Introduction to Weather-Resistive Barriers and Flashing
Water will always find its way behind siding. Wind drives rain through gaps, and solar radiation pushes moisture vapor inward through the cladding. If siding is pressed tight against plastic housewrap or applied directly over foam sheathing, the front of the siding tends to dry while the back stays wet — accelerating rot, cupping, cracking, and mold growth. The solution is not caulk or sealant, which inevitably deteriorates, but a properly designed weather-resistive barrier (WRB) system combined with correctly installed flashing. This field guide covers WRB types, rainscreen design, flashing details, and best practices for every type of cladding.
Weather-Resistive Barrier (WRB) Basics
A WRB serves as the drainage plane behind the exterior cladding. However, the WRB alone does not create a drainage plane — you must provide a gap between the WRB and the siding. This is most effectively achieved with a rainscreen system: vertical battens or a drainage mat that creates an air space behind the cladding. The space should have vents at the top of the wall to promote convective airflow and weep holes at the base to let water drain out.
| Material | Perm Rating (perms) | Drainage | UV Resistance | Tear Strength | Typical Cost |
|---|---|---|---|---|---|
| Standard polyolefin wrap | 5-15 | Poor (needs battens) | 30-90 days | Moderate | $ |
| Wrinkle-wrap (drainable) | 10-20 | Good (3D channels) | 60-120 days | Moderate | $$ |
| Fluid-applied WRB | 8-15 | Depends on substrate | Excellent | Excellent | $$$ |
| Building paper (Grade D) | 5-10 | Poor (needs battens) | Good | Good | $ |
| Self-adhered membrane | <1.0 | N/A (air/water barrier) | Excellent | Excellent | $$$ |
Rainscreen Design Principles
The fundamental principle of rainscreen design is to create a ventilated cavity between the cladding and the WRB. This cavity allows any moisture that penetrates the cladding to drain freely and dry through natural convection. For best performance, the cavity should be a minimum of 3/8 inch deep, with larger cavities (up to 1-1/2 inches) providing better drying potential.
The rainscreen cavity must be open at both top and bottom. Inlet vents at the bottom allow air to enter, and outlet vents at the top allow warm, moist air to escape. This chimney effect drives continuous airflow behind the siding, keeping the back of the cladding and the WRB surface dry. Screened vent openings prevent insect entry while maintaining airflow.
Drainage Plane Details by Siding Type
Different cladding materials require specific drainage plane details. For board siding, vertical battens create the space between the siding and sheathing wrap. Vents at the base of the wall provide drainage and airflow. Nailing siding to battens has the added advantage of reducing nail penetrations through the housewrap. For stucco, which is highly porous, a three-dimensional wrinkle-wrap type housewrap is recommended to create a small drainage plane. These wraps must be covered with building paper or paper-backed mesh to create a bonding surface and keep the drainage channels clear.
Vinyl siding requires loose nailing to accommodate thermal expansion, which inherently creates some airflow behind the panels. However, the minimal space beneath vinyl siding is often insufficient for proper drainage. Installing vinyl over vertical battens or a drainage mat significantly improves performance. Brick veneer relies on the air space between the brick and sheathing for drainage, with weep holes at the base of the wall every 24 to 33 inches.
Flashing: The First Line of Defense
Flashing directs water away from vulnerable building intersections — roof-to-wall junctions, window and door openings, deck ledgers, and foundation-to-wall transitions. Flashing must be layered in a shingle fashion, with upper flashings overlapping lower flashings so water flows continuously downward and outward. Common flashing materials include galvanized steel, copper, aluminum, and flexible self-adhered membrane.
| Material | Thickness (ga) | Corrosion Resistance | Workability | Lifespan | Best Application |
|---|---|---|---|---|---|
| Galvanized steel | 26-28 | Good | Moderate | 20-40 years | Step flashing, counterflashing |
| Copper | 16-20 oz | Excellent | Excellent | 100+ years | Valleys, chimney flashing |
| Aluminum | 0.019-0.032 | Good | Easy | 25-40 years | Drip edges, window flashing |
| Self-adhered membrane | 40-60 mil | Excellent | Very easy | 20-30 years | Window rough openings, transitions |
Window and Door Flashing
Window flashing is one of the most critical and commonly misapplied details in residential construction. The sequence follows a specific order: install the sill flashing first, then jamb flashing, and finally head flashing — always shingle-lapped. Self-adhered membrane is the preferred material for window rough openings because it conforms to irregular surfaces and seals around nail fins. A properly flashed window should show no signs of water entry under testing at 8.5 psf (approximating wind-driven rain exposure).
Roof-to-Wall Flashing and Kickout Diverters
Where a sloped roof meets a vertical wall — at dormers, porches, or split-level additions — step flashing is required. Each course of roofing overlaps a piece of step flashing that extends up the wall at least 4 inches. Counterflashing built into the wall covering overlaps the step flashing below. At the bottom of the roof slope, a kickout diverter redirects water away from the wall and into the gutter, preventing water from running down the siding. The proper installation of kickout diverters is one of the most effective ways to prevent water damage at roof-wall intersections.
Common Installation Errors
Missing kickout diverters at the base of roof-wall intersections is among the most common and damaging flashing errors. Other frequent problems include using caulk instead of proper metal flashing at transitions, installing housewrap with inadequate lap (minimum 6 inches horizontal, 2 inches vertical), and failing to seal fastener penetrations through the WRB. Nailing siding too tightly to the WRB eliminates the drainage plane, trapping moisture against the sheathing.
Testing and Verification
After installation, the WRB and flashing system should be inspected before the cladding is applied. Look for tears, unsealed penetrations, and inadequate laps. A water hose test at suspect areas can identify deficiencies before they become costly callbacks. Many building codes now require third-party inspection of the WRB and flashing for commercial projects, and this practice is increasingly recommended for residential construction as well.
Conclusion
A durable weather barrier and flashing system is the most important layer in the building envelope. By understanding rainscreen principles, selecting appropriate materials, and following correct flashing sequences, builders can protect structures from water damage for decades. The small additional cost of a properly designed WRB and flashing system is one of the best investments in long-term building durability.