Cavity Wall Design
In short, a cavity wall is two wythes of masonry, separated by a gap of varying dimensions. A masonry wythe can be comprised of concrete, structural clay tile, or solid brick. The 'cavity' or gap within the wall itself can range from two to four and a half inches in width. A wall cavity may or may not be insulated depending on the project at hand. When all of these elements are combined together with appropriate details, sound structural design, high quality materials, and excellent craftsmanship the result will be a premium performance cavity wall.
4 Basic Cavity Wall Properties
#1 Moisture Penetration Resistance
Cavity walls are designed and built to become a moisture-deterrent system as no single, unreinforced, four inch masonry wythe is completely resistant to moisture penetration on its own. While moisture can and will penetrate into masonry wall hairline cracks (typically between mortar and unit), this system ensures that any extra moisture flows unobstructedly down the face of the cavity on the outer wythe as its design provides a clean air space and flashing. Flashing is installed at certain locations throughout the cavity itself and diverts all moisture back to the buildings exterior by weep holes. It is imperative that proper drainage occur within the cavity wall to avoid the risk of freeze-thaw damage or efflorescence.
#2 Thermal Energy Efficiency
Energy conservation is a major concern and should be considered in all constructs. Fortunately, insulated cavity walls work well at minimizing both heat loss and gain. Each wythe acts as a heat reservoir having a positive effect on both cooling and heating modes. As the air space isolates each wall (exterior and interior), heat is able to be absorbed and dissipated by the outer wythe before it can reach the inner wythe through the large cavity making it unable to reach the buildings interior. Thus, any building utilizing cavity walls stays warm in winter and cool in summer, basically on its own accord.
#3 Fire Resistance
The ASTM E-119 Fire Resistance Tests clearly reports that masonry cavity walls have superior fire resistance. Both the Fire Resistance Ratings (ASIG)3 and the Fires Protection Planning Reports (CMIPHC)2 contents address how all cavity walls have a fire resistance rating of 4 hours or more.
#4 Structural Properties
The structural potential of masonry material is outstanding and cavity walls are no exception. Three principle factors that contribute to the overall compressive strength of a cavity wall include mortar type, individual unit stability, and craftsmanship quality. Prism testing is encouraged to test the structural strength of larger projects since it can calculate actual values versus assumed strengths. Not only can cavity walls bear weight well, but they can also act as a backup system in the case of natural disasters. Every cavity was has two wythes by design and each wythe acts as a separate wall able to withstand heavy winds among other elements. Tests have reported that the joint reinforcement found within cavity walls provides a certain amount of transferable shock (around 20%-30%) across the wall cavity itself.
Cavity walls are extremely impressive structures. Their construct is well worth the extra time and effort as its design can not only withstand the elements but also conserve energy. Cavity walls are a win-win for everyone.