Weather-resistive barriers in stucco cladding systems

Recent moisture related building envelope failures in wood frame walls with stucco cladding provided a reason for a question if adjacent components such as wood-based sheathing board or stucco layer could affect the field performance of water resistive barriers (WRB).

This thesis either selected or developed a few experimental techniques for laboratory testing of WRB with the view to their use for evaluating the material performance. In particular, test methods such as air permeance test, modified inverted cup test, and water absorption test were used to examine the effects from WRB surroundings on air and water transmission through the materials.

Theoretically, WRB resistance to water transmission changes when chemicals e.g. surfactants or wood extracts are dissolved in the water. Yet, the measured moisture transmission did not indicate significant effects when wood extracts were dissolved in water or deposited on WRB surfaces. Neither four-month outdoor weathering nor laboratory mechanical stretching showed a significant effect in WRB performance when measured with typical test methods. This is because in most cases vapour transmission is a dominant phase of the transport.

On the other hand, when employing more discrimination test methods, the interaction of bentonite particles with the wood extract deposits was evidently found to alter the water transport rate.

Although this study did not address explicitly the durability of different WRB products, it showed the significance of an interactive weathering mechanism that could affect the long-term performance of some WRB products. By enhanced understanding of the interactions with the adjacent components of the wall, this study will assist in improved design of stucco cladding systems.