Windows and doors are incredibly dynamic elements to a home. They need to contribute to keeping a home cool in summer, warm in winter, provide security, fresh air and natural light.
Windows react thermally via direct and indirect sunlight. When sunlight hits a window, the heat does three things.
1. Reflection - is where sunlight strikes a window and some of the solar heat is reflected back.
2. Absorption - is the process of some heat being absorbed, this is emitted outside and emitted inside.
3. Transmittance - is where the remainder of the heat is transmitted through the window into the building.
Solar Heat Gain Coefficient (SHGC) is the measure of the percentage of solar heat gain that passes through a window. Reflection, absorption and transmittance are factors that affect the SHGC.
To make this clearer, in a cooler climate, windows which have a high SHGC allow a greater amount of solar radiation to pass through which heats up the home.
The other element to rating a window based on the frame and glass is known as the U-Value. A U-Value is the measure of how much heat energy is transferred through a window. Heat can be lost and gained through a window by the processes of conduction, convection and radiation. The lower the u-value, the better the window reduces heat transfer.
1. Conduction - is the route of the heat from the hotter to the cooler side of the home. High conductivity means heat moves quickly.
2. Radiation - is the energy radiated from hot surfaces for example sun and heating devices.
3. Convection - is where hot air rises and travels to cooler places. The transfer of energy.
High solar gain is important for cold climates. The aim is to reduce heat loss from the interior and allow solar heat through in the winter. U-Value should be low and SHGC is high.
Low solar gain is important for hot climates. Keep air-conditioned cool air inside but also reduce heat gain in summer. U-Value and SHGC should be low.
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