Plywood Vs. OSB or Is Cheaper Really Better? or How To Keep Your House From Exploding On the Highway

Today we went shopping for the final bits of lumber for our build: plywood sheathing. We decided to buy plywood from a lumberyard instead of the big box stores because the online reviews from contractors and homeowners for plywood all seemed to say the same thing: big box store plywood is a gamble. Sometimes you could find top grade plywood good enough for cabinets at an unbeatable price, but most of the plywood was the stuff that would delaminate the first time you sneeze on it. Since we knew we couldn’t tell the difference, we decided we didn’t want to take the risk. That’s why we went to our local lumberyard and paid a slight 3%-5% premium for the stuff contractors use.

When we arrived at the lumberyard, I spoke to an employee there and told him I’m building a tiny house and was looking for a few sheets of 15/32 plywood (used for the sheathing), and a few sheets of 19/32 (for the roof). The clerk thought this was pretty neat, because he was thinking of building a tiny house himself.

That was when he asked “So, I’m just curious, what are you guys going to use this stuff for?” I replied the plywood was going to be for framing.

“Interesting,” he said “have you guys thought about using OSB (oriented strand board)? It’s stronger. Might be better for a tiny house.”

“What?” I asked, perplexed. The plans we had called for plywood, not OSB. I didn’t remember why at the time, but that’s what it had said. That, and my research (which I had hopelessly forgotten when the clerk started talking to me) indicated that plywood was in fact better suited for tiny houses (more on that later). My dad, who was even a woodworker, said he wouldn’t trust OSB as far as he could chuck it. “But, I thought plywood was stronger?” I asked the clerk, trying to remember why I was buying plywood in the first place.

“No way,” his fellow co-worker said. “OSB is stronger. True story.”

Sierra and I looked at each other. I had this vision of our house speeding down I-40 hitting 60MPH and suddenly shredding into thousands of 4×8 slices of plywood, like a Webster’s Dictionary in a tornado. “What are we going to do?” I asked. She shrugged.

“OSB is less expensive than plywood too,” the clerk offered. “Might save you some money.” This struck me as a strange thing to say. Wasn’t he just trying to sell me on OSB? Shouldn’t he be trying to get me to pay for a more expensive product? Whatever happened to working for commission? I decided to go back to the truck and fetch the plans to see why they recommend plywood over OSB– take a break from their word-jazz.

When I got back, I read to the guys behind the counter how the plans recommend plywood because it is less prone to moisture problems. The clerk’s buddy said “It shouldn’t matter as long as you use the right vapor barrier.” I was getting a little freaked out. I kept trying remembering vague bits and pieces of why I wanted to buy plywood and not OSB, but I couldn’t remember any of them. OSB just seemed like a better buy.

That’s when I remembered something. I asked “What about weight?” The clerks looked at each other and said “Why don’t we run a weight loading comparison on the computer? The system has the weight written into it, right?”

It turned out, after comparing 1/2″ boards of plywood and OSB on the computer that OSB was about 13lbs. heavier per board. Wow! For the amount we were buying, that would have added up to about 350lbs. of dead weight. Based on that, and my sneaking premonition that OSB was somehow inferior, we agreed to just go with the traditional plywood. We loaded the van up on plywood and hitched it back to my dad’s shop. That sneaking feeling of suspicion stuck with me until I got home and remembered why we wanted to use plywood in the first place.

……………………………………………………………………………………………………………………………………….

Before we went, I read this great article about how to select high grade plywood. The article was written for cabinet makers who usually need the highest grade plywood products they can find, so a lot of it is written with the cabinetry market in mind, but it also touches on the top brands and overall trends in structural-grade plywood.

To be honest, I was pretty surprised by the article’s overall claim: plywood products are slowly becoming worse. It seems that the housing market’s trend towards affordability and price-point are driving down the worldwide quality of plywood products, and every industry involved is having trouble finding quality plywood.

