R-value is a measure of thermal resistance. It’s a measurement of how effectively a certain material can resist conductive heat transfer. A loose or porous material will have a low R-value, while a strong and durable material will have a high R-value. A lower R-value means the material allows for heat transfer more easily, while a higher R-value means a material is more resistant to heat transfer.   R-value is the reciprocal of U-factor (also known as U-value). U-factor is a unit of measurement describing how well a certain material conducts heat, while R-value is a measurement of how well a certain material resists heat conduction.