Transitional masonry construction gradually evolved into a new building style, that of lightweight "curtain wall" construction installed over internal structural framing. This construction was not only lighter than masonry, but slip connections to the structure allowed independent movement of the cladding and the framing.
While this greatly reduced the risk of cladding failure due to differential movement, curtain wall construction introduced a new set of problems. Unlike masonry, lightweight claddings typically have little or no storage capacity for heat and moisture. They rely on waterproofing and thermal insulation to control the elements.
Early curtain wall systems were built without redundancy; that is, they relied on the exterior surface of the wall being waterproof to prevent leakage into the building. As designers began to understand the differences between masonry and curtain wall construction, new designs developed that included multiple planes of protection.
The most common of these is the drainage-plane wall system in which the primary waterproofing is concealed within the wall construction. The cladding is not expected to be completely waterproof but acts as a rainscreen, preventing most rainwater from reaching the internal waterproofing. An air space behind the cladding promotes drainage of penetrating water, with flashing at floor lines and wall openings to collect and direct water back to the exterior.
Controlling water penetration is only one of several challenges that designers faced in the development of contemporary wall construction. The trend toward airtight buildings led to a variety of potential moisture problems unrelated to exterior water leakage. This is worsened by the fact that many modern materials such as wood fiberboard and paper-faced gypsum products are, unlike traditional masonry materials, easily damaged by water.
The introduction of insulation in curtain wall assemblies created cold planes within the wall system that are subject to condensation due to air leakage and water vapor diffusion. Similarly, the design of air-tight enclosures without adequate mechanical ventilation can lead to excessively high interior moisture levels due to occupant respiration, artificial humidification, and so on.
Condensation on windows, doors, and other interior surfaces, combined with hidden condensation within the walls, can produce damage equal to or greater than that from exterior water leakage. Understanding the dynamic performance of these wall systems, the proper location of components such as insulation and vapor retarders, and the design of appropriate HVAC systems are required to prevent such problems.