The Role of Windows and Doors in Energy Efficiency
Windows and doors don’t use energy themselves so they’re considered passive products, but by allowing the transfer of energy they severely impact a building’s heating and cooling load. In fact, regardless of how well a building is insulated, windows are the most vulnerable to energy loss. This means it’s vital to give careful thought to windows and doors during the design and building stages of any project.
How is a window’s energy efficiency measured?
One of the most important ways to reduce a home’s energy consumption is to use the right windows to limit heat gain or loss. There are four main measures for a window’s energy performance, all of which take into account the whole window system (frame, glass seals, spacers, etc.).
U-value (sometimes referred to as heat transfer coefficient or thermal transmittance) is used to measure how effective the windows is as an insulator. That is, how effective it is at preventing heat from transmitting between the inside and the outside of a building. The lower the U-factor, the more energy-efficient the window.
SHGC – Solar Heat Gain Coefficient
Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window, either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun.
AI – Air Infiltration
Air Infiltration is the rate of air movement around a window, in the presence of a specific pressure difference across it. A product with a low air leakage rating is tighter than one with a high air leakage rating. Note that air leakage also depends on the proper installation of a window, which is assumed in all ratings.
VT – Visible Transmittance
Visible transmittance is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window should be determined by your home’s daylighting requirements and/or whether you need to reduce interior glare in a space.
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What affects a window’s energy efficiency?
A window’s U Value and SHGC are affected by the following factors :
Frame Material Conductivity
This is a material’s ability to conduct energy. The lowest rated is timber and the highest is aluminium, which is 1000 times more conductive than timber.
Window Design and Frame Size (height and depth)
Some window designs are inherently more energy-efficient than others. Even when same framing materials are used, the U value and SHGC will vary depending on the size and the design of the windows.
The importance of installation cannot be overlooked when discussing window efficiency. Even the most expensive window unit won’t perform effectively if it’s not installed correctly. However, the seals will only affect Air Infiltration, not the U value or SHGC.
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