U-factor, also known as U-value, is a measure of the rate of heat transfer through a building material or assembly, such as a wall, roof, window, or door. It quantifies how well a product prevents heat from escaping or entering a building.
A lower U-factor indicates better insulation and higher energy efficiency, meaning less heat is lost in cold climates and less heat is gained in warm climates. The standard units of measurement are BTU/hr·ft²·°F in the US or W/m² · K when working in international contexts.
U-factor often applies to an assembly (like a whole window unit) rather than just a single material, especially when discussing fenestration.
Here’s an example usage: “To meet the stringent energy code requirements for the new office building, the project team specified windows with a U-factor of 0.28 to minimize heat loss during the winter months.”
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