BS 5250:2002 pdf download

BS 5250:2002 pdf download.Code of practice forcontrol of condensation in buildings.
5 Causes of condensation
5.1 General
Two categories of condensation should be distinguished:
a) condensation on surfaces within the building; and
b) interstitial condensation within or between the layers of the building envelope.
Most materials will absorb water vapour from the environment: some, subjected to high humidity, ran absorb moisture sufficiently to cause damage even though no actual condensation has taken pLace. In
considering the risk of condensation, consideration should therefore also be given to the actual levels of humidity to which materials will be exposed.
Sources of water vapour include atmospheric moisture, construction water, the oupants and their
activities and any Wet processes within the building (see Table 11.1. Table 11.2, Table 13.3, Table 11.4 and Table 11.5 in Annex B).
5.2 Causes of surface condensation
Surface condensation occurs on surfaces, such as the internal surface of external fabric elements or cold pipes and cisterns, that are at or below the elewpoint of the’ air in contact with them, and is controlled by the temperature of the surface and the vapour pressure of the air.
The temperature of the surface depends on the following factors:
a) the type(s), amount, time and rate of heating of the building:
h) the ventilation rate:
c) the thermal properties and surface finish of the building fabric;
d) the external temperature.
The vapour pressure of the air is determined by:
a) the water vapour production within the building;
b) the ventilation rate:
c) the moisture content of the “replacement” outdoor air;
d) the ability of the building fabric and contents to absorb or desorb water vapour (sponge effect), This will reduce or increase the vapour pressure depending on whether the building is cooling or heating.
5.3 Causes of interstitial condensation
5.3.1 General
In the winter, the interior of buildings will usually be warmer and the air will contain more moisture (i.e. have a higher vapour pressure) than outside. Heat and water vapour will diffuse out through the materials of the structure and be carried by bulk air movement through gaps and cracks into and through the structure.
For diffusion, rates of flow will vary depending on the interior/exterior conditions and the thermal and vapour resistance properties of each part ofth,’ structure, For air leakage, rates of flow will depend on wind and stack pressures and on the dimensions of the openings, joints and cracks. Unless these gaps are sealed. it has been found that the tiominant internal/external transport mechanism of water vapour is usually by mass movement of air.
Interstitial condensation occurs within the fabric of a building when the temperature of some part of the structure equals the dewpoint at that point, which is determined by the balance of flows of moisture to and from the point. At this temperature, the air is saturated: thus further vapour passing through the structure will condense rather than increase the vapour pressure. Such condensation is more likely to occur on the surfaces of materials within a structure, particularly on the warm side of relatively vapour resistant layers. but it is possible to have condensation occurring within the material when the dewpoint and the structural temperatures coiflci(le throughout the material. It is also possible to have interstitial condensation on more than one surface in a structure due to moisture evaporating from one surface and recondensing on a colder one.
Although interstitial condensation usually oceure when water vapour is diffusing out from the interior of a building, there are circumstances. eg. an air-conditioned building in warm, humid weather, in which the interior is cooler and drier than outside: water vapour will then enter the structure from outside. In this document, reference will be made to the warmer sides and cooler sides of the structure; in all but the moat exceptional circumstances, these will correspond to the higher and lower vapour pressure sides respectively. In spring and autumn as well as summer, even though the external air temperature may be lower than inside, the external surface of south facing walls, which might have been wetted by driving rain, can be sufficiently heated by the sun to cause water vapour to diffuse into cooler areas where it can condense (reverse condensation).