R-value (insulation) - Internationally

Internationally

Around most of the world, R-values are given in SI units, typically square-metre kelvins per watt or m²·K/W (or equivalently, m²·°C/W). In the United States customary units, R-values are given in units of ft²·°F·h/Btu. It is particularly easy to confuse SI and US R-values, because R-values both in the US and elsewhere are often cited without their units, e.g., R-3.5. Usually, however, the correct units can be inferred from the context and from the magnitudes of the values. United States R-values are approximately six times SI R-values .

Heat transfer through an insulating layer is analogous to electrical resistance. The heat transfers can be worked out by thinking of resistance in series with a fixed potential, except the resistances are thermal resistances and the potential is the difference in temperature from one side of the material to the other. The resistance of each material to heat transfer depends on the specific thermal resistance /, which is a property of the material (see table below) and the thickness of that layer. A thermal barrier that is composed of several layers will have several thermal resistors in the analogous circuit, each in series. Like resistance in electrical circuits, increasing the physical length of a resistive element (graphite, for example) increases the resistance linearly; double the thickness of a layer means half the heat transfer and double the R-value; quadruple, quarters; etc. In practice, this linear relationship does not hold for compressible materials such as glass wool batting whose thermal properties change when compressed.