Achieving Heat Insulation in Buildings through U-values, R-values and K-values

U-values, R-values, and K-values are known as thermal coefficients in buildings. To insulate the interior of buildings against high external temperatures, it is necessary to ensure that the materials used in making the heat transmitting mediums in buildings such as walls, windows, doors, plasters, partitions etc. are well insulated. This insulation is more important at this critical stage where the world is experiencing global warming which is due to large amounts of entrapped heat on the earth’s surface due to large concentrations of greenhouse gases on the surface. The insulation capacities or heat transfer capacities of exposed elements of building materials are usually measured with the means of U-values, R-values, and K-values.

U-values also known as thermal transmittance measure the amount of heat transmitted per unit area of a particular thickness per unit temperature difference between inside and outside environments.  It is the reciprocal of thermal resistance (R-values) and it is denoted by W/m²K (or W/m^2oC). Materials with high U-values are poor insulators while materials with low U-values are good insulators. Materials with low U-values should preferably be used in buildings.

Mathematically,

Where,

R_SI = thermal resistance of an internal surface

R_SO = thermal resistance of an external surface

R_A = thermal resistance of air spaces within construction

R₁, R₂, R₃, etc. = thermal resistance of successive components

R-values also known as thermal resistance measure how well a particular thickness of material resists the passage of heat by conduction. Materials with high R-values are preferable in building construction. R-values are denoted by the unit m²K/W (or m^2oC/W).

Mathematically,

K-values also known as thermal conductivity measures the heat (W) transmitted through a unit area (m²) of a material of unit thickness (m) for unit temperature difference (K) between inside and outside environments. K-values are usually expressed as W/mK (or W/m^oC). Materials with low K-values are preferable in buildings because low K-values imply that the materials do not easily allow heat energy to pass through.

Typical Values of U-values, R-values and K-values

Table 1: Standard U-values for new construction elements

Table 2: Standard R-values

Table 3: Standard Densities and K-values for typical building materials

 References

Baden-Powell, C. (2011). Architect’s Pocket Book (4^th edition). Elsevier Ltd, Burlington, USA