The principles of heat transfer help to understand how insulation works. Heat flows from warm to colder surfaces until the temperature of both is equal.
These flows can take three forms:
Conduction is the direct transfer of heat between adjoining molecules. A warmer molecule transfers some of its energy to colder neighbours. A good example is when one sits down on a cold metal chair, one can feel the cold from the chair as heat from the warmer body is quickly transferred by conduction to the chair.
Convection is the transfer of heat through liquids and gases. An example is warm air rising from a hot surface and being replaced by cooler and denser air that sinks. Heat is carried away from the surface by the warm air.
Radiation is the transmission of energy through space by electromagnetic waves. Radiated heat moves at the speed of light through the air without heating the space between, just as one feels the warmth of the sun on one's face, heat radiates from the sun to earth without heating the space between.
Mineral wool thermal insulation prevents convection by holding air still in the matrix of the wool. Still air is a good insulator. Mineral wool also stops radiation and limits the conduction of heat through the body of the insulation. The effectiveness of the mineral wool in reducing heat transfer depends upon its structural properties such as density, thickness, composition and the fineness of the wool as well as the temperature at which it is used.
The heat transfer through insulation is a combination of solid and gas conduction, convection and radiation. This gives a thermal conductivity versus density characteristic which is non-linear and has a minimum.
How well a material transmits heat through itself is known as thermal conductivity.
Thermal conductivity, l, (lambda, measured in watts per meter per degrees Kelvin, W/mK) of a material represents the quantity of heat that passes through a meter thickness per square meter per time unit with one degree difference in temperature between the faces.
The Lambda value compares the ability of materials to transmit heat through them under these fixed conditions. The lower the lambda value the better the insulator the material will be. (Lambda values of typical materials are for example, Copper 380 W/mK, Aluminium 210 W/mK; Steel 46 W/mK; Wood 0.21 W/mK; Mineral wool 0.045 W/mK; Air 0.026 W/mK).
For construction purposes a material is defined as insulating if it's thermal conductivity is less than 0.065 W/mK. A typical mineral wool has l of 0.035-0.040.
The insulation capacity of the mineral wool products is based on the low thermal conductivity of air in the pockets of the wool material.
Thermal resistance or R value, is a measure of the ability of a given thickness of a material to prevent the passage of heat. The thermal resistance, R, of a material with thickness, d (meters) and thermal conductivity, l, is equal to R= d/l (Units are square meters degrees Kelvin per Watt (m2K/W).
Thermal resistance, R, is the inverse of the coefficient of thermal transmission whilst thermal conductivity is an inherent property of a material.