A Cup of Joe: Stucco Rainscreens Explained
Stucco Rainscreens 101 with Dr. Joe Lstiburek
Watch stucco rainscreens explained with Building Science Corporation’s Dr. Joe Lstiburek, where he’ll take us back to the 1940’s and the first iteration of the rainscreen. He then expands on the evolution of the system, and where we are today in stucco rainscreen design.
We’ve also summarized Dr. Lstiburek’s presentation below.
Moisture control in the 1940’s
The first iteration of the rainscreen goes back to 1946. The Vikings (not the ones from Minnesota, but from Norway) are credited with conceptualizing the simple, elegant solution for moisture control.
The general idea was to have a secondary line of protection for rainwater. The Norwegian Vikings believed some water would pass through the outer skin of the building, and concluded that a form of control for the water was necessary.
That means you have drainage, as well as back ventilation. This allows the wall to dry from the inside out into the cavity, and be ventilated outward.
It also allows the rendering – in this case, stucco – to dry into that cavity as well, and redistributed to the outside, so you’re getting both drying of the backside of your cladding, and drying to the outside of the wall assembly behind your cladding.
Reservoir claddings and inward vapor drive
And that’s the rainscreen system, right?
Not quite.
Sometimes these materials store and absorb water. These are known as reservoir claddings, which includes both stucco and brick.
Reservoir claddings get ‘charged’ when rain falls; think of them like moisture capacitors that discharge when they’re hit with, for example, sunlight.
This creates concern about inward moisture drive. Let’s say it rains on stucco or brick, and they get wet. When the sun comes out, it elevates the temperature of the water and the brick – driving the water inward and outward.
Side note: This is why you never want to paint your stucco with something that’s a vapor barrier, because it’ll bubble and blister. If that happens, that’s your wall telling you, ‘You messed up,’ by not picking a permeable paint.
The water that’s driving outward is exactly what we want. It’s the moisture that’s driving inward you need to worry about.
One way of preventing that is to put a condensing surface there. A vapor barrier, for example, can be used to prevent inward vapor drive; however, you’re preventing the ability for the assembly to dry too deep from the inside to the outside.
If you’re going to have a vapor control layer to combat inward vapor drive, you want an air gap behind it. That way, you can back ventilate that vapor control, making for an effective strategy.
You might also like…How to Control High-Risk Moisture Environments
What you’re doing is ‘uncoupling’; you disconnect the reservoir cladding from the rest of the assembly with this vapor control air, but you couple it with an air gap that controls hydrostatic pressure that provides drainage as well as back ventilation. That allows the wall assembly from the inside to dry into this space, with the water dissipated to the outside.
That is the art of the rainscreen, and how rainscreens have become one of the best tools for moisture management in high-performance building.
Rainscreens have come a long way since then.
Check out one of the more advanced rainscreen systems available today: DELTA®-DRY & LATH, a 2-in-1 product that combines the proven technology of the DELTA®-DRY rainscreen with an innovative fiberglass lath for a complete, one-step moisture control and lath system.
About Dr. Joe Lstiburek
Dr. Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. Dr. Lstiburek holds a Bachelor of Applied Science in Mechanical Engineering, a Master of Engineering in Civil Engineering, and a Doctor of Philosophy (Ph.D.) in Building Science. Get the full scope of Dr. Lstiburek’s work, accolades, and contributions to the industry over at Building Science Corporation.
