A drainage plane is a layer of water-resistant material between the cladding and the framing that catches water and makes it drain to the exterior. This keeps water that leaks in -- around windows, doors and cladding -- from wetting the framing and insulation. For a drainage plane to work, there should be enough space between the water-resistant material and the cladding to allow the water to drain freely and not be caught by capillary action. Layers must be lapped properly so water draining from higher points on the house is directed over (not behind) lower layers.

Remember, leaks happen. Plan for them.

• A drainage plane can be created by using housewrap or building paper installed shingle fashion or by sealing the seams of foam sheathings with foil tape. Non-perforated housewraps are recommended behind wood siding and brick veneer because wood and mortar can leach surfactants that allow water to penetrate the tiny perforations. Special attention is needed to make sure the drainage plane provides complete protection at windows and doors.
  
  
• Window and door flashing: All openings in wood frame walls must be properly flashed, especially window sill corners, but this is often omitted or not detailed correctly. Water should not be able to drip behind the housewrap or flashings or seep into the corners of a window or door opening. Shingle-fashion installation must be maintained throughout. For example, the top of a window flange (nail-on frame) should be slipped behind the drainage-plane housewrap or paper, but the bottom flange should be on top of the paper. Recently developed window and door flashing products and techniques provide fast, easy and dependable long-term performance. Effective sill flashing systems should include a back-dam or slope to drain outward under the window unit, seamless corner protection and shingle-fashion integration with the housewrap.
  
  
• Vented space behind cladding: In rainy climates, all walls (except those protected by deep porches and solid masonry “reservoir” construction systems) should have a continuous drainage plane and a drainage space (a gap) behind the cladding. (Drainage Plane & Space Behind Cladding -- see Figure 1). In high-rainfall areas, the drainage space should be vented to speed drainage and reduce pressure that can drive water vapor into a wall. (Vented Rain Screen System -- see Figure 2)
  
  
• Brick veneer walls with an air space (not obstructed with mortar) and weep holes at the bottom course are a good example. Including vents at the top and bottom courses of the brick veneer with a clear 1-inch minimum air space creates an especially effective system that drains rapidly and relieves vapor pressure.
  
  
• Sidings: Wood or fiber cement siding should be primed on the back side and installed with spacers, on furring strips or over a rain screen drain mat to create a drainage space behind it. Vinyl or aluminum siding has a built-in drainage space.
  
  
• Portland cement stucco and synthetic stucco exterior insulating finish systems (EIFS): Both must include a drainage plane and space if there are any penetrations in the wall (windows, doors, etc.). Barrier-system EIFS (adhered to the sheathing) are not recommended except under deep porches and in dry climates. A water-managed EIFS can be a good choice that provides a stucco appearance with added insulation and good drainage. If using real stucco, a drainage space is still needed. It can be created by using a crinkled housewrap (providing drainage gaps) with a a second layer of building paper or two layers of building paper so the stucco does not stick to the drainage plane.

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