Wall Sheathing Options
Wall Sheathing Options
Choosing between OSB, plywood, fiberboard, rigid foam, diagonal boards, and fiberglass-faced gypsum panels
For the past 30 years, the majority of new homes in the U.S. have been built with wood-framed walls sheathed with oriented strand board (OSB). Most builders are so comfortable with OSB wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. that they never consider using an alternative material.
In fact, a wide range of materials can be used to sheathe a wood-framed wall. In addition to OSB, builders can choose plywood, fiberboard, rigid foam, diagonal boards, and fiberglass-faced gypsum panels. If you’re a dyed-in-the-wool OSB user, it might be time to consider some of the available alternatives to OSB.
It’s sheathing, not “sheeting”
Before we compare different sheathing materials, we need to clear up a common misconception. Plywood and OSB are examples of sheathing, not “sheeting.” The word comes from the verb “to sheathe,” which means to encase something in a protective covering (as a dagger is protected by a leather sheath).
Builders who talk about “sheeting” apparently assume that the word is derived from the word “sheet” (as in, “a sheet of plywood”). These builders are wrong.
Sheathing serves many functions
No one has yet invented the perfect sheathing material. Ideally, a sheathing material should:
- meet code requirements;
- be affordable;
- be strong enough to brace a wall (provide racking resistance);
- hold nails and screws well without being so dense that nails and screws have a hard time penetrating the material;
- be airtight (or able to be rendered airtight fairly easily);
- be vapor-permeable;
- be easy to install quickly;
- be a "green" material that can be produced locally from recycled or renewable raw materials.
If you know of any material that complies with these properties, send me an e-mail.
OSB. OSB panels are made of large wood chips and glue. OSB is strong enough for wall bracing, and holds fasteners well. The main advantage of OSB over alternative products is its low price.
If OSB can be kept dry, it performs well. In cold climates, most types of wall sheathing go through regular cycles of wetting and drying; the moisture content of sheathing is typically relatively high in February and relatively dry in May. Some building scientists wonder what 50 years of this type of moisture cycling will do to the structural integrity of an OSB panel; time will tell.
If OSB gets wet and is unable to dry out fairly quickly, it swells irreversibly and then begins to rot. Of course, most types of wall sheathing — including plywood sheathing and board sheathing — will rot if they get wet and stay wet. But when exposed to persistent moisture, OSB rots faster than plywood.
In a paper on mold by Joseph Lstiburek, Nathan Yost, and Terry Brennan (“Mold: Causes, Health Effects and Clean-Up”), the authors explain, “Mold … likes processed wood better than it likes real wood. So mold likes oriented strand board (OSB) better than plywood and plywood better than a stud or a joist.”
Lstiburek expanded on this point in a Fine Homebuilding article called “The Mold Explosion: Why Now?”, noting, “OSB is the Spam of mold food. Because we peel and flake the tree, we can use smaller trees that are faster-growing and contain mostly sapwood. If you’re mold with the choice between 2x4s and OSB, which are you going to choose? OSB, every time.”
Builders who have had problems with OSB rot sometimes switch to one of the high-quality OSBs manufactured by Huber Engineered Woods. Huber makes both AdvanTech sheathing and Zip System sheathing, either of which can get wet without swelling like ordinary OSB.
Zip sheathing is a type of OSB with a special exterior facing (an overlay of kraft paper impregnated with water-resistant resins). Zip sheathing is much more water-resistant than other types of OSB, and that’s a good thing. Moreover, the Zip System approach includes a high-quality tape for OSB seams; once taped, the Zip sheathing layer is close to airtight and can even function as a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB).
However, AdvanTech OSB and Zip OSB cost quite a bit more than ordinary OSB, so anyone who specifies one of these Huber products isn’t getting the same cost advantage that most OSB purchasers enjoy.
Plywood. In the U.S., plywood was the dominant wall sheathing material for wood-framed walls from the early 1950s until the late 1980s. Like OSB, plywood is strong enough to brace a wall and holds fasteners well. Plywood has a proven track record for use as sheathing.
