The thermal transmittance, commonly represented by a value, measures the rate at which heat transfers through a structure. Specifically within window assemblies, it indicates how well the window insulates. A lower value signifies better insulation, meaning less heat escapes during cold weather and less heat enters during warm weather. For example, a window with a rating of 0.30 allows less heat transfer than a window rated at 0.50.
This metric is vital for energy efficiency in buildings. Windows with favorable ratings contribute to reduced heating and cooling costs, improved comfort levels within the building, and decreased reliance on energy-intensive systems. Historically, advancements in window technology, such as low-emissivity coatings and multiple panes of glass with gas fills, have steadily improved these ratings, resulting in significant energy savings over time.
