Although Oriented Strand Boards, or OSB, have become a popular building material, it remains poorly understood. While some builders swear by OSB, others remain wary and critical of it as is a top-shelf building material. It’s commonly used as an exterior sheathing and also offers benefits in a garage but can you actually use OSB for interior walls in your home?
OSB can be used on interior walls since it is strong, durable, and provides structural support. The wood strands add a chic decorative finish if you are wanting the walls to stand out. Leave them unpainted for a raw decorative element or seal and paint them for stylish textured walls.
This really comes down to style preference. I’ve seen people make courageous design decisions using OSB. Just realize that it could potentially impact the appeal of your home to others should you ever put it up for sale.
Given the substantial financial implication when building a dream home, the last thing you want is to make the wrong choice of building materials. Read on to learn why using OSB on your interior walls is safe and won’t compromise your house’s integrity.
What Is OSB?
It may lack a sleek polished veneer but exceptional engineering goes into making OSB. That said, OSB has a riveting texture that resembles a flattened hay bale, thanks to its unique construction process.
Each oriented strand board sheet comprises compressed wood strands held together with adhesives and resin. To produce oriented strand boards, manufacturers grind logs into wood strands.
Once they’re dry, the rectangular strands are mixed with adhesives and synthetic resin and molded into thick mats, then hot-pressed in sheets.
Here’s a video detailing the OSB-making process:
Unlike other scrap wood products, the strand piles in oriented strands boards aren’t randomly placed; instead, they’re oriented. The strands are placed as alternating layers running parallel to each other; hence the name oriented strands.
In essence, OSB uses the same manufacturing process as wafer boards, but the structure mimics the plywood. That’s because OSB panels are engineered to rival plywood’s stiffness and strength.
The engineering prowess that goes into making OSB makes it a more consistent and dependable construction material. Since each pile in OSB comprises bound stands, you can never have a soft spot on the panel.
Soft spots are common in plywood when two knots overlap. On average, an OSB panel is about 50 strands thick, giving more layers than plywood.
OSB is made from small, resinous fast maturing trees such as southern yellow pine, poplar, aspen, and some soft hardwoods, making it an eco-friendly building material. Soft hardwoods refer to physically soft hardwood species.
Types of Binders in OSB
Since interior oriented strand boards use phenol-formaldehyde and methyl diphenyl diisocyanate binders, these boards are non-toxic and environmentally-friendly. Despite the scary-sounding name, phenol-formaldehyde isn’t toxic and doesn’t gas off as urea-formaldehyde used in other panels such as medium density boards (MDF).
Types of OSB
Oriented strand boards fall into five broad categories classified by mechanical performance and moisture resistance:
- OSD/0 – No formaldehyde
- OSB/1 – General-purpose panels for use in dry conditions and interior fitments such including furniture.
- OSB2 – Excellent for structural, non-load-bearing use in dry conditions
- OSB3 – Great for load-bearing projects in humid conditions
- OSB4 – Heavy-duty load-bearing panels for humid conditions
OSB’s Moisture Permeability
Vapor control is one of the primary goals of any building material used in the interior sheathing. Traditionally, fiberglass insulation and polyethylene were the go-to solutions. But OSB panels are increasingly becoming the most favored vapor barrier in the internal sheathing.
In the US, a building material’s vapor permeance is expressed as perms, as in how many perms of moisture can pass through it under set conditions. The Canadian vapor barrier benchmark is expressed in Nanograms (NG). 1 US perm equals 57 NG, making for easier comparison.
The vapor permeance of a building material accounts for the effective wet cup permeability of both the sheathing and cladding. There are four categories of vapor permeance:
- Vapor impermeable: 0.1 perm or less
- Vapor semi-impermeable: 1.0 perm or less but more than 0.1 perm
- Vapor semi-permeable: 10 perms or less but more than 1.0 perm
- Vapor permeable: greater than 10 perms
For a material to qualify as a vapor barrier, it should allow less than 60 NG (nanograms) or one perm of moisture to permeate under specific conditions.
A sheathing assembly using foil-faced isocyanurate material is classified as vapor impermeable regardless of the cladding installed on the outside. OSB and plywood sheathing overlaid with house wrap or building paper and vinyl sidings are considered vapor semi-permeable.
Installing a vapor barrier on the warm side of your home’s insulation is crucial in restricting moisture movement. In the cold winter months, it keeps the moisture from moving through the walls.
In the summer months, the hot and humid day temperature coupled with the air-conditioned and dry interior, reverse the vapor drive. The reversal can force the moisture-laden air into the house through the insulation, where it’ll condense on the cold, impenetrable vapor barrier.
Simply put, you need a vapor barrier that allows the interiors of your house to dry as much as possible without compromising the winter performance.
Vapor barriers whose permeability rates are closer to 60NG or one perm get you the best of both worlds. Polyethylene is rated as 3.4NG, while a ¾ inch (1.9 cm) OSB sheathing comes in at 40NG (⅔ perm), making it a better vapor barrier in home construction.
Since sheathing provides structural strength to house frames, it can either be outside or the inside. But installing the sheathing on interior walls is more advantageous since it provides structural strength while acting as both a vapor barrier and an air barrier.
The ¾-inch (1.9 cm) OSB meets the technical specifications of building codes in both vapor and air permeance. Therefore, using OSB on the sheathing interior walls gets you a vapor and air barrier while saving you the need to purchase additional materials for the job.
Since OSB panels are made of up to 50 layers of wood strands, they make strong, solid walls. Unlike a foil or polyethylene air barrier, OSB interior walls aren’t easy to damage. It’s unlikely that you’ll accidentally pierce through OSB walls with a sharp object.
OSB on Interior Walls and Blower Door Tests
A building’s air seal is measured in Air Changes per Hour (ACH), determined by a blower door test. During the test, a building is depressurized by placing a fan in the door to measure the air leakage rate.
A home using the conventional exterior wall sheathing has an air leakage rate of 3.5 ACH. Under normal air pressure conditions, the entire air volume in such a home leaks out and is replaced almost four times a day.
In contrast, using interior sheathing with appropriate materials such as OSB panels improves a building’s air sealing capacity to reduce the leakage rate to a mere 0.4 ACH.
An interior wall sheathing allows you to seal your home tightly, which improves your air exchanger’s efficiency. It also makes it easier for electricians, HVAC contractors, and drywall crew to carry out their work without punching holes into your air barrier.
Prioritizing air barriers and ensuring proper sealing of any breaches in the walls improves a home’s insulation capacity. A well-insulated house lowers your heating and cooling bills since it doesn’t tax and overwork your air conditioning unit.
Oriented Strands Boards (OSB) are an excellent choice for the internal walls of your home. Thanks to their solid construction and impermeability to moisture, these panels provide structural support and double up as air and vapor barriers.
Interior OSB uses non-toxic phenol-formaldehyde and methyl diphenyl diisocyanate binders that don’t pose a health hazard. The binders also make OSB impermeable to moisture while extending its strength and durability.
Since the wood strands add to OSB’s aesthetic appeal, you can leave the internal walls unpainted or paint them for a beautiful textured finish.
So, here’s the bottom line. Can you use OSB for interior walls? Yes, you absolutely can. Should you? Well, that depends on your style preferences.