Katie, the Go-to Girl for Windows, writes...
The U-value is an assigned number that states a object's (or window's) ability to keep inside a building warm or cool. The U-value states the amount of thermal energy transported through a window. For homeowners residing in a place with a warmer climate like Florida, the U-value is an indicator of how long a home can be kept cold.
Good U-values are an effective way to maintain the amount of energy used to keep a home at an ideal temperature. The larger the U-value, the greater amount of heat will flow through an object such as a window. An efficient window has a low U-value.
Technically, this value is reached through the use of the formula W/( m²K). In this formula, W refers to thermal energy in Watts, and m²K refers to window with the size of 1 square meter [m²] at a temperature difference of 1 Kelvin [K] (=1ºC). Without remaining too technical, a beneficial U-value, to put it simply, should be less than 1.0 W/(m²K). Of course, this number can be obtained through glass and other window energy efficiency options.
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