Problem: Premature deterioration.
Wood siding deteriorates prematurely because of moisture-induced wood rot.
Cause: Condensation form air leakage and vapor diffusion on the back face of siding can lead to premature deterioration. The movement of moisture through wood siding can create external stains and mildew and cause paint to peel. Similarly, inadequate clearance of siding from grade level or roofing can result in constant saturation of the siding and lead to damage.
Prevent the damage caused by condensation.
- When humid indoor air leaks through the exterior wall and condenses on the back face of wood and hardboard siding, it creates ideal conditions for the growth of fungi. Take the following precautions:
- Install a continuous air-barrier system to minimize air leakage from the interior of the building.
- Reduce vapor diffusion through the wall by providing a vapor barrier (polyethylene or vapor-barrier paint) on the inside surface of the exterior walls.
- Use a single layer of 15 lb. sheathing paper between the sheathing and the siding, lapped at 4 inches (100 mm) around the edges.
Prevent rot by keeping siding at least 8 inches (200 mm) above grade as shown in Figure 36 or 2& inches (50 mm) above the abutting roof junction as shown in Figure 37.
Figure 36 - Keep Siding Above Grade
Figure 37 - Keep Siding Above Roof Junctions
- Note 1: Lap sheathing paper over base flashing.
- Note 2: Provide 2 inches (50 mm) minimum between siding and roof covering.
- Increase the drying potential of the siding by providing increased drainage, more air circulation on the back side as previously discussed in the rain-screen section and capillary breaks on the lower edges. Install siding over burring strips.
Avoid the use of inferior-grade wood siding and siding that has been damaged on the site.
- Use clear lumber, free of checks, cupping, splits and knot holes. To use other than good quality lumber is a false economy.