The capacity of a building material to store heat. Materials with a high thermal mass absorb heat, store it and then release it later on. This can help to smooth out extremes in temperature inside a building, helping to maintain a comfortable internal environment and reduce the need for heating. Heavyweight construction materials like concrete and bricks have more thermal mass than lightweight materials like timber. Thermal mass is particularly important in climates where there is a large difference between daytime and night-time temperatures, which isn't the case in the temperate Irish climate, though thermal mass can still be beneficial in Irish building - it's of most benefit when buildings are well insulated and occupied consistently throughout the day.