Effective rainwater management depends on a combination of limiting the wetting of components that are susceptible to deterioration and enhancing the drying of components that become wet. The rainscreen principle is a design approach that achieves the first half of the combination by limiting the wetting of wall components. However, an unresolved issue is whether it will enhance the drying of these components, i.e., achieve the second half of the combination. An experiment was conducted to measure the effect of drainage cavity design on the drying of water from the insulated stud cavities of wood-framed, plywood-sheathed walls. Five test specimens, all finished with stucco cladding, were included in the experiment. At the start of the experiment, a measured quantity of water was injected into the stud cavities. During the experiment, temperature and vapor pressure differences were controlled across the specimens, and the drying of the specimens was monitored. After 5 1/2 months the specimens were examined for evidence of moisture absorption and decay. The following conclusions were drawn from the experiment:
• The drying process for all specimens was very slow and took months to achieve any significant effect.
• The rainscreen design does not enhance drying of water that penetrates into the stud cavity, nor is the drying rate affected by cladding design or by drainage cavity design.
• Moisture movement within the specimens was very limited. From a practical perspective, where water initially accumulated is where it stayed.
The experiment showed the importance of designing for effective rainwater management since it is essential that little, if any, water should be permitted to get into the insulated stud cavity.

Authors: Mark D. Lawton, P.Eng.; William C. Brown, P.Eng.; Andy M. Lang

Citation: Thermal Performance of the Exterior Envelopes of Buildings VIII

Keywords: December, Florida, 2001