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.
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