The purpose of windows is to provide light and/or a view out for the users of the building. When a window is installed, it creates an opening in the wall which affects the indoor environment. This post is meant to offer a brief explanation on some important metrics that governs the windows performance. The three main metrics of the windows are the U-value (R-value in north America), the G-value (UHGC in north America) and the transmittance of visible light, Tvis.
The U-value
The U-value of a window is a metric describing the thermal heat transfer through the window, having the unit of W/(m2K). A lower U-value equates to a lower thermal heat transfer. Factors that affect the overall U-value of a window is the material and construction of the window frame, number of glass panes in the construction, the use of selective coatings (Low E-coatings) and the use of different gases between the panes. The overall performance of the window is also affected by how air-tight the window and the window connections are.
A simplified illustration of the heat transfer through a 2-pane window is shown below. (Main differences between the illustration and reality are that radiation is also reflected, absobed and reemitted by surfaces, the solar radiation has been omitted, and the air leakage is shown as going from the interior to the exterior. While the air leakage would occur in this direction if the only factor was the indoor and outdoor temperatures, in general a ventilation system keeps an under-pressure in buildings and outdoor air is then sucked into the building rather than being pushed out.)
Illustration of heat transfer through window
A typical U-value in newer windows is between 1,0 and 1,5 W/(m2K). An older window has a typical U-value of 1,8-3,0. Renovating an old 2-pane window where one pane is replaced with a pane using a Low E-coating the U-value falls somewhere between a new and an old window. This can be an interesting proposition in cases where it is important to keep the windows appearance. Often it is a less expensive option compared to a full replacement of the windows.
The benefit of a lower U-value is not only a more energy efficient building envelope, but in colder climates it also reduces draft around the windows as the temperature of the interior pane is higher. A phenomenon that can be experienced with newer windows in cold climates is fogging on the outside pane. This is due to a lower temperature on the outer pane, introduced by a lower heat transfer through the window. The fogging is most prevalent after clear nights where the windows radiate heat towards the cold clear sky. The phenomenon of radiant losses to a clear sky is known as sky cooling. Some typical U-values for windows are listed below.
- Single glazing: 5,8 W/(m2K).
Old double glazing: 2,6-3,0 W/(m2K).
Renovated old 2-pane windows with energy glazing: 1,8-1,9 W/(m2K).
New double glazing: 1,3-1,5 W/(m2K).
Old triple glazing: 1,8-2,0 W/(m2K).
New triple glazing: 1,0-1,3 W/(m2K).
Low energy triple glazing: 0,7-0,9 W/(m2K).
The glass itself is very thin and have a high lambda-vale, meaning the glass itself contribute little to the overall R-value of the window. Instead, the insulation is made up of airgaps between the panes. For a single pane of glass it is worth noting that almost the entire insulation is made out of the interior and exterior surface resistance, Rsi and Rse:
The values previously mentioned refers to the U-value of the glazing (Ug) and is the value which are presented by manufacturers. There are however two additional components to a windows U-value (Uw), which are the heat loss through the frame (Uf) and the thermal bridge caused by the glazing edge (Ψe). The equation for calculating the U-value of the window is seen below:
In addition to these factors there is also a thermal bridge introduced when installing a window as the construction around the window is generally less insulated than the wall. Having fewer larger windows rather than many small windows reduces the length of these thermal bridges compared to the window area provided.
As a final note on the U-value of windows; certain elements can be added to improve the energy performance of the window as part of a system. Shutters, drapes, curtains, shades and blinds all affect the thermal performance. A venetian blind for example can reflect heat back into the room while a cellular shade or drape can create an additional insulating layer and airgap. If energy is the main motivator behind a window replacement it would be wise to consider these options.
G-value
The G-value is a fraction of how much energy is being transferred through the window when irradiated by sunlight. A G-value of 0,5 equates to 50% of the energy irradiating the window surface being transmitted into the room. The G-value holds significance in both warm and cold climate. In a warm climate it is often not desirable to transmit large amounts of solar energy into the building as this either increase the indoor temperature or increase the energy use of the buildings AC-system. In a cold climate the solar energy is on the contrary often a welcome contribution that lowers the heating demand of the building. However, even in a cold climate the added solar energy can be too much in the summer months. Factors that affect the G-value of a window are the number of windowpanes and the use of coatings to reduce solar radiation. Electrochromic coating can be used to change the G-value of the window depending on outdoor conditions. The disadvantage of using window coatings is that it affects the view out of the window as well as reduces the transfer of visible light through the window. In colder climates using a fixed coating can be beneficial in the summer but a detriment in the winter where sun and light are sought after. A low-E coating reduces the heat transfer in the form of infrared heat but does allow light in the visible spectrum. Low-E coatings used together with tinted windows can significantly reduce the G-value.
A shading system can block excess radiation during the summer while allowing more sun to irradiate the window during winter. The shading can be placed on the exterior of the building, between the windowpanes or in the interior of the building. Exterior shading is more effective than interior ones as with interior shading will absorb some heat which will be transferred to the room by convection. Light coloured interior shadings are more efficient than darker ones but if the shading is partially translucent a very white interior shading can act as a diffuser and cause visual discomfort in the room.
Tvis
While the G-value describes the % of total energy being transmitted through the window the Tvis describes the % of visible light being transmitted. Like the G-value, Tvis is affected by the number of panes and the use of coatings on the window. This should be a consideration when selecting windows. For example, replacing a 2-pane window with a 3-pane window will reduce the amount of visible light that gain access to the room. While Low-E coatings used to reduce the U-value are selective and not intended to block visible light they do have a small impact on Tvis. Coatings intended to reduce the G-value of the window on the other hand can have a large impact in the Tvis of the window. Another consideration is the cleaning of the windows as the Tvis of the windows gets lower over time due to accumulation of particles on the exterior and interior surfaces of the windows.
From an energy perspective the natural light from the windows can be used to replace electric light indoors. This can be through manual control, where the user of a room simply opts to not turn the light on, or an automatic system controlled by a light sensor. Moreover, the luminous efficacy of natural light is generally quite high compared to electric light sources. In other words, using natural light from windows will result in a lower internal heat gain compared with providing the same amount of light from electrical light sources. This can create savings not only from reduced electric light use but also from reduced AC use. To achieve the reduced AC-use in the northerns hemisphere south-facing windows needs to be fitted with proper shading devices to reduce excess solar heat gain. Here it should be noted that, for example, a 1 m2 window with a Tvis of 80% and a 2 m2 window with a Tvis of 40% does introduce the same amount of visible light into a room but the smaller window will create a higher contrast with the surrounding walls and a narrower light distribution in the room. Smaller windows will however be cheaper than larger ones and introduce lower heat losses due to their smaller area.
Conclusion
Windows and the light they provide are crucial for inhabitants of buildings. With knowledge about their properties and how they affect the indoor environment better choices can be made both for the wellbeing of the residents of the building, the economy, and the environment.