The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
Three different mechanisms convey heat — conduction, radiation, and convection — and in many instances, all three may operate together.
Conduction is the transmission of heat through a material by contact between particles of matter without their displacement.
Metals are excellent heat conductors relative to most other materials. Air and other gases are poor conductors because heat transfer can only take place by movement of their molecules, which are much further apart than those of metals or solid objects.
The amount of heat that flows through any substance by conduction depends on the time, area, temperature differential, and the type of material. Because still trapped air is a poor conductor, it acts as an insulator, which is why fibrous and cellular insulation materials are efficient insulators. Free air, however, conducts heat by convection.
Radiation is the transmission and emission of infra-red energy in the form of electromagnetic waves when heat is transferred from a warm body to a colder one without heating the substance it passes through.
Bodies that absorb thermal rays from the sun are also radiating bodies. That means much of the heat absorbed in daytime are radiated at night, unless cloud cover reduces radiation. When roof cladding radiates its heat, it is known as 'night sky radiation'. (See 9.3.8 Night Sky Radiation)
All bodies are emitting and absorbing radiation continuously, and attempting equilibrium. Which is why a hot body radiates heat to all those colder than itself and also why it often feels uncomfortably cold in a room with cold walls, floor, or roof.
Convection is the transportation of heat within a gas or fluid by the circulatory motion that occurs at a non-uniform temperature owing to the variation of their density and the action of gravity.
The law of gravity acts so that hot air rises because it is less dense than cold air, for example, a hot air balloon. Convection currents produce ventilation, which is known as passive ventilation because only heat and space are required for air movement to occur.