The R-Value is the
measurement of insulating ability of the material. The higher the R-Value, the greater
the insulation value.
The U-Value is the
measurement of heat loss through the material. The lower the U-Value, the less heat is
Many hobbyist greenhouses are
classified as season extenders and are not designed for year round growing if you live
in areas where it's cold and snows. For cold conditions select a greenhouse with the
thickest panels and also use a gas or electric heating system.