U-value measures the rate of heat flow through the glass by conduction, convection and re-radiation.
Glass U-value is usually expressed in terms of Ug-value, is measured in Watts per square metre per degree Kelvin (W/m2 K) difference. The lower the Ug-value, the better the glass is at preventing heat loss and the higher its thermal performance.
The U-value is also the reciprocal of the “R” value and either can be calculated from the other e.g. U = 1/R or R = 1/U.
Solar factor (g-value) or Solar Heat Gain Control (SHGC)
The g-value, also called a Solar Factor, is the coefficient commonly used in Europe. The solar factor (g-value) is expressed as a decimal number between 0 and 1—The lower the g-value or SHGC, the less solar gain admitted into a building.
This is the percentage of total solar energy (or heat from the sun) able to pass through the glass. This can be either directly transmitted and absorbed or radiated inward.
Solar control glass comes along with a special coating that assists in reducing the heat entering your homes and offices. This type of glass can also absorb and reflect heat and be useful in reducing the need for air-conditioning.
UV (ultraviolet) transmittance. Reducing UV rays means less fading.
Visible Light Transmittance
This term is used to describe the percentage level of interior daylight that directly transmitted through the glass. The higher VLT (visible light transmission), the brighter the interior will be.
Low-e (low-emissivity) coating
Low-e coating is a microscopic coating that improves a glass’ energy efficiency. The glass which has a low-e coating called as low-e glass.
Two type of low-e coating lass:
- passive low-e: suitable for cold climate
- solar low-e: suitable for hot climate