The U-factor of windows and doors is a measure of how well they insulate against heat loss or gain. It’s calculated by considering several factors, including the materials used in the window or door, its size, and its overall design.

To calculate the U-factor of a window or door, you need to measure the rate of heat transfer through it. Specifically, you need to measure the amount of heat that is lost or gained through the window or door per hour per square foot per degree Fahrenheit.

Let’s say you want to calculate the U-factor of a single-pane glass window that’s 3 feet wide and 4 feet tall. The temperature inside the building is 70 degrees Fahrenheit, while the temperature outside is 30 degrees Fahrenheit. You measure the rate of heat loss through the window to be 2000 BTUs (British Thermal Units) per hour.

Using these measurements, you can calculate the U-factor of the window using the following formula:

U-factor = (Rate of Heat Loss) / (Area x Temperature Difference)

U-factor = 2000 BTUs / (3 feet x 4 feet x 40 degrees Fahrenheit)

U-factor = 2000 BTUs / 480 square feet per degree Fahrenheit per hour

U-factor = 4.17 BTUs per hour per square foot per degree Fahrenheit

This U-factor tells us that the window is not very efficient at insulating against heat loss or gain. A lower U-factor would indicate a more energy-efficient window.

In India, there are several standards and regulations that must be considered when calculating the U-factor of windows and doors. The Bureau of Energy Efficiency (BEE) Energy Conservation Building Code (ECBC) sets minimum energy efficiency standards for buildings, including requirements for windows and doors. The U-factor of windows and doors must meet these standards to be considered energy-efficient in India.

In practice, window and door manufacturers in India will often have their products tested and rated for their U-factor by certified testing agencies to ensure compliance with these standards and regulations.