Technical Discussion

Understanding Condensation in Autoclaved Aerated Concrete and Applied Coatings



The natural movement of water and vapor is illustrated in nature’s hydrologic cycle and consists of several processes: evaporation of water from rivers, streams and lakes; condensations of water which produces clouds and precipitation of rain, snow or ice. In building physics, the movement of water vapor takes place in much the same manner.

Water vapor is absorbed in air and the extent to which water can saturate the air is a function of temperature. When air has absorbed all the water it can hold at any temperature it is said to be saturated, or at dew point, or at 100% humidity. Air at higher temperatures can hold more moisture than air at colder temperatures. As a result, vapor pressure is a measure of the force imparted by water saturated in air and is directly related to temperature and humidity.

In a steady state situation, water vapor then moves through a building system by diffusion from an area of higher vapor pressure to an area of lower vapor pressure. When a moisture-laden vapor comes in contact with a colder surface, condensation takes place because new colder air cannot hold as much water. This change of state from gas to liquid or condensation is one of the most destructive forms of water damage in a building.

A paramount concern for engineers, architects and building owners, therefore, is the desire to control and eliminate the destructive forces of condensation in a building. When examining the wall system, extensive calculations can be performed to determine where condensation will occur and how to mitigate its effects with the use of vapor barriers.

Autoclaved aerated concrete wall systems are fortunate in that they do not suffer the condensation problems that can exists with many other wall systems. Because AAC is a highly insulating monolithic wall system, it is virtually impossible for condensation to occur within the wall in either cold or hot environments. Calculations for sample conditions can be provided.

Though autoclaved aerated concrete is a monolithic material and coatings are typically applied for decorative purposes, these coatings can alter the performance and overall effects of vapor permeability and condensation and therefore are important to the performance of the system.

The effects of coatings can be seen for both hot and cold environments.

COLD ENVIRONMENTS:

In cold areas, vapor moves from the warm inside humid environment through the wall and to the cold drier outside. A vapor barrier on the inside, either over furring, or an impermeable interior paint can lower the rate of vapor passing into the wall. In nearly all cases, however, excellent results are obtained by letting the vapor transport into the wall and to escape through the permeable outside coating. If however, a low permeability coating is used on the outside significant problems can develop including a slower rate of drying of the AAC after construction and a gradual build up of moisture inside the wall. Both cases can lead to failure of the coating and damage to the aerated concrete.

HOT ENVIRONMENTS:

In hot humid environments, vapor is mostly driven from the hot outside environment into the cooler inside environment. It may be suggested that it would be beneficial to apply a vapor impermeable coating to the outside to keep vapor from entering the wall. This should be avoided for several reasons.

First, and very importantly, AAC contains large amounts of moisture during construction. The drying process after construction may take as long as one to two years for the wall to reach equilibrium. In this situation the wall will "dry out" after construction with the vapor being driven outside due to solar radiation. If an impermeable coating is applied on the outside this will retard the drying out process and possibly have a ruinous effect on the coating. Secondly, if for some reason water moisture gets in to the AAC from unsealed areas, ground moisture, cracks, or other situations, the units will need to dry out and will therefore need a permeable exterior coating to pass vapor out of the wall. Because steady state conditions normally drive vapor through the AAC from outside to inside in hot humid climate, impermeable coatings should never be applied on the interior walls for these areas of use.

Finally, the question of the possible deleterious effects of vapor passing from the outside into the structure need to be examined. By using vapor permeance calculations it can be calculated that during extreme conditions of high humidity and temperature outside and low air conditioned temperatures inside, a mass of moisture vapor can theoretically pass into the inside living area. This amount of water vapor passing through does not take into account the solar radiant "drying out" effect of the AAC or the mechanical cooling inside which increases the interior pressure. Both will reduce the theoretical effects of vapor passing to the inside. As a result the amount of vapor passing to the inside is negligible.