Plywood mills require high-quality logs with few knots. To make CDX plywood — the most common sheathing grade — softwood logs are rotated on their axis as a long, very sharp knife is used to slice off thin veneers. These veneers come off the log like sheets being pulled from a roll of paper towels. The thin veneers are then flattened and glued together, with the grain of each layer oriented at 90 degrees to the adjacent layer.
When exposed to enough moisture for a long enough time, plywood will rot — but not as fast as OSB will. Manufacturers of OSB point out that inexpensive brands of plywood can have problems with delamination or voids. That’s true, but these problems are rare with the more established plywood brands. In general, plywood has fewer performance problems than OSB. Of course, it also costs more.
Fiberboard. According to the North American Fiberboard Association, fiberboard “is a fibrous-felted, homogeneous panel made from ligno-cellulosic fibers – usually wood – which has a density of less than 31 lb/ft3 but more than 10 lb/ft3. Fiberboard is characterized by an integral bond which is produced by interfelting the fibers, but which has not been consolidated under heat and pressure as a separate stage in manufacture.” Many brands of fiberboard sheathing are impregnated with asphalt. Fiberboard has a measurable R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. ; most manufacturers report that 1/2-inch fiberboard is rated at about R-1.3.
Ordinary fiberboard sheathing isn’t strong enough to be used for wall bracing. There are fiberboard products rated for wall bracing, however; these are referred to as “structural fiberboard.”
North American manufacturers of structural fiberboard include:
- Blue Ridge Fiberboard (manufacturer of SturdyBrace)
- Building Products of Canada (which refers to its fiberboard product with an imaginative name — the company calls it “sheathing”)
- Georgia-Pacific (manufacturer of Stedi-R and QuietBrace)
- Homasote (manufacturer of N.C.F.R. Homasote)
- International Bildrite (manufacturer of Bildrite fiberboard)
One of the main advantages that fiberboard has over OSB or plywood is that fiberboard is more vapor-permeable. Depending on the brand, fiberboard has a permeance of between 5 and 28 perms. This high permeance allows a fiberboard-sheathed wall assembly to dry to the exterior more quickly than an OSB-sheathed wall or a plywood-sheathed wall.
One of the main disadvantages of fiberboard sheathing is its tendency to belly out when a builder installs dense-packed cellulose or blown-in fiberglass between the studs. In 2011, I interviewed Mark Dixon, who helped install fiberboard wall sheathing on a PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. residence in Olympia, Washington. Dixon told me that the fiberboard wall sheathing was unable to resist the pressure of the dense-packed blown-in fiberglass insulation. After the insulation contractor finished insulating the walls, the fiberboard had bellied out and was bulging as much as 3/4 inch in some stud bays. The workers eventually managed to force the bellies back, at least partially, but the experience revealed one potential drawback of fiberboard sheathing.
A Maine builder, Chris Corson, has this to say about fiberboard wall sheathing: “It worked; it did its job. But it’s difficult to work with. It’s hard to install. It’s dirty. It smells like asphalt. When we started installing the cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., it really bellied out. Fortunately we caught the bellying before it became a big problem. We finished the job by watching it closely and babysitting the insulation contractor.”
Fiberboard sheathing can’t hold nails or screws, so siding needs to be installed with long fasteners that extend back to the studs, or to vertical furring strips installed over the studs.
For more information on fiberboard sheathing, see The Klingenberg Wall.
[Photo credit: HP Builders / YouTube]
Rigid foam. Builders who install a continuous layer of rigid foam insulation on the exterior side of their walls often install the foam over OSB or plywood sheathing. However, it’s also possible to skip the OSB or plywood, and to install rigid foam as the only sheathing material on the wall.
The main advantage of this approach is that the rigid foam provides a continuous layer of insulation that interrupts thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the studs. It’s also an affordable approach, because you don’t need to install any OSB or plywood.
