A measure of a given material's ability to resist the absorption or conduction of heat energy, a characteristic also referred to as thermal resistance. R-value is most commonly used to classify insulation. The higher a material's R-value, the better that material insulates against thermal energy losses. R-value is calculated based on the material's thermal resistance per inch of thickness, and the R-value of most insulation materials used in construction depends directly on how well they trap air. Styrofoam has a much higher R-value than fibreglas batting because it completely traps most of its air in tiny bubbles surrounded by styrene plastic. Mats of glass fibre allow some air to flow through the mat, so it takes several inches of fibreglas to provide the same R-value as an inch of styrofoam.

The relative R-values of solid materials that don't trap air tend to match their electrical conductivity. Thus glass, which is an excellent resistor of electrical current, has a far higher R-value than the same thickness of steel, which is an excellent conductor of current. Insulating materials that use a vacuum or partial vacuum have the highest R-values of all, because energy cannot pass through a vacuum. This is why partial-vacuum dual-pane windows and vacuum flasks used to store liquids have such high comparative R-value ratings.

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