Chris Matthews of GCI Consultants hosted a recent Everything Building Envelope podcast, with guests Bret Taylor and Jason Bondurant on hand for a comprehensive discussion of how walls are constructed to ward off moisture. Both Jason and Bret are senior consultants at GCI. Bret transitioned from structural engineering to a focus on the building envelope and forensic investigations at GCI; Jason is also a forensic investigator, dealing mostly with existing buildings.
The experienced trio delivered a useful tutorial on wall barrier systems, reviewing construction methods, pros and cons of drainage wall and barrier wall systems, and more.
Drainage Wall System
The drainage wall system is based on the premise that exterior cladding will be able to keep water away from the structure for the most part, but that some water will nevertheless work its way behind the cladding and needs to be managed. Typically, that involves flashing details behind the cladding to redirect water away from the structure and back to the exterior
Any cladding system over a back-up wall should have a drainage wall system, including masonry veneer, stucco and modern brick structures, metal panel systems, and flowstone veneers.
Barrier Wall System
A barrier wall system is designed to stop all water at the exterior face of the wall, relying on coatings, sealants, and sealant joints to prevent any infiltration behind the wall. It’s a “zero tolerance” wall system, Jason says.
Stucco directly applied over concrete or CMU (concrete masonry units) is a common type of barrier wall in South Florida; the system is also used precast panels and tilt-up walls that are typically seen in large commercial and institutional structures.
Pros and Cons of Drainage Wall System
One of the strengths of the drainage wall system is that redundancy is built into design program, which assumes incidental water will find its way behind the cladding at times. This is an asset in storm events that exceed the capacity of windows to resist water, or where other penetrations or defects in the wall permit infiltration; excess moisture may then be drained back to the exterior through the drainage system.
But, as Bret says, the devil is in the details. A high level of craftsmanship and understanding of the components, including through-wall flashing and the weather-resistive barrier (WRB), is critical and the detailing adds to the cost. Architectural elements, such as bump-outs, corner detailing, patios, and decks, increase the challenge.
Drainage walls are designed to shed incidental water, not the bulk water of extreme storm events that may find a way behind the wall. When moisture penetrates the wall, finding the source of the problem often requires destructive testing and correcting it tends to be expensive.
Pros and Cons of Barrier Walls
Barrier walls have the benefit of simplicity. Waterproofing the exterior wall is straightforward and the wall can be monitored by viewing the condition of the coatings and sealants that resist moisture penetration.
But it’s a single line of defense, without a redundancy factor. Minor deficiencies in the coatings or sealants, or the finish layer of stucco, mean water is entering the building and that is immediately a problem. For large buildings, where thousands of feet of sealant may be applied, it’s not unusual to have issues with the product or its application.
The sealants and coatings also need to be regularly replaced, where a drainage wall, if assembled properly, can continue to function for years without needing attention.
Chris, Jason, and Bret have much more to say on the topic of barrier walls, including which types are prevalent in different regions of the US, and variations they’ve found in other countries. Hear the entire conversation at Everything Building Envelope, and be sure to subscribe when you’re there. You don’t want to miss any of the animated discussions between leading experts in the construction industry.