In most old homes windows are one of the largest sources of heating during the winter do to their low insulating ability combined with high air leakage. During the summer they are also a major source of additional unwanted heat. This results in windows being typically net energy loser, that can be responsible for 25% to 50% of the energy needed to heat and cool your home. There is hope in the form of new and improved windows and giving proper consideration for the actual placement of them can result in making them a positive in helping to maintain comfortable home environment.
If you are considering replacing or adding new windows there are some basic factors to consider. They can include glazing type, orientation, shading, total area of windows and their insulation values, plus the actual installation method. You can access the National Fenestration Rating Council (NFRC) directory of windows and doors by clicking here. They are a non-profit organization with a directory of over 1.6 million products and 500 manufactures that have been rated and tested. They provide ratings that indicate the whole window U-value. This not only includes the insulating of the glazing and frame, but also low e-glazing, gas fillings, tinting and films. The information includes product types, number of glazing layers, presence of low-e and value, spacer type, and gap fill and spacing.
Some other considerations besides that of the actual windows chosen are air leakage and placement/orientation. Air leakage cannot only occur around but through your windows and can have serious effect on your energy efficiency and comfort level. Weatherstripping and caulking can be done around existing windows to improve the air tight seal desired. New windows are routinely checked for air leaks between their frame and sash. Leakage rates between .01 and .06 cfm/ft of perimeter are considered better windows. Windows types with lower rates of air leakage are hopper, casement, and awning then double hung or sliders. Check information provided by the manufacturer for the information on air leakage.
A design considerations that can provide a net energy gain for your windows is to allow the sun light to enter the windows during the winter and block it during the summer. As the sun is lower in the winter then in the summer placement and depth of windows along with awnings can have a huge impact on your heating and cooling. You want to reduce the area of windows on the east and west sides to help avoid summer heat gain. Use overhangs, awnings or vegetation, such as trees, to shade your windows. Limit your use of skylights as in summer they add heat while in winter not enough.