Thermal conductivity : λ (lambda)
A material’s thermal conductivity is its ability to transfer heat. The lower the thermal conductivity, that is, the smaller the value of λ, the greater the material’s insulating power. Thermal conductivity is measured in watts per metre kelvin (W/m.K – 1 K is equivalent to 1° Celsius).
Thermal resistance : R
Thermal resistance, R, is another indicator of insulating power. It expresses a material’s ability to resist heat flow. The higher the R value, the greater the material’s insulating power. A material’s R value, measured in metres squared Kelvin per watt (m2.K/W), is obtained by dividing the material’s thickness by its thermal conductivity.
NB: In practice, it is not enough to compare the R values of different materials because R values are stated for equal thicknesses of material. In fact, what the R value tells us is that you will needed greater thicknesses of some materials than others in order to achieve the same insulating power. For example, a 1-cm-thick layer of glass wool will provide the same amount of insulation as a 30-cm-thick brick wall.
Thermal transmittance: U
Thermal transmittance, U, is the rate of heat transfer through one square metre of a material. It is measured in W/m2.K.
Recommended R values to achieve a good level of insulation for roofs, walls and floors:
Roofs: 4.5 (minimum) to 8 (ideal)
Exterior walls: 2.5 to 4
Floor directly on the ground: 1.5 to 3
Floor above an unheated space: 3.5 to 6.