When people ask me to ball park a price on their heating or cooling system, they usually tell me how many square feet the house or addition will be. Information which means nothing to me.
If they told me how many square feet of windows the building would have, then I would know quite a bit because that is where most of the energy goes, thru the glass.
Storm windows are the minimum, double glazing is acceptable. Low-e is even better. Leaky panes, or windows that won't lock are almost open.
Window efficiency has improved dramatically in the last twenty years.
Glass layers are known as glazing. Single glazed means one layer of glass. Double glazed means two layers of glass, etc. Argon gas is used to replace air between the layers of glass because it is dry and inert, meaning it will not corrode or decay. The inside of one layer often has a metallic coating which gives the glass an E for emitance value. Low-e glass has an improved insulating value over regular glass.
There are three basic styles of low-e glass. High solar gain low-e, moderate solar gain low-e and low solar gain low-e. Their application depends on how much solar heat you wish to enter the building. In warmer climates, where cooling is the primary concern, low solar gain would be chosen over high solar gain. High solar gain would be more useful in colder climates where one would want the heat from the sun to enter the building.
The direction a window faces is also a factor. Even in warmer climates, high gain windows may be installed on south facing walls, thus allowing winter sun to shine in. These windows will not increase summer air conditioning loads because the sun is higher in the sky during the summer and its rays will not shine thru. The east and west facing windows can be low solar windows for the opposite reason.