Windows make up on average 15 to 40 percent of the wall area in a house, and have the potential to allow large heat losses. In the average house, heat losses through windows can represent 22 to 37 percent of the total heat lost. Careful window selection when building a new house or renovating an old one can make a significant difference by helping to reduce heat loss.

If you are replacing or adding windows, take time to research the selection available to you. If you're considering new windows, it really makes sense to look for your best options in energy efficiency. Windows come in a variety of shapes, sizes, and material combinations. Each type of window design has different attributes that affect the overall performance. Knowing what to look for in a window is not an easy task, given that window technology has undergone rapid developments in the past few years.

The efficiency of a window is impacted by:

• The size of the window
• The performance of the glazing
• The frame construction
• The spacers

The standard double-glazed sealed unit is quickly being replaced by windows with improved thermal performance, which are known as high-performance windows. There are several features to consider when selecting high-performance windows. You can choose from single, double and even triple pane windows, as well as the insulating gas used in between the varying window panes.

A couple terms you may hear include Low-E and Argon filled. Low-E glass prevents the heat inside a room from escaping. The Low-E coating also filters out most of the damaging ultra-violet sunlight, which helps to reduce fading of furniture and drapes. Highly efficient windows may use argon gas to fill the space between panes of glass in double and triple glazed windows. The gas has a higher insulating value than air.

Following is a table setting out the R value associated with different types of windows:

No matter how much you spend on the windows, if they are not installed properly, snugly and insulated, they won’t work at their full potential. What this means is that the window frame has to fit snugly with the jam. By keeping this gap as small as possible, you will need to fill in less space with insulation. This insulation can be foam or fiberglass, but you have to have it put in underneath the window’s trim; both inside as well as outside.