Brick, Stone, and Stucco Assemblies – Danger, Will Robinson…Danger*

Brick, stone, and stucco are proven claddings…except when they are not. They used to be, but not anymore. What changed? A bunch of things – one of which was/is the effect of inward vapor drive. 

Inward vapor drive is an issue where a “reservoir” cladding is located on the exterior of a wall assembly. What is a reservoir cladding? Ah, Grasshopper,** it is a cladding that stores moisture when it gets wet after a rain event, hence the term “reservoir.” Typical reservoir claddings are brick, stone, and stucco.

Reservoir claddings can overwhelm typical wall systems. Consider the case where a brick veneer becomes saturated after a rainstorm. The brick acts as a reservoir storing water. When the sun shines on the rain-wetted brick veneer, it raises the temperature of the water stored in the brick. This water is now driven out of the brick in both directions (Figure 1). The outward drive does not hurt the assembly, but the inward drive can. The water vapor driven inward can pass through the air gap, vapor permeable housewrap, and vapor semi-permeable sheathing into the wall cavity.  It is possible to drive sufficient moisture into the assembly to create problems. 

Figure 1

One effective way of addressing this issue is to have a cavity behind the brick veneer free of mortar droppings that is vented at the top and bottom. The moisture driven inward out of the brick can then be intercepted by a moving stream of ventilation air that dries the assembly to the exterior. The problem with that approach is getting a cavity free of mortar droppings.

A more effective way of addressing the issue is to install an exterior vapor barrier that is back ventilated and provides drainage; one such as DELTA®-DRY.

What is nice about this approach is that it also allows outward drying through the sheathing and a vapor-open water control layer on the sheathing. The wall can dry into the air gap and the water vapor can be vented to the exterior.

If the wall also does not have an interior vapor barrier, we have a classic “flow-through” assembly. It dries in both directions, even with a reservoir cladding like a brick veneer. It does not get better than this.

Stone veneers, stucco veneers, and manufactured stone veneers (aka “lumpy stucco”) have similar issues to brick veneers. And the issues can be addressed in the same manner – an exterior vapor barrier that is back ventilated and provides drainage: DELTA®-DRY.

This was never a problem when brick was installed over masonry cavity walls…or when stucco was installed over multi-wythe brick mass/storage walls, or when we had solid stone walls. Masonry, brick, and stone covered with brick, stone, and stucco did not care if they got wet from inward vapor drive. Today, oriented strand board (OSB) cares very deeply and passionately, as does gypsum sheathing, when it is covered with a reservoir cladding. Caring and passion go into overdrive when we have wood or steel studs insulated with fluffy stuff coupled with the OSB and gypsum sheathing. And when we add interior vinyl wall coverings or interior plastic vapor barriers…. Wow – passion overload, and the wrong kind. We need to uncouple the reservoir. Let me repeat: We need to uncouple the reservoir.

By George, she’s got it…***

*Phrase uttered by Robot B9 from the TV series Lost in Space when someone is about to do something stupid. Robot B9 is not to be confused with Robby the Robot from the movie Forbidden Planet. Robby the Robot came to life in 1955…the same year I did. Robot B9 was on television in 1967…the last year the Toronto Maple Leafs won the Stanley Cup.

**Phrase uttered by Master Po, a blind monk, to his student Kwai Chang Caine just before he says something profound from the TV series Kung Fu. The series originally aired in 1972…a year that the Toronto Maple Leafs did not win the Stanley Cup.

***Phrase uttered by Professor Henry Higgins when Eliza Doolittle says, “The rain in Spain stays mainly in the plain” in the musical My Fair Lady, released in 1964…a year that the Toronto Maple Leafs did win the Stanley Cup. Eliza Doolittle should have said “You need to drain the rain on the plane.” She apparently did not know about drainage planes.