Heat Transfer: How we lose and gain heat through the building fabric
• Heat will naturally flow from a hotter to a colder object (2nd Law of Thermodynamics)
• ‘Heat Transfer’ is important because it’s the mechanism by which our homes lose heat energy
• The greater proportion of energy loss is by heat transferring through the walls, roof and ground-floor. The remaining heat loss is mainly due to air leakage through windows, doors and through outlets in the building envelope.
• When we lose heat it needs replacing using radiators, fires, under-floor heating etc.
A heated house in a cooler climate, such as found in the UK, will always lose heat from a warm interior to the cooler outside. How quickly the heat is lost is determined by:
- The materials and construction methods used in builidng the house and
- The temperature difference between the inside and outside - which is why our heating systems work harder in winter.
The opposite happens where a cool house interior is heated by a warmer climate such as found, for example, in the Mediterranean.
Conduction, Convection and Radiation
In physics, heat can be transferred in three ways: Conduction; Convection; & Radiation
All of these methods occur to a greater or lesser extent when a house loses or gains heat
Conduction is the main method of heat transfer within solid objects or between solid objects in contact with each other.
It is the main way by which heat is transferred through the ‘building envelope’ (walls/roof/ground-floor)
Molecules in the hotter part of the object vibrate faster than the molecules in the cooler parts. The faster moving molecules transfer part of the energy to their slower moving neighbours – so transferring heat through the object
A ‘steady state’ is achieved when the heat entering the object at one side is balanced by the heat being emitted from the other side. Hence throughout a period of time, the object’s heat remains constant.
Metals are highly conductive whereas gases are not.
In the wall of a house, conduction can be found in solids such as brick, block or plaster
Using radiators, our homes are heated through Convection. Currents of air are repeatedly heated and cooled to maintain a constant temperature.
Convection is a circular motion / current that happens when warmer air or liquid — which has faster moving molecules, making it less dense — rises, while the cooler air or liquid drops down
In the wall of a house, convection will typically be found where cavities occur. Convection currents are set up by the warmer side of the cavity and as the heat moves around to the colder side it is cooled, thus transferring heat across the cavity.
All objects absorb and emit thermal radiation (aka infrared electromagnetic radiation).
In Radiation, heat energy is transferred via waves, not as in molecules as occurs in Conduction.
Thermal radiation is emitted as a result of the random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles (protons and electrons) their movement causes the emission of electromagnetic radiation, which carries energy away from the surface of the body.
In a temperate climate, such as in the UK, transfer of heat through a wall/roof/ground floor is only in very small part through radiation.
The Earth is heated through thermal radiation from the Sun. Ultimately all the energy we use originates from the Sun's radiation.
In hot climates, experiencing long periods of direct sunshine, the outside surface of the wall is heated by radiation from the sun (put your hand on a wall during the heat of the day). Though limited now, it is likely to be increasingly manifest in the South of England as the climate warms. Traditional house construction is ill-prepared for this eventuality.
Conversely in a cool climate where the house loses heat, objects (such as us or the furniture) within a heated room emit small amounts of radiation which contributes in small part to the heating of the inside surface of the wall - the beginning of heat transfer to the outside.
Thermal radiation is absorbed well by :
Water, water vapour, glass, wood, brick, stone, concrete, asphalt, copper,
Thermal radiation is reflected well by :
- Aluminium foil (hence the sometimes doubtful attempts to include it in insulation products)