R-Value & U-Value

R-Value is a measure of thermal resistance used to compare insulating values. The higher the R-value number of a material, the better its insulating properties and the better the window will retain heat on a cold day or cooling on a hot day.

Many windows are rated in U-Factor (or U-value) or a measure of how well heat is transferred by a window, thus affecting heating and air conditioning costs. U-factor is the inverse of
R-value. The lower the U-factor, the better its insulating properties. 

The U-value equals one divided by the R-value (U=1/R).

SHGC

SOLAR HEAT GAIN COEFFICIENT (SHGC): The percent of solar radiation that actually enters a building through a window as heat gain. A high (up to 1.00) coefficient means high solar heat gain. The smaller the number, the better the product is at preventing solar heat gain. If shading coefficient values are required, take SHGC value and divide by 0.87.

VT

VISIBLE LIGHT TRANSMITTANCE (VT): The percentage of light that is transmitted through glass in the visible light spectrum (380 to 720 nanometers). The higher the number the higher the percentage of visible light transmitted through the window.