Book - School of Science and Technology

6 Fundamentalsas insulators have a low conductivity. The conductivity of many materials varies widelywith temperature and thus only those values that fall within the range to which they applyshould be used. Thermal conductivity, which is the property discussed here, shouldnot be confused with electrical conductivity which is a quite separate quantity.As far as building materials are concerned, those having higher densities are usuallyhard and are not particularly good insulators. Porous materials are bad conductors whendry and good conductors when wet, a fact which is sometimes overlooked when a newlyconstructed building is occupied before the structure has been able to dry out properly,which may take some months in the winter or spring.ConvectionConvection is a process in which heat transfer involves the movement of a fluid mediumto convey energy, the particles in the fluid having acquired heat by conduction from a hotsurface. An illustration commonly used is that of an ordinary (so-called) radiator whichwarms the air immediately in contact with it:this expands as it is heated, becomes lighterthan the rest of the air in the room and rises to form an upward current from the radiator.A second example is water heated by contact with the hot surfaces around the furnace of aboiler, leading then to expansion and movement upwards as in the preceding instancewith air. A medium capable of movement is thus a prerequisite for conduction, whichcannot, in consequence, occur in a vacuum where no such medium exists.RadiationThis is a phenomenon perhaps more familiar in terms of light and was, in Newton's time,explained as being the result of bombardment by infinitesimal particles released from thesource of heat. At a later date, radiation, whether of light or heat, was supposed to be awave action in a subtile medium (invented by mathematicians for the purpose) known asthe ether. In the present context, it is enough to state that radiation is the transfer ofenergy by an electromagnetic process at wavelengths which correspond to, but extendmarginally beyond, the infra-red range (10 6 to 10 4 m). Radiation is completely independentof any intermediate medium and will occur just as readily across a vacuum as acrossan air space:intensity varies with the square of the distance between the point of originand the receiving surface. The amount of radiation emitted by surfaces depends upontheir texture and colour, matt black surfaces having an emissivity rated as unity in anarbitrary scale. Two values only need to be considered in the present context:0.95 whichrepresents most dull metals or the materials used in building construction and 0.05 whichapplies to highly polished materials such as aluminium foil. Surfaces which radiate heatwell are also found to be good adsorbers; thus, a black felted or black asphalt roof is oftenseen to be covered with hoar frost on a cold night, due to radiation to space when nearbysurfaces having other finishes are unaffected.Air volume and movementAlthough these two subjects interact, their characteristics may be considered separately.VolumeIt is necessary, when discussing the matter of air volume, to make a clear distinctionbetween the total quantity in circulation and the proportion of it which is admitted fromoutside a building. It was traditional practice to refer to the latter component as fresh airbut, since pollution in one form or another is a feature of the urban atmosphere, the term