Zippity Do Don’t

Volume 1   Issue 5

In the two hundred year evolution of sheathing for houses, we’ve gone from boards to plywood to OSB (oriented strand board) and the latest entry, water-resistant treated OSB. The latter is sold as the ZIP-WALL System (a roof sheathing version is also available) and is marketed aggressively by Huber EngineeredWoods as the perfect substitute for the more conventional sheathing and house wrap system. Basically the product replaces the water-resistant/vapor-permeable house wrap barrier with a factory applied coating followed by a site applied joint tape.

Let me list all the reasons that I just love this system…….. (sound of lone chirping cricket).

Okay, let me tell you the reasons I don’t love this system by going down the list of glowing accolades provided by the manufacturer.

Added to what? It is OSB. The literature goes on to say that it provides unmatched strength…Huh? Here’s the truth about OSB: In racking and shear strength testing it is about equal to plywood of the same thickness. But the testing is done on new sheets, before they take on the moisture they are so vulnerable to. Anywhere OSB is subjected to humidity or direct contact with even minimal amounts of water, it starts to deteriorate quickly. If testing was done on plywood vs. OSB after a few years of service, the claims for OSB’s equivalency to plywood would not hold up. Bottom line: the best and most durable sheet panel product for sheathing is plywood.

Again, superior to what? The building science behind all water resistant barriers (WRBs) whether they are building wraps or sheet goods such as ZIP-WALL System, is that vapor gets through, bulk water does not. We need vapor to pass through so that moisture can be carried through to the outside. The ability of a product to do this is measured in perms. The higher the perm rating the more vapor that can pass through. Most building wraps have a perm rating in the twenties. ZIP-WALL’S perm rating (per the manufacturer) is 12-16. But even that reduced rating is questionable, as a third party testing result showed it as less than one! I don’t have any information on the third party testing, but even ZIP WALL’S own perm rating of 12-16 compared to building wrap’s twenty-something calls into question the claim that its product has superior moisture protection while still allowing the panels to properly dry.


This one might be true, but for the wrong reason. The job of any WRB is to let vapor through but be an air barrier. Any product with a low perm rating will be a superior air barrier. If blocking air was the only job, we’d wrap the house with plastic, which has a perm rating of zero, but we all know what a disaster that would be. If the third party testing mentioned above happened to be accurate and the ZIP WALL’s in-place rating really was “one”, it’s getting mighty close to being a plastic wrap.

There is none. Well, actually the manufacturer says the surface texture is a drainage plane…sorta.  water vapor moves to the colder surface between the sheathing and the siding, it condenses and turns to water droplets. They need a pathway on which they can be carried away. That pathway is a drainage plane, and many building wraps have a built-in drainage plane. Some are simply a deformed surface that creates enough irregularities to allow water droplets to be pulled to the ground. Others have capillaries that absorb and drain water away. In what is a pretty disingenuous attempt to address the lack of a true drainage plane, Huber’s on-line video shows a test of its product that purports to show that it addresses the drainage plane issue. It clamps a piece of Plexiglas to the ZIP surface and applies water to the top edge showing that water trickles down behind the Plexiglas. It then shows a piece of non-ZIP OSB with a flat piece of house wrap and the same Plexiglas clamped to it showing less water movement. The claim is that there are enough irregularities on the surface of the zip to create a drainage plane. If that was truly the case, then the control test should have been with a building wrap with a built-in drainage plane to see how they compare. Furthermore, even if a case can be made for some water movement on the surface, I’m betting that the same Plexiglas clamped to the horizontal smooth ZIP tape would show a greatly reduced if not a total stoppage of drainage plane action.

Any well-seasoned builder will tell you that to keep water out of a building, you need to shed and re-direct water away from the building. Sealants, caulks and adhesives take a different approach by plugging the seams, gaps and holes defying water to breach its fortress until the sealant ultimately fails and breaks down, as they always will. Conventional house wrap sheds water in a shingling effect in that upper layers overlap lower layers, always directing the water away from the building and into the drainage plane. Zip Wall tape, no matter how good the adhesive, is essentially a reverse shingle that relies on adhesive to overcome the practical advantage of shingling overlay. When the adhesive breaks down, water plants its flag in the opening, eventually claiming its victory, the spoils being rotted OSB and framing beyond. This same reliance on sealant rather than shedding happens at the top of all window and door openings where Zip tape is the last line of defense for the relentless onslaught of water. I don’t doubt that the Zip tape adhesive is darn good as adhesives go. The whole system has a thirty year warranty. But wait... what happens in year thirty-one? I guess if you are using a low quality siding that needs to be replaced every thirty years or less, you can re-tape the joints. If you’re using a high quality siding though, you’re on your own when the tape fails and the siding hides the sudden breach of your tape defense and your OSB is signing a truce with the victorious water that includes terms allowing a permanent hostile occupation.

