Thermal Conductivity

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.

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Conductance Vs Conductivity

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)
Dry Wet
Metal Aluminium 2800 204 204
Copper 9000 372 372
Lead 12250 35 35
Steel, Iron 7800 52 52
Zinc 7200 110 110
Natural stone Basalt, Granite 3000 3.5 3.5
Bluestone, Marble 2700 2.5 2.5
Sandstone 2600 1.6 1.6
Masonry Brick 1600-1900 0.6-0.7 0.9-1.2
Sand-lime brick 1900 0.9 1.4
  1000-1400 0.5-0.7  
Concrete Gravel concrete 2300-2500 2.0 2.0
Light concrete 1600-1900 0.7-0.9 1.2-1.4
  1000-1300 0.35-0.5 0.5-0.8
  300-700 0.12-0.23  
Pumice powder concrete 1000-1400 0.35-0.5 0.5-0.95
  700-1000 0.23-0.35  
Isolation concrete 300-700 0.12-0.23  
Cellular concrete 1000-1300 0.35-0.5 0.7-1.2
  400-700 0.17-0.23  
Slag concrete 1600-1900 0.45-0.70 0.7-1.0
  1000-1300 0.23-0.30 0.35-0.5
Inorganic Asbestos cement 1600-1900 0.35-0.7 0.9-1.2
Gypsum board 800-1400 0.23-0.45  
Gypsum cardboard 900 0.20  
Glass 2500 0.8 0.8
Foam glass 150 0.04  
Rock wool 35-200 0.04  
Tiles 2000 1.2 1.2
Plasters Cement 1900 0.9 1.5
Lime 1600 0.7 0.8
Gypsum 1300 0.5 0.8
Organic Cork (expanded) 100-200 0.04-0.0045  
Linoleum 1200 0.17  
Rubber 1200-1500 0.17-0.3  
Fibre board 200-400 0.08-0.12 0.09-0.17
Wood Hardwood 800 0.17 0.23
Softwood 550 0.14 0.17
Plywood 700 0.17 0.23
Hard-board 1000 0.3  
Soft-board 300 0.08  
Chipboard 500-1000 0.1-0.3  
Wood chipboard 350-700 0.1-0.2  
Synthetics Polyester (GPV) 1200 0.17  
Polyethene, Polypropene 930 0.17  
Polyvinyl chloride 1400 0.17  
Synthetic foam Polystyrene foam, exp. (PS) 10-40 0.035  
Ditto, extruded 30-40 0.03  
Polyurethane foam (PUR) 30-150 0.025-0.035  
Phenol acid hard foam 25-200 0.035  
PVC-foam 20-50 0.035  
Cavity isolation Cavity wall isolation 20-100 0.05  
Bituminous materials Asphalt 2100 0.7  
Bitumen 1050 0.2  
Water Water 1000 0.58  
Ice 900 2.2  
Snow, fresh 80-200 0.1-0.2  
Snow, old 200-800 0.5-1.8  
Air Air 1.2 0.023  
Soil Woodland soil 1450 0.8  
Clay with sand 1780 0.9  
Damp sandy soil 1700 2.0  
Soil (dry) 1600 0.3  
Floor covering Floor tiles 2000 1.5  
Parquet 800 0.17-0.27  
Nylon felt carpet 0.05    
Carpet (with foam rubber) 0.09    
Cork 200 0.06-0.07  
Wool 400 0.07