What's Better, Plywood or OSB?

In the waterproof balcony and deck coating industry, we continue to support education in various areas to better protect the investments we make in homes, apartments, condos, and commercial buildings. In an effort to do our part in educating the construction sector, we need to put a spotlight on OSB or "oriented strand board" (a hot topic for most in the business of new construction and reconstruction). It’s important to understand the differences between plywood and OSB, the pros and cons of OSB as well as how the two products stand up in comparison testing.  

Pros and Cons of OSB 

Most professionals in the construction industry agree that OSB is great for shear walls and interior flooring as a lower-cost alternative to plywood; it could even be argued that it is more rigid or sturdy than plywood. Yet, when it's installed on the exterior of a home or building and moisture accumulates around it (which is inevitable in damp or humid climates), OSB can and will swell. This swelling will put stress on the deck coating or roofing material which was applied to protect the OSB and then cause cracking. This problem is exacerbated when contractors cut the OSB on site, revealing a raw surface that has no factory protection. 

Waterproofing Issues with OSB 

One can help combat this swelling (moisture getting to the raw edges) by applying reinforced waterproof seam treatment and, of course, completely waterproofing the OSB; however, this won't completely solve the problem when you are in a climate that is 95% humidity such as in many parts of the South, Northeast, Midwest, and of course the Pacific Northwest. As a matter of fact, the better we waterproof the OSB, the worse the rotting can be, as water cannot easily escape and moisture still gets in from the un-waterproofed negative side and through unprotected seams. Inevitably, we can't stop moisture from traveling throughout our homes and buildings. If we could, there wouldn’t be mold in our homes or buildings, right?  With all of this said, venting or ventilation is helpful to combat mold and wood rot.

Comparison Testing of OSB vs. Plywood 

For over 30 years, we have seen the difference in the performance of OSB vs plywood. We have even conducted our own research comparing the two materials. We did this by cutting and placing both OSB and plywood in water overnight and then measuring the swell the following morning (a so-called “Swell Test”). From these tests, we found that the OSB does swell quite a bit, mimicking what other tests have found. After placing the samples in water, the OSB initially swelled about 11%, whereas the plywood only swelled 9.5%. After 24 hours emerged from water, the numbers were 12% for OSB and 3.8% for plywood. One week later, the OSB was still swollen to 5.7% and the plywood had returned to about 1%. Considering these results, doesn’t it make sense to use the more reliable, traditional exterior plywood or even CDX plywood (both of which can handle a little more moisture and dampness without swelling as much or rotting as quickly) instead of OSB? 

 Some Final Thoughts 

I am not in the plywood sales business, nor do I have malice towards the OSB industry. In fact, I am sure it’s a great product when used in desert climates and the interior sections of buildings. However, is it worth the savings to use OSB on the exterior sections of houses, apartments, and buildings when there is reliable proof that plywood performs better in various swell tests?