Heat Flows in Buildings
Objects of any kind at different temperatures can exchange heat energy in three ways: conduction, convection and radiation.
Heat exchange (both heating and cooling) between bodies of air and thermal mass elements like floor or walls rely on conduction, convection and in particular, radiation.
Heat flow by Radiation:
Heat flow by radiation occurs in a roof for instance where the warmer part of a ceiling radiates heat to the cooler underside of the roof. The more reflective the surfaces, the lower the heat flow by radiation (Emissivity).
Reflective foils are low emissivity surfaces (typical 0.03 - 0.05). Reflective foils as per AS 4859 must be labelled with their corresponding emissivity (e). Heat flow by radiation depends not only on the thermal properties of the product itself but also on its orientation and environmental conditions (temperature).
Heat Flow by Convection:
Heat flows through the movement of a gas or liquid and occurs when heat flows through the external building envelope due to a temperature difference between the temperature of the inside and outside air.
Convection (both temperature and wind driven) typically occurs in building elements in the following ways:
In an attic space or subfloor temperature, driven convection occurs when air in contact with a warmer surface heats the air causing it to rise and then release its energy to surfaces at lower temperatures. In a roof for instance, air is warmed by contact with the upper surface of the ceiling and rises in the attic space. It then comes into contact with the colder underside of the roof, loses some of its heat to them and falls.
In wall structures, wind driven convection may occur if the air space in the wall is open to the subfloor or attic and through vents and other gaps in the structure.
Conduction is regulated by the use of insulating materials with low conductance and high resistance or R-value.
Heat Flow by Conduction:
Conduction occurs between objects in contact with each other. Within the plasterboard of the ceiling for instance the lower layer is heated by contact with the warmer air in the room. This layer heats up and releases some of its heat to the next layer and so on.
Heat Flow Between a Building and its Surroundings
The rate of heat flow (from objects with a higher temperature to a lower temperature) between a building and the outside depends on the temperature difference and the thermal properties of the building envelope.
Ventilation Gains and Losses
Ventilation may occur due to either intentional opening of doors and windows for instance or through infiltration which is uncontrolled leakage through a building fabric, in particular around gaps and cracks around windows and doors.
Framing & Thermal Bridges
Sometimes heat is allowed to bypass insulation through direct thermal connections in the building (window frames for instance). Direct thermal connections should be avoided by the use of counter battens made of non conducting material as a thermal break.
Solar Gains
A large proportion of solar radiation (direct, diffuse and reflected) passes through glass but also through exposed roof, wall and other surfaces. These solar gains change as solar position changes throughout the day and year and means that different orientations receive different amounts of radiation at different times of the year.
Internal Gains
Internal gains such as heat from people, lighting and electrical appliances all contribute to increase in internal heat gains.
