U-Factor
Simply put, the lower the U-Factor, the better.
According to Wikipedia.org "The U-value (or U-factor), more correctly called the overall heat transfer coefficient, describes how well a building element (in our case; a window) conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions."
Here's where it can get confusing. Window manufacturers invent unique names for their U-Factor ratings using words that include ultra and super put together with the word glass, develop a specialized logo and trademark the combination making it appear that they have a product no one else can offer. We aren't saying that's a bad thing. What we are saying is "pay attention to the numbers." The lower the U-Factor, the better.
When you are comparing U-Factors, just as when you are comparing R-Values, especially on fancy comparison charts (there's a reason we don't have comparison charts here at Windossier.com), note that each window company carries several lines of new replacement windows. Be sure apples are being compared to apples and oranges are being compared to oranges.
Window manufacturers generally carry multiple product lines providing a range of window qualities within those lines to meet the variety of needs we have as individual consumers. Charts can be deceptive. Make sure you aren't comparing the U-Factor of a brand name window from a company's high end line to the brand name of another replacement window from a company's low end line.
Remember that R-Value is the opposite. The higher the number is better for a replacement windows R-Value, the lower number is better for U-Factor; and while U-Factor rates how much heat enters and exits through a window, R-Value rates a windows resistance to heat.
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