Rain does not need a dramatic hole to get behind siding. Wind, joints and ordinary wetting make that part of a wall's job. The useful question is what happens next: can the water reach a drainage surface and leave, or does it find a shelf where it can sit?
A rainscreen space separates the cladding from the wall's water-management layer. Depending on the designed assembly, that space also assists drying. It is not a reason to leave window flashing unresolved, and it does not turn a leaking wall into a waterproof one by itself.
Follow one drop of water
Imagine water passing a siding joint. Behind it should be the intended drainage route, connected to flashings that direct water outward at interruptions. The route must continue past ordinary obstacles rather than ending on a horizontal block of wood or a misplaced bead of sealant.
Entry
Assume some rain can pass the exterior finish.
Drainage surface
Provide the specified water-resistant layer and connected flashings.
Continuous space
Resolve supports and penetrations without creating unintended dams.
Exit
Keep designed drainage openings clear and properly protected.
Decision guide; not a construction detail or a substitute for the project specifications.
CertainTeed CertaWrap weather-resistive barrier describes a weather-resistive barrier as a water-management layer with vapour-permeance properties. It is one component. The complete assembly still needs compatible joints, penetrations, flashings and attachment. A housewrap brand on the invoice does not establish that those connections were built correctly.
NRCan: insulating walls explains the importance of drainage cavities in existing wall constructions and warns against blocking their drainage. The size and arrangement of a new siding cavity must come from the selected system and project requirements. There is no useful universal gap measurement that covers every cladding and retrofit.
Ask for three small drawings
Before work starts, ask to see the bottom of the wall, a window head and a roof-to-wall intersection. Those details tell you more than a large elevation coloured to match the new siding. Each drawing should show the water-management layer, any cavity, flashing, attachment and the intended exit.
At the base, ask how water leaves while insects and other pests are managed. At a window, ask where water arriving from above is directed. At a roof junction, ask how the wall drainage connects to roof flashing without discharging into the structure. Our kickout flashing guide explains one especially troublesome junction.
Cladding orientation changes the support arrangement. Horizontal and vertical siding may need different batten layouts to provide attachment while keeping drainage continuous. Do not rotate a familiar detail and assume it still works. Ask the designer or installer to show the route through the actual proposed layout.
Watch the interruptions
Outdoor taps, receptacles, vents and lights interrupt more than the visible surface. Their mounting blocks and flashings need to work with the layers behind. Decide who handles each service before the siding crew arrives, especially when another trade must disconnect or relocate it.
Insulation added outside the sheathing changes the thickness and fastening arrangement. That can be worthwhile, but it makes a complete attachment and opening detail more important. A pile of longer screws is not a design. Ask for the specified assembly rather than assembling one from unrelated online diagrams.
During installation, request photographs before the cladding hides the drainage surface. Useful photos show transitions and openings with enough surrounding wall to locate them later. A close-up of tape without context is surprisingly hard to interpret when a problem appears two winters later.
Do not seal the exit as a finishing touch
Homeowners often want every visible opening tidied away. Some openings are intentional. Before caulking a suspicious line, identify its purpose in the product instructions or ask the installer. Siding weep holes and caulk covers the smaller openings that are particularly easy to obstruct.
If you already have staining or water indoors, investigate the entry and drainage path rather than drilling random holes in finished siding. The defect may be above the visible stain, or at a flashing concealed behind the trim. A planned opening and repair gives you a much better answer than a fresh line of sealant over every seam.
The finished gap should mostly disappear from everyday attention. Its job is quiet: let water leave, preserve the intended drying path and keep the wall assembly working behind the colour you chose.