Thermal conductivity is a material property. It will not differ with the dimensions of a material, but it is dependent on the temperature, the density and the moisture content of the material. The thermal conductivity of a material depends on its temperature, density and moisture content. The thermal conductivity normally found in tables is the value valid for normal room temperature. This value will not differ much between 273 and 343 K (0 — 70° C). When high temperatures are involved, in ovens for instance, the influence of temperature has to be taken into account.
Generally light materials are better insulators than heavy materials, because light materials often contain air enclosures. Dry still air has a very low conductivity. A layer of air will not always be a good insulator though, because heat is easily transferred by radiation and convection.
When a material, for instance insulating material, becomes wet, the air enclosures fill with water and, because water is a better conductor than air, the conductivity of the material increases. That is why it is very important to install insulation materials when they are dry and take care that they remain dry.
Conductivity (k) is a material property and means its ability to conduct heat through its internal structure. Conductance on the other hand is an object property and depends on both its material and thickness. Conductance equals conductivity multiplied by thickness, in units of W/m²K. As conductivity is the reciprocal of resistivity, the total resistance of a material can therefore be given as its total thickness divided by total conductivity. The table below shows a list of building materials and their thermal conductivity for dry (indoor) and wet (outdoor) conditions.
|Group||Material||Specific mass (kg/m3)||Thermal conductivity (W/mK)|
|Natural stone||Basalt, Granite||3000||3.5||3.5|
|Pumice powder concrete||1000-1400||0.35-0.5||0.5-0.95|
|Synthetic foam||Polystyrene foam, exp. (PS)||10-40||0.035|
|Polyurethane foam (PUR)||30-150||0.025-0.035|
|Phenol acid hard foam||25-200||0.035|
|Cavity isolation||Cavity wall isolation||20-100||0.05|
|Clay with sand||1780||0.9|
|Damp sandy soil||1700||2.0|
|Floor covering||Floor tiles||2000||1.5|
|Nylon felt carpet||0.05|
|Carpet (with foam rubber)||0.09|