Huber claims that the ZIP-WALL SYSTEM saves 40% on labor. This is the claim that really sells this product for most builders. There’s a video online produced by Huber that shows what Huber calls the “…amazing labor savings” discovered in a side-by-side demonstration pitting the ZIP-WALL SYSTEM against a standard sheathing and house wrap system. It’s a two story structure, maybe 2500 SF, and two “experienced” crews go at it with the fury that only time-lapse photography can simulate, and the “amazing” result is that four hours and twenty minutes are saved by the
ZIP WALL crew. As an added bonus, to drive home the point that taping joints is a one-man operation while applying house wrap is a two, there is a follow up real-time segment in which a single ZIP tape dispenser operator who looks like he was just fetched off an NFL football field, gracefully and athletically applies ZIP tape to the seams, while the non-ZIP participants are played by what looks like two geeks from accounting with hard hats who wrap the house and themselves in a hopeless frenzy to keep up with Mr. ZIP. (I admit to some embellishment here of the video portrayal, but I wanted to make a marketing point and since I don’t suspect that Huber is one of the three people who read my blog regularly, I believe I’m safe from industrial reprisal.)

There’s a second video. And the results are not good for the ZIP-WALL System narrative. This one was done by the National Association of Home Builders. I suspect that the NAHB entered the fray because any seasoned building expert with a high level of practical experience is going to view the claim for labor savings of ZIP over the conventional house wrap with a raised eyebrow. The NAHB trials showed that in several tests, there was no discernable difference in labor between the two systems, and in fact, some trials had the house wrap system beating the ZIP-WALL System. For a three man crew that’s six raised eyebrows.

Even accepting the four hours and twenty minutes of labor saved in the original test puts an approximate value of the “amazing” ZIP-WALL System at about $250. But wait, we’ve got to pay for the ZIP panels, which on a 2500 SF house is about an additional $720 over standard OSB. Heck, it’s even $360 more than ½” CDX five-ply fir plywood sheathing, which is truthfully the best and only sheathing that should be on a house, and is what we use as standard on a Connor Homes!

So how can it be that the most acclaimed reason for using the ZIP-WALL System (labor and cost savings) doesn’t even exist? Now that’s amazing!

ZIP-WALL System also has a roof sheathing system that’s even more puzzling, because unlike the wall system that addresses the complex building science conundrum of keeping bulk water out while allowing vapor to pass through, the ZIP roof system pretty much only addresses the one job required by a cold roof, keep the water out. And the system only has to do that job for a short period of time until the roofing goes on. So the only reason to justify the added cost is if you think you can tape OSB joints faster than you can roll out roof underlayment. I see raised eyebrows. More importantly, if it is a sin to use OSB on sidewalls, it’s condemnation to hell to use it on a roof. Some roofing materials such as wooden roofs carry no warranty if applied over OSB. This is because the long-term nail holding strength of OSB does not equal that of plywood. And one more damning piece for OSB: the International Association of Certified Home Inspectors rates the service life of OSB to be 60 years, plywood to be 100+. Other rating agencies have varying but similar numbers. If you’re building the 40-50 year house as many building codes promote and allow, then I guess you’re covered. But if you are building homes that are expected to last at least a hundred years and more, well…

If you asked a hundred builders what they thought of the ZIP-WALL System, I’ll bet ninety of them would endorse it. Such is the power of marketing that keeps up a steady drumbeat of “facts” that slowly but steadfastly become imbedded in a narrative that literally is too good to be true. I’ll also bet that the ten who don’t endorse it are seasoned and experienced and hardened by previous claims by other manufacturers of various building products that just didn’t pan out.

Huber Engineered Woods makes some good building products. The ZIP-WALL System is not one of them.

 - M. Connor



1 Comment

  1. Sidney on March 12, 2018 at 1:16 pm

    Brilliant! “…if it is a sin to use OSB on sidewalls, it’s condemnation to hell to use it on a roof.” I’ve always been suspicious of using “scotch tape” as a building detail to keep water out. Better to use gravity, capillary breaks, etc.

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