The U-value is a measure of how many watts (representing the rate of flow of energy) pass through one square metre (m2) of a construction detail (such as a wall) for every degree difference in temperature between the inside and the outside. Temperature is measured in Kelvin, where 1K = 1°C (degree centigrade).
As an example, a U-value of 6.0W/m2K (that of a single glazed window) means that six watts will be escaping through each square metre of glass when the temperature difference is one degree. If it is 20°C in the house and 0°C outside, then the heat loss is 20 x 6 = 120 watts per square metre.
U-values are generally used to describe the thermal performance (heat loss) for a section of construction that involves several materials - such as a wall made up of timber, insulation and plasterboard.
For individual materials, such as a type of insulation, you will come across the term ‘thermal conductivity’, also known as a k- or λ-value. This is the rate at which heat flows through a particular material, and good insulation will have a low thermal conductivity. It is measured in Watts (heat flow) per metre (depth of material) per degree difference (inside to outside), so the unit is W/mK.
Most natural insulation materials (hemp, wool, recycled paper or textile) will have a thermal conductivity of about 0.035 to 0.040 W/mK, which is similar to the performance of conventional mineral wool insulation. The foil-backed plastic foam insulation boards are slightly better, with thermal conductivity about 0.023 W/mK. So about 100mm of the plastic foam board will give equivalent insulation value to about 150mm of the various woolly types.