R-value is a term that is used rather often in construction applications as it applies to materials, but when it comes down to it most people don’t have a basic understanding of what it means. The concept that a higher R is better in terms of insulation performance is rather intuitive, but how the values are arrived at is another matter.

To grasp this concept, we’ve got to go take a look at the opposite of R-value; K-value (which is sometimes referred to as “U-value”). K-value is the value assigned to a material that quantifies (under uniform conditions) the property of a material to conduct heat.  In Imperial units thermal conductivity is measured in BTU/hr•ft•F°. To simplify, it’s how much heat a material can transfer over a given time frame. A material that is a good conductor can move a lot of heat very quickly. For example, if you touch two different objects that are the same temperature but made from different materials, say a t-shirt and a frying pan, the frying pan will feel colder to your touch. The reason it feels colder is that it allows more heat to move from your hand through its mass in a given time that the t-shirt material does.

R-value is the inverse or reciprocal of K-value. Being the reciprocal of K, its unit of measure is in hr•ft•F°/BTU. When purchasing building materials, the R-value is usually referred to without the units of measure, for example R-14. The higher the number, the greater the thermal resistance of the material. R-value can also be multiplied by increasing the material thickness, for example if you double the thickness of your attic insulation from 4 inches to 8 inches, your R-value is doubled.

Thermal resistance is exactly what it sounds like; the resistance to the flow of heat energy. While a good thermal conductor will move heat well, a good thermal resistor will not. It’s important to remember that this physical property works both ways. A good thermal conductor will move heat easily, so it heats up quickly and cools off quickly. A good thermal insulator doesn’t move heat easily, so it heats up slowly and cools down slowly. Take a look at Figure One above. This material is what the heat shield tiles on the space shuttle are made from. The sample in the picture is hot enough to give off light, which means a minimum of 950°F, but the man in the picture is holding it in his hand. It’s such a good insulator (which means a bad conductor), that it moves the heat so slowly that it can be briefly picked up without thermal protection.

Simplified, R-value and K-value are the exact opposite of one another. One moves heat well, the other slows the transfer of heat down. Now you know what those values mean on the materials you buy, which will help you when you’re Thinking Thermally in Building Applications.