I certainly thought this must have been the case when I recently traveled to Washington state and found that just about every single new housing project I saw on the Pugent Sound was made with OSB. I thought this was crazy, especially considering what I had heard about OSB crumbling in a matter of a few years when exposed to high humidity. I thought the developers were just trying to cut costs, but that was before I went shopping for plywood myself and spoke with those two lumberyard employees who claimed that OSB products, which actually used to be inferior to plywood, had recently surpassed plywood in terms of quality and affordability.

While in Washington, what surprised me more was the type of housewrap they were using on top of the OSB: it was all Tyvek. From what I had read in online forums about Tyvek was that it worked alright in concept, but that it worked differently in practice, especially with cedar siding. This made it all the more important to pair the right sheathing material with the right house wrap. This, and the fact that Tyvek is plastic is what made Sierra and I choose the more breathable tar paper over plastic products. I can speak more to this decision in another post, but our whole anti-plastic thing is a topic of its own.

Tyvek’s claim to fame is its ability to allow water vapor through its membrane, but reject liquid water from reaching your sheathing. However, the million dollar question is: what if liquid water somehow, someway, makes its way past the housewrap? This is not supposed to happen, but from what I’ve seen in recent home building practices in my neighborhood alone, it probably could.

Dow Chemical, the manufacture of Tyvek, recommends that all seams be taped with a special Tyvek tape that protects the seams from water leakage. However, most of the new houses I see using Tyvek almost never tape the seams. And, if they do tape the seams, I’ve certainly never seen anyone tape over the staples used to hold the housewrap in place. That means many houses wrapped with Tyvek, or some equivalent, are probably filled with hundreds of tiny holes where liquid water could bypass the housewrap. Not only that, but the sugars and tannins found in common siding materials like cedar or douglass fir can actually act as a surfactant in water droplets, lowering their surface tension thus making them able to pass through an otherwise impassable house wrap.

At that point, from what I have heard some professionals say, the water is trapped and has ample opportunity to soak into your sheathing. And if your sheathing is the supposedly moisture-prone OSB, you’re in trouble. OSB, as it ages, and especially as it is exposed to moisture has the tendency to release a fine dust, which over time accumulates at the bottom of the housewrap where it acts like a moisture sponge holding on to large amounts of stagnant water. At this point, it acts kind of like a petri dish: an isolated sample of moisture sitting for long periods of time in a dark, damp place. That’s a renovation waiting to happen. That’s why not only are Sierra and I planning on using tar paper, we are also planning on making a rain-screen to allow the siding and the sheathing to breath and dry out when it gets wet.

The problem was, Sierra and I learned all of this after we had already purchased six Zip panels for attaching underneath the subfloor. Zip panels, for those of you who don’t know, are a new product where 4×8 sheets of OSB are painted with an acrylic finish that acts as a moisture barrier, not unlike Tyvek housewrap. The difference being that there is no air gap between the OSB and the painted surface, making it less prone to the problems posed by traditional housewrap products. The seams are taped together by a special Zip tape that prevents water and moisture from passing through.

We thought this system, which seemed to be used successfully by many tiny housers, would be a good way to seal the undercarriage from moisture damage. The problem though, was that it was still made of OSB, and the unfinished edges of the boards would still face moisture problems. So we decided to tape all of the edges and the nail-holes too, and as an added barrier, we plan to also use the aluminum flashing you see most tiny housers use. We’re just going to put that underneath the Zip panels to keep rodents, insects, and great slopping splashes of water from ruining all of our hard work. However, we’re going to go one step further and seal all of the seams of the aluminum and the nail-holes with heavy duty, weather rated, exterior grade aluminum tape. That, we hope, should do it.

So anyway, this whole plywood experience has been pretty interesting, and we’ve learned a lot so far about building while scanning articles and internet forums. What have you seen or learned? Please let us know if you have any relevant experiences!

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