Thermal Properties of Building Materials

Thermal Conductivity, Thermal Conductance

Fundamental to thermal performance lies the thermal properties of various building materials.

Each building material’s Thermal Conductivity (W/mdegC) denotes how well the material conducts heat and Thermal Resistivity (mdegC/W) measures the thermal insulating property of the material. Both measures remain constant regardless of the thickness of the material. Glass for instance has a Conductivity of 1 whereas Steel has a Conductivity of 200. Thermal capacity and Specific Heat are also important characteristics of the material in determining its behavior.

Thermal Conductance(W/m2degC) measures how quickly heat energy flows through a surface and varies with thickness of the material.

It is hence seen that the difference between conductance and conductivity is that conductance involves area while conductivity involves length of a material. Metals have high thermal conductance and heavy building materials such as concrete, brick and stone are moderate conductors of heat. Timber and fabrics have low thermal conductance while bulk insulating materials(e.g fiberglass) have even lower conductance again.

Thermal Resistance

Each building product (building material and insulation product) as well as the film of air on the external and internal surfaces of a building has a thermal resistance to conducted heat flow, so called R-Values (m2degC/W) and are quoted for a specified thickness of the material.

The Total R-value of a building element takes into account resistance from specified construction materials , internal air gaps, thermal bridging, insulation materials and air films adjacent to solid materials. The higher the R-value the greater the resistance to heat flow.

Thermal Transmittance ‘

The overall thermal transmittance or U-Value is also referred to as the heat transfer coefficient and denotes the thermal transmittance of that element including air films and air speed.

U-Values are generally used for homogenous materials such as wall, ceiling and glazing systems. For glazing systems most of the U-value is due to the air films next to the surface. This means that the U-value of the glass material has such a small contribution to the overall U-value of the glass window, hence double and triple glazing will always be more effective versus a thicker glass.

Resistance of Surfaces

A material’s thermal performance is effected by its Absorptance, Emittance and Reflectance. The reflectance of most building materials to solar radiation is highly dependent on the properties and color of the surface. Shiny surfaces and light colors have a much higher reflectance (e.g new and bright aluminimum~0.95 reflectance, 0.05 emittance) than matt and darker surfaces (Unpainted concrete ~0.4 reflectance, 0.9 emittance ).

Thermal Capacity

Thermal capacity is an important characteristic when analysing the thermal impacts of high mass materials and should be used together with properties such as resistance or U-value. Thermal capacity is used in Building Thermal Performance assessments when comparing elements.