rvalues

R-values are a measure of thermal resistance used to describe how well a material or assembly resists heat flow. In building insulation, a higher R-value indicates greater resistance and typically translates to lower heat transfer. The R-value can apply to individual materials, such as insulation batts, or to an entire wall or roof assembly.

In the SI system, the unit is square meter kelvin per watt (m^2·K/W). For a homogeneous layer, R = L / k, where L is thickness and k is thermal conductivity. For multi-layer assemblies, the total R-value is the sum of the individual layer R-values, while the overall heat transfer is described by the U-value, which is the reciprocal of R_total.

In practice, building codes and energy guides use R-values to guide insulation requirements. In many regions, climate zones determine recommended or required R-values for walls, roofs, and foundations. Per-inch performance is material dependent; for example, fiberglass and mineral wool have lower per-inch R values than closed-cell foams.

Limitations include that R-value primarily measures conductive resistance and does not directly capture air leakage, moisture effects, or thermal bridging, all of which can reduce real-world performance. Radiant barriers and ventilation also influence heat transfer beyond nominal R-values.