Any of the three most common types of rigid foam — polyisocyanurate, extruded polysturene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.), or expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) — can be used as well sheathing. Because of its relative fragility, EPS is rarely used for this purpose unless it is fully supported by OSB or plywood. Green builders prefer polyisocyanurate to XPS, because XPS is manufactured with blowing agents that have a high global warming potential. Foil-faced polyiso is the easiest type of rigid foam to tape, which is another reason it is often chosen for use on walls.
Builders in cold climates need to make sure that any rigid foam wall sheathing is thick enough to keep the interior surface of the rigid foam above the dew point during the winter. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Anyone who wants to install rigid foam wall sheathing without any underlying OSB or plywood needs to come up with a method for bracing the walls. Here are four possibilities:
- You can install a few strategically placed pieces of OSB or plywood (usually at the corners) to stiffen the wall.
- You can install 1x4 let-in bracing.
- You can install L-profile metal strapping.
- You can install a few inset shear panels.
For more information on these bracing methods, see Four Options for Shear Bracing Foam-Sheathed Walls. Note that most of these methods require the help of an engineer, and that some of these methods don’t meet code requirements in high-wind areas.
For more information on fastening details for rigid foam and flashing details for foam-sheathed walls, see How to Install Rigid Foam Sheathing.
Rigid foam won't hold fasteners. To facilitate the attachment of siding to a foam-sheathed wall, most builders install vertical furring strips between the rigid foam and the siding. (Of course, the furring strips also create a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap which helps the siding dry quickly.) These furring strips are installed at 16 inches o.c. or 24 inches o.c., and are fastened to the studs with long screws. The siding is fastened to the furring strips.
Some types of siding (including vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding) can be installed directly over rigid foam, as long as the foam is no more than 1 1/2 inch thick. With this type of installation, the siding is fastened through the rigid foam to the studs with long nails or screws.
Builders who are considering the use of rigid foam sheathing may also want to consider the use of nailbase panels. For more information on this topic, see Nailbase Panels for Walls.
[Photo credit: Robert Swinburne]
Diagonal boards. Diagonal board sheathing was quite common in the U.S. before World War II, and the method is still used occasionally in rural areas of the U.S. In northern New England, affordable rough-cut spruce, fir, or pine boards — usually 1x6s or 1x8s — are commonly available at local sawmills.
Diagonal board sheathing has a lot of advantages for green builders: it’s usually produced locally; it’s strong; it’s vapor-permeable; it resists rot better than OSB; it holds nails well.
Diagonal board sheathing has three main disadvantages: it’s labor-intensive to install; it can’t be used as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.; and many building inspectors won’t accept the use of boards that lack a grade stamp.
Builders who install diagonal board sheathing usually let the boards run wild at window rough openings and corners. Once the wall is sheathed, the builder snaps vertical and horizontal lines where necessary and cuts off the wild ends neatly.
Fiberglass-faced gypsum panels. While fiberglass-faced gypsum panels like DensGlass Gold have long been used on commercial jobs, this type of sheathing is relatively rare on residential jobs.
Fiberglass-faced gypsum panels are structural (they can be used for wall bracing), fire-resistant, and vapor-permeable (23 perms). Some brands of fiberglass-faced gypsum, including DensGlass Gold, can be exposed to weather for up to 12 months. Because this type of gypsum panel is paperless, it is unlikely to be attacked by mold.
Unlike OSB or plywood, fiberglass-faced gypsum panels are not designed to hold fasteners: “All siding must be attached through the DensGlass sheathing and into the steel or wood framing.”
A recent web search shows that DensGlass Gold costs about $20 per sheet.
No sheathing material is perfect
All of the sheathing materials mentioned in this article a can be successfully used to build a quality home. The key to a durable wall is good detailing.
Good wall design requires careful consideration of the vapor permeance of all the layers making up the wall assembly. Dry sheathing is happy sheathing. Walls need to be detailed to limit moisture accumulation (either from the exterior or the interior) and to encourage rapid drying. Good wall details include high-quality flashing and a ventilated rainscreen gap between the sheathing and the WRB.
Martin Holladay’s previous blog: “Where Does the Air Come From?”
- Image #1: Lunenburgfirehouse.blogspot.com
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