Heat - Conduction, Convection & Radiation

• Heat is energy constantly seeking to dissipate and spread until everything is the same temperature.

• Heat energy flows from warm (more energy) to cold (less energy).

• In temperate countries like ours, we need our houses to be warmer than outside most of the time, so we artificially heat them and then have to find ways of slowing down the flow of heat to the outside world

Heat moves in three ways: conduction, convection and radiation.

How Insulation Works

Conduction is heat flow through a solid material, from molecule to molecule. Generally, the closer these molecules are packed (the denser the material), the more effectively heat can pass between them.

Conversely, the more they are spaced out, the harder the heat to flow. This is the fundamental principle of insulation, which seeks to ‘trap air’ or, in other words, spread out the molecules of the solid material as far from each other as possible, separated by pockets of still air.

Thermal Conductivity (lambda value)

Different materials allow heat through to different degrees, known as their thermal conductivity or lambda value. Better insulating materials have lower lambda values. Here is a table of common materials and indicative lambda values for comparison.

When looking at tables like this and considering, the temptation insulation choice is to opt instantly for the insulation with the lowest number. Still, so many other variables are at play that resisting is important! Quite apart from cost (not surprisingly, some of the much lower lambda materials are correspondingly expensive), there are issues such as fire resistance, performance with moisture, ease of use, durability, availability and environmental impact to consider, which are briefly discussed in the box overleaf.