Sheathing

Sheathing adds strength and wind resistance to house walls. Depending on construction techniques and materials used, sheathing can:

• add rigidity and strength to a wall (wind bracing)
• add insulative value to a wall
• act as a wind barrier to minimize infiltration
• act as a stiffener behind thin siding materials (such as vinyl)
• provide a surface to lean a ladder against before the siding is installed
  

Sheathing was not used when 2x4 construction was first developed in the early 1800s. As a result, many old buildings have leaned out of square because of a lack of bracing. Without sheathing, houses were less resistant to the wind—it simply blew right through the cracks into the living space.

 

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After time, builders began to use horizontal boards for sheathing. While this technique improved wind resistance, it didn’t provide optimal bracing. Diagonal sheathing eventually replaced the placement of horizontal boards, giving buildings much more rigidity.

Because sheathing is used under siding and is within the wall itself, it has less potential to cause issues for those with chemical sensitivities. Still, if a wall isn’t constructed in an airtight manner, air currents flowing through random cracks and gaps can carry pollutants outgassed by these materials into the living space. Concerns about outgassing from sheathing can be minimized by using low-polluting materials and tight construction.

Today a wide variety of materials are used for sheathing. Each has advantages and disadvantages related to strength, cost and insulation properties.

Wood

Solid board sheathing is no longer used because of its high cost. To add strength to the structure a solid wood diagonal board is sometimes nailed into the studs at the corner of a house; the wall itself is then is covered with a non-structural sheathing.

Plywood

Plywood and oriented-strand board are widely used for sheathing. If nailed properly, they provide the rigidity necessary to brace a house against the wind and keep it square. In many cases, these materials are used only at the exterior corners of a house—for strength—and a weaker non-structural material is used on the rest of the wall.

Both exterior plywood and oriented-strand board contain a phenol-formaldehyde glue that is not considered a significant formaldehyde emitter. Still, many sensitive people are concerned about using such products in construction.

Fiberboard

Fiberboard sheathing is composed of various plant fibers held together with an asphalt binder. It provides more insulating ability than plywood or oriented-strand board, and is lower in cost, but also has less strength. If fiberboard is used, other materials or techniques must be added to provide wind bracing and rigidity to the building. Fiberboard can outgas small amounts of asphalt odor, especially when heated by the sun.

Foam

When the energy crunch hit in the 1970s, insulating foam boards began to replace plywood and oriented-strand board as sheathing. Foam sheathing adds no strength to a wall, so corners must be stiffened with diagonal boards, or wood or metal strapping set into diagonal notches.

Several types of foam boards are used for sheathing—urethane, isocyanurate, extruded polystyrene and expanded polystyrene. These materials tend to be the most expensive sheathing materials, but they add the most insulating value. All foams boards have some outgassing characteristics, but this doesn’t seem to cause significant problems for sensitive people. Some foam boards release small amounts of chemicals that can damage the ozone layer.

Gypsum board

Gypsum-board sheathing is similar to drywall or plasterboard, but it is designed to be used on the outside of a wall. To withstand the weather until it is covered with siding, it contains a small amount of asphalt to resist moisture absorption from precipitation. It is not widely used on residences, but can be special ordered. Gypsum sheathing will add strength to a wall, but has little insulative value. It outgasses less than asphalt-impregnated fiber board.

Cardboard

Foil-faced cardboard can also be used for sheathing. Surprisingly, if nailed properly it can usually provide the strength necessary for wind bracing. It doesn’t add much insulating value however, and its cost is moderate. It outgasses very little, but in a few rare cases sensitive people have been bothered by the printing ink on the surface which advertises the manufacturer’s name.

Because aluminum foil is a very good diffusion retarder, foil-faced sheathing is risky to use on the outside of the wall in a cold climate because it can lead to unwelcomed moisture condensation inside the wall cavity. These products are most often used in warm climates.

Elimination of Sheathing

Where building codes allow it, it’s possible to eliminate sheathing altogether. Sheathing should be eliminated only if the functions it provides are addressed by other materials or construction techniques. For example, diagonal wood or metal bracing can give a wall sufficient rigidity and strength (but not as much strength as plywood); insulating sheathing may not be needed if sufficient cavity insulation is used; a housewrap can be used as a wind barrier and some sidings are stiff enough to not require a backing material.

 

 

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