Open-joint cladding systems have been getting a lot of attention from architects, in part because they allow the addition of texture and depth to walls. On a recent project, I had a lot of fun designing the exterior façades, as I was no longer limited to the repetitive pedestrian vernacular of typical cladding materials. Mixing the open-joint system with a panel system, I could play with an almost-Mondrian aesthetic; and the open-joint areas offered a distinct warm and patterned texture. While this may be liberating for designers, it can be intimidating for builders unfamiliar with the system, who often express concern with the performance of open-joint claddings, which stems from a widespread misunderstanding of what claddings are.
Cladding Performance
The primary function of a cladding is as a kind of armor. That’s still the case with open-joint cladding. The outer boards protect the building from lawn mowers, baseballs, and tree branches, and from destructive ultraviolet light. However, with an open-joint system, we are no longer pretending the exterior cladding is a water barrier.
Water control. Almost no cladding—certainly none of the usual suspects used for residential work, such as fiber cement, wood, engineered wood, stucco, or brick—is a true barrier. They all leak! Most of us know we need to create an effective water-control layer behind the cladding to protect walls from water damage. Many of us have been installing ventilated rainscreens as a regular practice behind our cladding for a long time. These work incredibly well even if the water-resistive barrier isn’t perfect, for two reasons:
- Water that gets behind the cladding drains very fast (96% in seconds vs. 80% after hours).
- The gaps at the bottom and top of the wall allow roughly 80 to 90 cfm of air to flow across the back surface of the cladding. This helps evaporate any droplets of water clinging to the WRB or a tape edge.
I contend that an open-joint system works even better than other ventilated rainscreens because now you have hundreds of cfm of airflow across that surface. It’s impossible to keep it wet. This massive airflow also helps deter insects, I believe. For some reason New Englanders and Northern Minnesotans see open-joint claddings and start asking about bugs and critters. I’m not sure which bugs they are worried about, but given the absence of protection from the rain, the massive amount of air moving through the voids, and the absence of moisture, I’m not sure which critter would find this habitat attractive.
Kryptonite.
Any great super hero must have a weakness. Open-joint systems are more vulnerable to degradation from UV light. Anyone who’s seen black paper turn brittle and crack from a month or so of exposure to full sunlight knows any WRB is susceptible to damage from sunlight. Eyebrows go up when we talk about creating gaps between cladding boards from 3/4 inch to 2 inches. While UV degradation might be a concern, it’s manageable. To better understand, let me explain a system I put together for the project shown below, which I think mitigates all concerns and creates a resilient system I expect will last 100 years or more, which is the standard I strive for when designing and building all my projects.
Recipe for Resilience
Working from the exterior inward, here are the parts and pieces of my open-joint system:
Cladding. The horizontal cladding boards are TimberTech Azek decking—a four-sided cellular PVC- capped board that is a full 1 inch thick. I used all three available widths—3 1/2-, 5 1/2-, and 7 1/4-inch-wide boards—to create a long, seemingly random pattern on the open-joint areas of the wall. We installed the boards with Cortex screws so the fasteners disappear on the face.
I have, in the past, expressed my dislike of vinyl siding from a sustainability perspective (I still don’t like the stuff), but this is different. This cellular PVC cladding is 100% recyclable, requires zero maintenance, and has very low embodied energy. Keep in mind that a deck, which this material was made for, sees substantially more sunlight than any cladding, and has snow and water laying on it for long periods, to say nothing of the impact abuse it is designed to withstand—conditions that these boards simply aren’t going to see on a vertical wall. I have no doubt it will withstand the elements for as long as just about any material available.
Battens. We used Benjamin-Obdyke’s Batten UV corrugated plastic battens. These 3/8-inch-thick by 1 1/2-inch-wide polyethylene battens are black; they are designed to disappear from sight in an open-joint cladding system. I like the corrugated material, which has open cells across the width of the batten that allow for cross ventilation. Polyethylene is impervious to water, so any moisture that might collect on the leading edge of the cladding boards at each batten will simply evaporate; it won’t lead to rot as it will over time with wood. The plastic battens are also very light, so they can be attached to the sheathing with a few staples prior to cladding installation. The fasteners for the cladding, which are driven into the sheathing, hold the battens in place. You don’t need to line up the battens on the studs.
Invisible layer. Along with the battens, we opted for Benjamin-Obdyke’s InvisiWrap UV—a flat-black nonwoven building wrap. We taped off this wrap with Benjamin-Obdyke’s HydroFlash UV+ seam tape and flashed penetrations through the wall with 4- and 6-inch HydroFlash UV+ flexible flashing. These all have an acrylic adhesive, and they are flat black to blend in with the InvisiWrap, essentially disappearing from sight behind the open joints in the cladding.
Backup water-control layer. In this wall system, the primary water-control layer is the fully taped Invisiwrap UV WRB, but we’re designing for the long haul, so we have a backup; the taped WRB facing on Zip R-Sheathing. From a bulk water-control perspective, I have a redundant system —a seat-belt and air-bag approach, as I like to say. This is what gives me confidence in the UV protection. Benjamin-Obdyke provides a 25-year warranty when the whole “UV-stabilized” system (wrap, tape, and battens) is used. While I’m not a fan of warranties, they are helpful to understand what the manufacturer sees as product limits. In this case, the warranty allows for a 2-inch gap and allows one full year of total exposure before cladding is installed. The gap I used on this project is 3/4 inch and the cladding is a full 1 inch thick, and the materials saw less than a month of exposure—all are variables that will affect the longevity in service. I am relatively certain that it will take a very long time for our invisible layer to experience any degree of UV degradation. Even if it were to become brittle after 40 years or so, I know that a tear in that membrane will only expose a pristine Zip System that is itself fully taped and flashed. This wall assembly is not likely to ever see any water infiltration or damage … ever.
Bottom Line
Open-joint cladding systems work. With the introduction of PVC decking as cladding as an alternative to tropical hardwoods, I’m confident in the long-term viability and sustainability of the cladding, and with the first residential UV-stable system from a single manufacturer, I’ve got an affordable, resilient WRB and batten that lets me create beautiful homes with confidence.
