Ambient heat transfer can occur via three methods; convection, conduction and radiation. The weakest resistance to these mechanisms, in a building envelope, is the glazing material.
By nature, a single piece of glass has little resistance to either heat gain or loss; however, an insulated glass unit (IGU), comprising of two or more panes of glass with hermetically sealed air spaces in between, slows down the rate of heat exchange between the ambient hot and cold air masses on either side of the unit. The resultant reduction in heat transfer minimizes the capital and subsequent operating costs of heating or cooling equipment.