R-Value (Thermal Resistance)

R-value is the reciprocal of U-value (R = 1/U for a single homogeneous layer). For composite assemblies (e.g., a wall with multiple layers), total R-value is the sum of individual layer R-values plus surface film resistances. Typical values: single brick wall R-0.4, 100mm mineral wool R-2.8, 50mm XPS foam R-1.75, double glazing R-0.35, single glazing R-0.15. ASHRAE 90.1 and BNBC specify minimum R-values by climate zone and assembly type. Insulation R-values degrade with moisture absorption, compression, and age — installed performance may be 10-20% below rated values. Thermal bridging through framing members reduces effective R-value of wall assemblies by 15-25%.

R-value directly affects heating/cooling energy consumption, thermal comfort, and condensation risk. In Dhaka's cooling-dominated climate, roof insulation R-value has the highest impact on energy performance. Inadequate insulation is one of the most common deficiencies in Bangladesh commercial buildings.