Water and water vapour have been surrounding us for ages and are among key factors that condition living processes of humans, animals and plants. It is impossible to live without constant access to water. Living in house is not an exception, constant water supply enables basic living processes from drinking to hygienic purposes. One can say that water is omnipresent under its different forms. The molecules of water steam in air are around us.

The sources of water stream in house are (among others) the results of usual choirs made by human. Choirs such as cooking, washing, cleaning, drying cause the emission of large quantities of water steam. Moreover, human generates even several litres of water vapour daily during his living processes. Low air humidity is no good for dwellers and is usually balanced by popular moisturising systems from very simple water containers to very advanced devices steered electronically.

Assembly moisture

“Assembly moisture” is the effect of evaporating water contained in building materials used for construction and finishing works. Those are mainly floor covering with concrete, mortars, plasters, finishing coats, paints and other mortars used as a solutions of water, loose mineral mixtures and upgrading admixtures. The amount of water used for forming mortar is so big that it would fill a water basin of medium size. For this reason it is so important to ventilate recently finished interiors. Such ventilation will be especially effective in warm periods – spring, summer and autumn when low temperatures (that carry the danger of dew point) are rather rare.

The closing of recently finished house with roof and putting in window frames and door, require ensuring the proper ventilation of finished interiors. Postponing of this moment for late autumn or winter will cause rising damp – with large amount of water vapour in air and its lower temperature the dew point will be localised rather low in house interior what will cause vapour condensation.

Common practice is the closing of roof before winter and conducting of insulating and finishing works during winter. In attic which is not heated soon can occur unexpected events of water vapour condensation. In such attic dew point is relatively low, often in thermo-insulation layer or even lower. In result condensed water may appear on roof membrane surface, thermo-insulation wool or finished surfaces. It is more intense when assembly works are taking place in attic – people generate large amounts of water vapour that in low temperature has no chance to diffuse outside. It condenses when touching cold elements of roof structure. One should avoid closing of roof before winter and afterwards doing finishing works. Such works should take place in warm months with ensuring optimum ventilation. In this way one can avoid problems connected with consequences of assembly moisture.

Water vapour diffusion

Depending on temperature and air humidity, on both sides of building barrier are vapour pressures of different values. Because of difference in vapour pressure, steam comes from warmer side (higher pressure) to colder side (of lower pressure). This occurrence is known as water vapour diffusion.

It is of significant importance in case of roof ant attic. Taking into consideration the fact that materials used in roof and attic are based on timber as well as the hygroscopic materials for thermal insulation (glass and rock wools), the dampness of roof barrier may cause irreparable gradual decay. The moisture in thermo-insulating materials not only causes the lowering of their thermo-insulating values and cooling of attic but also may bring about moulds and decay of roof barrier. Water vapour alone does not endanger the structure until it reaches dew point.

Dew point

Water vapour is in air that surrounds us. The cooling of air to proper temperature causes the state of its saturation with vapour. Below this temperature supersaturation takes place and vapour condenses. With large air humidity on warmer side the condensate of vapour may appear inside the building barrier as dew (moisture). When this moisture has a possibility of evaporation then there is no problem. It is worse when moisture accumulates because this may cause serious building damage.

In order to prevent forming of vapour condensate in interior side of roof barrier the temperature should be higher than dew point air. Properly insulated and protected attic should protect from dew point. Facebook B2B Platform