sheathing materials
A good sheathing of the exterior walls gives structural strength to them, controls air infiltration, mold and mildew, and deflects water, allowing it to drain out of the walls. But it should also provide insulation value, which doesn't happen with traditional sheathing materials.
Traditional materials: Plywood, OBS, blackboard
The traditional sheathing uses plywood panels and OBS (Oriented Strand Board). Blackboard (asphalt impregnated sheathing) and fiber cement and gypsum boards are also common sheathing materials...
Advantages and disadvantages of traditional sheathing
Builders use plywood sheathing and other traditional materials because they are cheap, and because they offer rigidity and structural strength to the walls, allowing an easy use of common fasteners (nails, staples…) for the application of the siding.
But these materials are also poor insulators, allowing hot and cold air to pass through the walls.
More exactly: even if the house has a good cavity insulation, that may not block unwanted heat losses or gains. Hot and cold air can pass through studs, pipes, ducts and similar elements in the walls. Cavity wall insulation is necessary, but it will not fix thermal bridign problems.
See: Plywood sheathing
New energy efficient sheathing materials
To overcome the limitations of plywood, OSB or asphalt sheathing materials, there are now several alternatives.
One of them is to add rigid foam insulated sheathing to the traditional materials. This foam sheathing comprises molded polystyrene, extruded polystyrene and rigid polyurethane or polyisocyanurate (polyso) boards.
In this case, the plywood (or other traditional sheathing material) provides the structural strength, while the foam boards provide insulation value.
Other more modern and sophisticated solutions include some proprietary materials embodying the structural strength of sheathing materials like plywood with the insulation power of rigid foam boards: something often known as Structural Foam Insulated Sheathing.
Though these choices are more expensive than traditional sheathing, they typically deserve the investment. New sheathing materials provide huge energy savings and lower energy bills...
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