Windows are openings in walls, doors, or vehicles that allow the passage of light. Windows are important for a household's energy efficiency because they're how homes let heat and light in and out. Proper window placement and type are essential for energy efficient building design making it a crucial part of the building envelope.
Operable windows can be opened and closed and also allow the passage of air and sound through them from one environment to another. Some examples of operable windows include casement and hopper windows. Windows that don't open are called non-operable (fixed) windows. Operable windows are more desirable in residences as they can be opened to help ventilate the home.
There are many parts that come together to make a window. Each part must be able to perform its own unique function as well integrate into the window as a whole.
|Frame||Creates an opening that houses the window components and consists of the head, jambs and sill|
|Glazing||The term used to refer to the transparent material of a window. The glazing can sometimes be covered in different coatings that can alter its energy efficiency.|
|Grilles/Grid/Muntin||Divide the glass of the window into smaller panes. In modern windows they are primarily used for aesthetic purposes only.|
|Head||Forms the top of the window frame|
|Jamb||Forms the vertical sides of the window frame|
|Sash||A framework that holds the various panes of a window within the window frame itself|
|Sill||Forms the bottom of the window frame|
With relation to windows these types of heat loss occur as described below:
Thermal Radiation - The glazing of the window absorbs heat and re-radiates it to the internal or external environment, whichever is cooler
Conduction - Heat moves through solid parts of the window such as the jambs, head, sill, and glazing
Convection - Heat that is lost from air movement near to and in the space between the glass
Air Leakage - This accounts for the heat transferred when air moves through the gaps in the frame due to shortcomings in the installation or manufacturing of the window
There is no way to completely stop heat loss through windows but there are ways to significantly reduce it.
For more detailed information about how to make windows, doors, and skylights as energy efficient as possible, see Natural Resource Canada's guide book here.
One way to reduce the heat lost due to air leakage is by weatherstripping the window. Weatherstripping is the use of a seal between the frame and the sash of the window. There are three basic type of weatherstripping seals: brush, sweep, and compression. Most operable windows use compression seals as the primary air barrier.
In addition to decreasing the air leakage of windows, weatherstripping also keeps rain out and helps in reducing noise exchange from the surroundings with the interior of the home.
The glazing is the glass in the window. Using glazing with various properties can drastically affect the performance of the window.
Typically air occupies the space between sheets of glass in a double or triple glazed window. However, special gases such as Argon or Krypton can replace the air to improve its thermal efficiency. Filling the space between glazing layers with these inert gases will reduce conductive heat losses because they have a lower thermal conductivity than air. The use of these gases will also lower convective heat losses because they are denser than air and will suppress gas movement between the panes of glass. Argon has 67% of the thermal conductivity of air, is non-toxic, and is relatively inexpensive, making it a common filler gas. Krypton is a more effective insulator than Argon but is more expensive resulting in it being less commonly used as a filler gas.
A special coating called an e-coating can reduce the amount of light and heat transferred as a result of thermal radiation.
Kits can be purchased that contain sheets of plastic and a means to attach these sheets to an existing window. To save money, the option also exists to use everyday saran wrap and a hairdryer to perform effectively the same task. Kits generally include a roll of thin, clear plastic and double sided tape to attach the film to the windows, preventing cold air drafts from entering through older windows when a blow dryer is used to shrink the wrap onto the window. This method of preventing cold air from entering the home is especially effective in the winter months.
The energy performance of a window depends on how much light and heat are allowed to move across the window. There are a few metrics that can be used to measure the energy performance of windows. The ones that are most often used in Canada are:
The U-value depicts the rate at which heat is transferred from warm to cold areas. The lower the value the higher the energy performance of the window. The units of the u-value are watts per square meter Kelvin. (W/m2K)
R-value is a value of thermal retention, and stands for resistance value. Housing insulation is rated in R-value, so rating windows in the same manner allows for easier comparison. The R-value is the reciprocal of the U value (R-value = 1/U value).
The solar heat gain coefficient (SHGC) is a ratio that represents that amount of sun’s heat that can pass through the window over the total incident solar radiation (insolation). The lower the value the less solar heat the window will transmit. This can be modified with a low emissivity coating.
Visible light transmittance is a ratio that measures the amount of visible light that can pass through the window over the total incident visible light. The higher the value the more visible light can pass through it.