Residential and commercial buildings need windows to improve occupant comfort. This is especially true in commercial office buildings where people spend long hours sitting at one place. The occupant comfort factor has to be balanced with energy efficiency of the building as some 30 percent of the energy used to heat or cool a building is lost through its windows. Double pane windows, made of two sheets of glass separated by a half inch air gap set into a common window frame, helps reduce energy loss. Changing single pane windows to double pane can help reduce energy costs and help reduce the carbon footprint of the building.
Energy efficiency of windows
Conduction and Convection losses
The National Fenestration Research Council (NFRC) has established a set of rating factors to evaluate different designs and makes of windows for energy efficiency. The most important of these is the U-factor, which is a measure of heat loss through a window due to conduction through the metal window frame and by air convection currents on the window pane surfaces. NFRC ratings for the U-factor range from 0.2 to 1.2 with a low number indicating lower heat loss.
When a building is heated in the winter months, the temperature difference between the inside and the outside could be between 30 and 50 degrees. This temperature difference would cause heat flow from the inner parts of the the window frame to the surfaces exposed to outside air. In modern window frames, polymer shims and plastic connecting components are used to break up these thermal conduction paths. Windows for residential applications sometimes use wooden or vinyl polymer frames that have low thermal conductivity but for the large size windows used for commercial buildings, aluminum or steel frames are standard.
In addition to heat loss through conduction, losses take place through convection currents. The air in contact with the cold outer glass pane, warms and floats up and gets replaced by colder air. This continuous flow of air causes convection losses. The opposite effect happens when the building is cooled in summer. The outer parts of the window frame are some 20 to 30 degrees warmer than the inside surface and heat flows inwards, increasing the cooling load. Convection air currents are formed on the inside window pane surface. A double pane window with the air column between the two glass panes does not have the large temperature differential that a single pane window has and consequently has less convection loss.
A single pane window typically has a U-factor of 1.07 whereas a double pane window from the same manufacturer has a U-factor of 0.49. These numbers indicate that heat loss through a double pane window is 50+ percent lower.
Solar Heat Gain
In daylight hours, sunshine provides not just light but also radiates heat into the building. A part of the radiant heat is reflected by the window pane but a substantial amount of the heat does get through. This radiated heat could help reduce heating load in winter months but adds substantially to the cooling load in summer.
For single pane clear glass windows, the Solar Heat Gain Coefficient (SHGC) is 0.86. When this is changed to a double pane window, the SHGC reduces to 0.76, suggesting a 12 percent reduction in solar heat gain. If the outer glass pane has a reflective coating, the SHGC factor goes down to 0.6.
The let through of visible light
Reducing the solar heat also ends up reducing the visible light let through that is needed to cut back on electric lighting. The Visible Transmittance rating (VT) is the measure of the percentage of sunlight falling on a window that goes through the window pane. A single pane clear glass window has a VT factor of 0.9, which means that 90 percent of the light falling on the window pane goes through. When a second glass pane is added, the VT factor reduces to 0.81 and adding a reflective coating to the outer pane lowers it further to 0.75.
Light coming in through window panes is also influenced by geographical location. In most parts of the US, north facing windows do not receive direct sunlight but only light reflected from the sky that does not have the heat component. Most south facing windows would receive direct sunlight and heat. East and west facing windows would get direct sunlight for certain periods of the day and that would change with seasons. In the UK, the sky is overcast for most of the year. In tropical countries, windows would be recessed or covered with shades to reduce solar heat and light. Good building design would factor these location factors into the architecture.
Other benefits of double pane windows
One additional benefit of double pane windows is noise reduction. Large commercial buildings are often located adjacent to busy roads and traffic noise can be a major irritant. A second benefit is reduced moisture condensation on the window pane. When there is significant temperature difference between two sides of a glass pane, water vapor from the air condenses on the colder surface. This condensation can cause dirt streaking on the window pane and damp patches on the window sill and the adjacent floor. In a double pane window, the temperature difference between the two sides of the window pane are smaller and there is less condensation.
Double pane windows clearly offer benefits over the single pane windows in reducing energy loss through windows in heated building and through reduced solar heating in cooled buildings. The actual savings to be realized in a commercial building depend on building design parameters. For a benchmark, it has been estimated that a 4000 square foot single family home in the US could save over $350 per year in energy costs and thereby achieve a carbon emission reduction of 680 kg per year by changing to double pane windows.