Studies conducted by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) show that windows, depending on their performance, can account for 25-30% of the total heating and cooling energy use in a home. Replacing windows just to save energy generally isn’t cost effective due to high unit and installation costs. However, when windows are old, worn out or broken, it can definitely be cost effective in the long run to replace them with the most appropriate units for your home based on energy efficiency, solar exposure, usability and aesthetics.
When learning about your options when it comes to windows it is important to start by doing a little research. Since there a large variety of window products are being offered on the market today, start by learning some of the most critical window assembly parts. A diagram of a standard window is shown below:
- U-Value is a measurement of the flow of heat through a material. The lower the U-value, the better insulating ability of a window assembly.
- Solar Heat Gain Coefficient (SHGC) is a term describing the percent of heat gain through the glass from solar radiation. Measured at a value between 0 and 1, the lower the SHGC the less solar heat the window transmits.
- Visible Transmittance is the property pertaining to the amount of light that is able to pass through a window.
- Condensation Resistance measures how well a window assembly resists the accumulation of moisture. Scored from 0 to 100, the higher the value the lower the susceptibility of water build-up.
- Air Leakage is the rate at which air is able to pass through different parts of a window. The lower the air leakage value, the less air leakage a particular window will experience.
Function and performance
Windows serve several purposes: to protect the home from the elements, to provide daylight and vision, to support occupant comfort, and to allow for natural ventilation. When windows perform poorly a variety of problems can occur:
- Water intrusion causes interior damage such as condensation and mold;
- Air leakage causes drafty and uncomfortable conditions for occupants;
- Excessive solar heat gain causes the air conditioning system to be less effective in cooling the home, increasing energy costs; and
- Excessive heat loss causes the heating system to be less effective in keeping the house warm, increasing energy costs.
If you are experiencing any of these issues, it’s likely your windows need repair or replacement. There is one caveat to this however, on the topic of air leakage. Often people think that when they stand near a cold window it means the window is leaking air, which is not always the case. When the exterior temperature is significantly lower than the indoor temperature like in the case of deep winter weather, convective looping of air as it hits the cold glass surface of a window creates a drafty feeling when, in reality, it may not actually be leaking. If you feel your windows are drafty and they appear to be performing properly visually (i.e. no broken panes, damaged sills, or moisture build-up), hire an energy auditor to test them for air leakage before moving further towards the decision of replacing them altogether.