Guide to Energy Efficient Windows
Multiple parameters impact a window's overall energy performance. Frame material and construction play an important role, as well as the glazing that is used. You also need to consider where your project is located because different metrics will be more important in different climates.
Measuring Energy Performance: U-Factor and Solar Heat Gain Coefficient (SHGc)
U-factor is a measurement for the amount of heat that leaks through a window. This is a number usually between 0 and 2 (Btu/h ft2°F). The U-factor of a window (Ut) is the weighted average of both the glass (Uc) and the frame (Uframe). Be mindful of these differences because manufacturers sometimes only list the Uc values on their websites. As a general rule, Acelab will always list the Ut value for the whole window if that data is available.
The Solar Heat Gain Coefficient (SHGc) is a measurement of the amount of heat from solar radiation that enters a building. Lower SHGcs will reduce the need for air conditioning (AC) and lower your electricity bills in warm climates. Like U-factors, SHGcs can be measured for the glass or for the window as a whole. In the latter case, the thickness of the frame and possible grids are also taken into account.
Products that meet the U-factor and SHGc requirements set by the Environmental Protection Agency (EPA) for their climate zone, receive an Energy Star Certification.
How Climate Zones Impact Energy Performance
To effectively estimate how much heat leaks through a window in a year, the U-factor and the SHGc of the glass need to be considered in relation to the climate zone. The climate zone affects the amount of energy needed for both heating and cooling which equals the total amount of energy spent. There are four main climate regions in the United States based on data from Energy Star: Northern, North-Central, South-Central, and Southern.
In the northern zone, where it’s cold, it's most important to prevent heat from leaking through a building’s walls, roof, and windows. The more heat leaks, the more heat needs to be produced. In northern climates, you want the U-factor to be as low as possible. Windows with lower U-factors will decrease your heating bills because they better trap heat inside of buildings. A low U-factor can be achieved by picking argon-filled glass or triple-pane glazing.
In the southern zone, it’s most important to prevent sunlight (solar radiation) from entering a building. When too much sun enters a building, it overheats causing more use of air conditioning which is both costly and requires a lot of electricity. The Solar Heat Gain Coefficient (SHGc) is a measurement of the amount of solar heat that enters a building. In southern climates, you want the SHGc to be as low as possible. This can be achieved by choosing triple glazing or glazing with a low-E coating.
In the north-central and south-central zones, both the U-factor and the SHGc are important and a good balance of the two is optimal.
In short: In cold climates, low U-factors and average SHGc are optimal. In warm climates, average U-factors and low SHGc are the best.
Frame Materials and Construction
The U-factor and SHGc of a window are determined by the performance of the frame and the glass. The performance of the frame is mainly determined by its material and internal construction. Frames that have a thermal break or a honeycomb or multi-chambered design will have better (= lower) U-factors. A thermal break is a material with a low thermal conductivity that’s placed in the middle of the frame in order to reduce the flow of heat through the frame. Thermal breaks come in many shapes and sizes — small, large or multi-chambered. Multi-chambered thermal breaks have a far superior thermal performance.
Aluminum Window without a thermal break (left), with a simple thermal break (middle) and a multi-chambered thermal break (right)
Frame material also plays a big role. Materials like vinyl or wood are better insulators than aluminum or steel. The graph below shows a distribution of U-factors by material for all 7,000 windows in Acelab’s database. Fiberglass windows have the best overall U-factors, followed by vinyl and wood windows. This is mainly because fiberglass is an excellent insulator. Additionally, vinyl and wood are generally better insulators than metals.
Among the metals, aluminum has the highest thermal conductivity (118 BTU/hr.ft.°F), as compared to steel (17 BTU/hr.ft.°F) and bronze (15 BTU/hr.ft.°F). In fact, aluminum transmits heat about seven times better than steel and bronze.
The cross-section of the frame also needs to be taken into account as well.
Aluminum frames are extruded and therefore, can have small flange thicknesses.
Steel can not be extruded. Cold-rolled steel frames are created from sheet metal that is bent, which makes the cross-section usually thicker. Warm-rolled steel frames are not bent but are solid profiles. This makes their cross-section much thicker as well.
For all metals, having a good thermal break is critical to achieving good thermal performance. This is why some aluminum products rank among the best, while others rank among the worst.
How Glazing Affects Thermal Performance
The window’s frame and glazing package also have a big impact on the thermal performance of a window. The number and thickness of the glass panes, their coatings, and the gas between the panes all influence this performance.
Glass with two panes (double-pane) is currently the most common type of window in the US. However, triple glass windows have a superior energy performance because it adds another air cavity and sheet of glass as thermal barriers.
Filling the cavities between the glazing panes with a low conductive gas like argon improves the thermal performance and lowers the U-factor.
Coating the interior of the glazing surfaces with a low-E coating makes the glass reflect more solar radiation back to the outside. This improves a window’s thermal performance in cooler climates by lowering the SHGc.
Because the U-factor and SHGcs of a window are weighted averages of the frame and the glass, a U-factor and SHGc will be listed per glazing option in Acelab’s database. Manufacturers usually offer multiple glazing options for one window frame.
Steps to Choosing the Most Energy Efficient Window
Consider what climate zone your project is located in
Determine whether U-factor or SHGc is the most important for your project
Pick a frame material and construction that best meet your requirements
