MHE International

Autoclaved Aerated Concrete - Material Advantages - Moisture Behavior

  • "Macro" pore structure results in minimal capillary action when unprotected AAC is exposed to water
  • AAC-compatible, vapor permeable coatings that allow internal moisture to dissipate should be specified for AAC products

Moisture from both external and internal sources can cause damage to buildings, therefore, moisture protection is a primary consideration.

External moisture sources include rain and water from the soil. Internal moisture, usually in the form of humidity, can cause condensation on the surface of the walls as well as condensation inside the wall itself. The goal when considering moisture protection details is to avoid allowing moisture, either from external or internal sources, to damage the building or make the building uncomfortable.

Moist walls can allow mold to grow, cause surface discoloration and cause damage to plaster and wall finishes. If the wall is permitted to retain internal condensation, the thermal resistance is decreased, thus reducing the thermal efficiency of the wall. Buildings with consistently moist walls and floors also allow for unhealthy environments since mold, mildew and bacteria can be present.

AAC has a very porous structure which is characterized by "macro" pores. Macro pores are small air bubbles evenly distributed throughout the material. The pores in a matrix characterized as "micro" pores are very small in relation to those in a "macro" matrix. Capillary action is very strong in structures characterized with micro pores as compared to AAC. Therefore, absorption of water into the AAC material through capillary action is minimal. Since this absorption of water is minimal, coating of the walls during the building construction can be scheduled at any convenient time or sequence. The interior elements and components of the building may be completed without concern of damage due to water migration through the AAC material.

All buildings must be protected from the influence of external moisture sources. The most common and effective ways are to apply water-protecting brick facades, stucco or coatings to the exterior surface of the walls. The protection against water influences from rain using these types of treatments are discussed earlier.
The protection of the building from water in the soil is effectively achieved by using a waterproof mortar on the floor slab under the first course of block or wall panel as shown in the construction details in our

Another very effective method is to place a horizontal moisture barrier in that first mortar joint.

Internal moisture and wall condensation in climates with high temperatures and humidity should be controlled by correct sizing and cycling of the air conditioners. Dehumidifiers may also be integrated into the air conditioning system.

For buildings with suspended ceilings, the ultimate goal for peak performance is to attain the same quality of air above and below the suspended ceiling. The entire space should be controlled to achieve a common environment - in terms of temperature, humidity, and air circulation. When necessary, fans may also be placed in the space above the ceiling in order to increase the air circulation.

When using AAC Wall and Floor/Roof panels together, a very airtight construction is achieved. Therefore, prior to enclosing the building, sufficient time to allow the solid AAC material to dry-out should be given. The most desirable and cost efficient method is to allow the environment to dry-out the building naturally. The building's supply air should be controlled using dehumidifiers. The air conditioners should control and recirculate the interior air only. They should not be used as a mechanism to introduce make-up or supply air into the building.

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