Knowing what R-value and U-values mean is key to following energy issues and to selecting products that best suit the climate zone you are building in. R-value is essentially a product’s resistance to heat flow. Adversely, U-value measures the rate of heat transfer. This means that products with a lower U-value will be more energy efficient. It is tempting to think that these two values are direct opposites of each other, but there are some important differences to note.