BS 5250:2002 pdf download – Code of practice for control of condensation in buildings
Although interstitial condensation usually occurs when water vapour is diffusing out from the interior of a building, there are circumstances, e.g. 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 most 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).
5.3.2 Hygroscopic materials Most building materials are hygroscopic, i.e. they have a porous structure that absorbs water vapour from the air, even before interstitial condensation has taken place. Water can therefore be built into a construction by:
— the water of hydration in cement, concrete or mortars;
— the inclusion of hygroscopic materials which have been stored outside undercover in humid conditions. For example, 25 mm plywood stored at 90 % r.h. will hold almost 3 kilograms of water in every square metre;
— rain impact during construction before the weatherproof layer is in place. For example, 10 mm of rain falling on an absorbent insulation layer of a roof will deposit 10 kg/m 2 . This water can then move through a structure under temperature and humidity gradients by a mixture of vapour diffusion and liquid flow through the pores and accumulate at impermeable layers. The absorption of water by hygroscopic materials can have a buffering effect, reducing the chance of interstitial condensation during short periods of cold weather, or on clear frosty nights, when the external surface can cool by night sky radiation. Many structural elements are subjected to significant diurnal temperature changes; the external surface temperature of a flat roof in spring or autumn can rise to 50 °C during a sunny day and fall to –10 °C on a clear night. This causes movement of water into the structure during the day and outwards overnight. The water that is initially spread uniformly through the structure at low concentrations can then become concentrated at interfaces raising the moisture content of vulnerable materials such as timber high enough to cause local problems of decay. High external surface temperatures due to solar gain can force water in through gaps in a vapour control layer during the day, giving rise to roofs that apparently leak only in hot dry weather.
5.3.3 Reverse condensation An excellent example of moisture movement in hygroscopic materials under temperature gradients is given by reverse condensation. This phenomenon is most frequently observed when the sun shines on damp walls. It is caused by the moisture in the wall being vaporized by the heat of the sun; the resulting pressure difference drives the water vapour towards the inside of the building. If a vapour control layer is included in the construction, interstitial condensation can occur on the outside face where it can run down to affect vulnerable materials. This is most likely to be observed in the thermal improvement of solid walls by the use of internal insulation systems. Although the severity of the problem is not known, it is more common in thin masonry walls, walls of an absorbent nature or on walls that remain saturated because of their exposure. A weatherproof treatment or system can reduce the moisture content of such walls and the consequent risk of reverse condensation. Weatherproofing should be applied to the outer surface of the wall and should be of low vapour resistance or be vented. NOTE This type of reverse condensation should not be confused with the problems of interstitial condensation that can occur in building elements, e.g. in cold stores or air conditioned buildings, where the internal conditions are colder and drier than outside. These complex phenomena, such as liquid water movement under temperature gradients, are becoming better understood and a number of computer models that can give reliable performance predictions are currently under development and in use by consultants. Work is under way to standardize these and develop a formal protocol for the assessment of structures.