Sheating Membrane

In conventional wood-frame construction, sheathing membranes are applied over the OSB or plywood sheathing in horizontal strips, shingle-lapped at the joints and stapled. When rigid or semi-rigid insulation is applied over the exterior of wall sheathing, the sheathing membrane may be applied over the outside face of the insulation (when the outboard insulation type must be protected from wetting) and fastened through to the frame wall. Alternatively, the sheathing membrane may be applied over the wall sheathing and on the inside face of the insulation (when the outboard insulation type does not need to be protected from wetting).

Some rigid insulation products such as extruded polystyrene are also relatively vapour impermeable. If used, care must be taken in the selection of cladding material and detailing to ensure that no significant amounts of water reach the interface between the extruded polystyrene insulation and the vapour permeable sheathing membrane. This risk may dictate the use of insulation products with high vapour permeability and low water retention, such as mineral fibre.

The sheathing membrane should be installed so that it intercepts any moisture that gets behind the cladding (the first plane of protection) and directs it outward over flashings, at openings, at changes in cladding materials and at the base of walls. The sheathing membrane must be permeable enough to permit water vapour to diffuse from a wall assembly to the exterior. One layer of sheathing membrane is generally used and may be applied horizontally or vertically with 100 mm (4 in.) or greater laps at the joints. A second layer may be required for added moisture protection.

Source : Canada Mortgage and Housing Corporation (CMHC)

The sheathing membrane forms part of a second line of defence against the entry of rainwater and may also be, in some cases, a primary component of the air barrier system. Since some moisture may condense in the wall space in winter, the sheathing membrane must be capable of allowing it to diffuse outward to prevent wood decay.

Several types of sheathing membrane are commonly used in wood-frame construction:

Building paper is bituminous-impregnated paper that is produced in 915 mm (36 in.) wide rolls and applied over the sheathing in horizontal strips, starting at the bottom of the frame wall. The paper is stapled to the sheathing, and each strip is shingle-lapped over the one beneath it. Spun-bonded polyolefin (SBPO), also known as “Housewrap,” is produced in rolls 2.74 m (9 ft.) wide by up to 45 m (150 ft.) long. It is applied over the sheathing in horizontal strips that cover the height of a storey and is stapled to the sheathing. SBPO is strong and flexible enough to be turned into window and door openings and integrated with the moisture management features at the openings.

Self-adhesive (“peel-and-stick”) membranes are used in areas subject to large amounts of water such as at door and window sills and heads. Peel-and-stick membranes are produced in 915 mm (36 in.) wide rolls. They are strong, flexible and can be formed to make sill dams, seal the joints in other sheathing membranes, and seal the joints between sheathing membranes and flashings.

Liquid-applied membranes are usually trowelled over the sheathing, with vertical grooves for drainage. They provide both an adhesive and a moisture barrier for the polystyrene insulation boards used in exterior insulation finish systems (EIFS).