Thermal conduction in buildings
Thermal conduction is the diffusion of internal heat within a static (rather than fluid) body as a result of a temperature difference across it. Heat will tend to diffuse from higher temperature parts of a body to lower temperature parts.
This is particularly important in buildings where there may be a temperature difference between the inside and outside, for example, in a heated building during the winter, or in a cooled building during the summer.
Conduction is one of the main potential heat transfer mechanisms by which the internal heating or cooling can be lost to the outside, resulting in high operating costs, high carbon emissions and occupant discomfort.
Simple conductive heat transfer (in watts) through a uniform body can be calculated from Fourier's Law:
q = k A dT / s
- A is the area of the body (m2)
- k is the body's thermal conductivity (W/m°C)
- dT is the temperature difference across the body (°C)
- s is the body's thickness (m)
To determine the heat transfer between the inside and outside of a building component, it may be necessary to calculate the conductive heat transfer across a number of layers, and the internal and external surface resistances. This is sometimes calculated using a U-Value. In simple terms, the lower the U-value of an element of a building's fabric, the less heat will transmit across it. U-values are expressed in watts per square metre per degree Kelvin (W/m2K).
It is sometimes thought that conductivity is described by the U-Value, however, U-values include inside and outside surface thermal resistances. Conductivity is more accurately expressed by a material's R-Value, which is the reciprocal of its thermal resistance and does not include a surface component. See U-Value for more information.
Conductive heat transfer is particularly high across narrow, highly conductive components such as windows. It can be inhibited by insulating materials which have a high thermal resistance. See insulation for more information. Typically conductive heat transfer is reduced by creating breaks in the continuity of a material, such as the air (or other gas) in insulation, or the air or gas filled space between the panes of glass in double or triple glazing. This breaks the conductive flow, replacing it with surface resistances and a convective heat transfer across gap.
A thermal bridge describes a situation where there is a direct connection between the inside and outside through one or more elements that are more thermally conductive than the rest of the building envelope. As a result, there will be wasteful heat transfer across that element, its internal surface temperature will be different from other, better insulated areas and there may be condensation where warm, moist internal air comes into contact with the, potentially cold, surface. This condensation can result in mould growth. See Thermal bridge for more information.
In practice, inside and outside temperatures do not remain constant (steady state) and the transfer of heat across a body is not instantaneous. Dynamic analysis of conductive heat transfer considers the changing temperature profiles on either side of a body and the time lag introduced by the rate of diffusion of heat through the body.
This time lag can be exploited by introducing thermal mass into the fabric of a building. Thermal mass describes the ability of a material to absorb, store and release heat energy. Thermal mass can be used to even out variations in internal and external conditions, absorbing heat as temperatures rise and releasing it as they fall. This can useful for evening-out and delaying extremes in thermal conditions, stabilising the internal environment and so reducing the demand for building services systems. See Thermal mass for more information.
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