Copyright Goodheart-Willcox Co., Inc. 288 Section 4 Construction Details Resisting Moisture Condensation Water vapor condensation can cause moisture problems in masonry walls and should be mini- mized where possible. When water vapor passes through pores of homogenous walls, which are warm on one side and cold on the other, the vapor may condense into water inside the wall. But if the flow of vapor is blocked by a vapor-resistant mate- rial in the wall, the vapor may not reach the point in the wall where the temperature is low enough to cause condensation. Condensation problems most frequently occur during the heating season in tight, highly insulated buildings. The interior air gains moisture, increasing the interior vapor pressure above the vapor pres- sure outside. The difference in pressures drives the vapor outward from the building through vapor porous materials in the wall assembly. A vapor barrier or retarder can be used to impede the migra- tion of the water vapor. The vapor pressure differen- tial across the wall section can be decreased through ventilation or dehumidification. In newer construction, high moisture locations, such as bathrooms or kitchens, may have mechanical devices that remove high moisture-laden air. Another device used to reduce heat loss in certain buildings is a heat exchanger. A heat exchanger takes moisture- laden air and exchanges it with outside air, which has lower moisture content. As part of the exchanger process, the higher temperature moisture-laden air transfers heat to the colder exterior air being brought into the exchanger. A barrier exists between inside and outside air so they do not mix together, but heat is transferred to the incoming air to increase energy efficiency and air quality. Water affects many building materials. For example, wood expands with increasing moisture content. Varying humidity conditions in different parts of the cross section of a single wood framing member may cause warpage. High humidity can also cause wood to decay. Water also promotes the corro- sion of metal. The insulating value of most materials is greatly reduced by the presence of free water. Changes in the volume of fired clay masonry units and concrete masonry units occur due to gains in moisture content. These changes should be considered in the design process. Alternate freezing and thawing of masonry units when saturated may eventually result in cracking and spalling. Moisture caused by condensation may contribute to efflorescence if soluble salts are present in or come in contact with brick. Reinforced Masonry Walls Reinforced masonry walls are walls built with steel reinforcement embedded in the masonry units. See Figure 14-23. The walls are structurally bonded by grout that is poured into the cavity (collar joint) between the wythes of masonry. The grout core seals the space between the wythes of masonry and bonds the reinforcing steel. Full bed joints are used for reinforced walls. Portland Cement Association Figure 14-22. Stretcher block being placed as the backup wythe. Goodheart-Willcox Publisher Figure 14-23. Reinforced masonry walls of brick and concrete block. Steel reinforcement Steel reinforcement Hollow Units Solid Units