficient as possible. New double glazing window units are a quick and relatively economical means of reducing energy costs. The energy savings of replacement windows will often payout in just a few years, or even less with rising energy costs taken into consideration. An added value is the reduction of outside noise and the lack of draftiness, two items that taken alone, justify the cost of replacing the old, drafty, energy leaking windows.
Before and after energy audits of homes with replacement windows and new door units, show a dramatic decrease in the heat transfer, making them nearly as energy efficient as well insulated walls and ceilings. So the real question is, not if you should replace your windows, but when you should. Talk to a window professional for assistance on what type is best suited for your building and your needs.