Two recent KCB projects that had stucco exteriors challenged the team to come up with the best solution to repair and preserve the material.

Stucco is thought to have arrived in America in the southwest, brought in by the Spanish. It has a pleasant look to it, with assorted textures to choose from, and it spread across the country in the early part of the 20th century. At that time, if homes had insulation, it was quite porous, allowing lots of air to move through the wall cavities. This meant that any moisture which found its way into the walls—and the stucco itself—could dry relatively quickly.

In modern times, homes have been renovated to include far more airtight assemblies with more robust insulation, leading to the trapping of moisture. With the drier, warmer climate in the southwest, there is less humidity and rain to become trapped, meaning the advent of tighter houses had less detrimental effects. But New England’s wet, humid summer conditions soak these structures, and with no air flow, don’t tend to dry as well. Add cold weather into the mix, and that water inside freezes—expanding and cracking the brittle stucco material. What is needed is a mechanism to promote drying.

Enter the modern rain screen: a wall assembly with an “air gap” in the middle, which allows air to flow through and dry things out. KCB recently had our first opportunity to implement this system with a stucco application.

Ezra inspects the rotted wood beneath the stucco

A home in Moss Hill had sprung a leak, with water entering the wall through a rotten window sill, living in there for years before making itself known to the inhabitants. KCB was called in to assess, and we needed only remove a small amount of existing stucco before discovering the massive extent of the rotten wood underneath.

In rebuilding this wall, we replaced the rot, then applied our usual water-resistive membrane to the wall sheathing, which will keep the water out of the wood frame of the structure. But it’s not enough to apply the stucco directly to this, because it won’t be able to “breathe” and dry out. The air gap was the next layer outbound, by way of a plastic, fibrous product, similar to a less-dense Brillo pad. In this application, we chose to follow this with a layer of cement board, which was then coated with a thin layer of stucco.

Key to solving the problem is allowing air to enter and exit the air gap. At the bottom of the wall, this was achieved with a termination bead to create a clean edge and to hold the stucco and cement board ¼” off of the flashing at the bottom of the water-resistive layer. At the top, we chose to use a specialty termination bead with a built-in vent, which neatly allows the air to flow through and exit the air gap.

With this pillow of air surrounding the cement board and stucco, the material will not remain wet for long after rain, the insulation can continue to do its work for the climate-controlled interior of the home, and the freezing action of our New England winters will have little impact.

We learned a lot about stucco that we can now apply to other projects.

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