Cladding is a screen, not a barrier
The intuitive model of a wall is that the outer surface keeps the water out. It does not, and no cladding material does, whatever it is made of. Wind drives rain through laps, joints and around penetrations, and materials expand, move and open small gaps as they age.
The model that produces durable buildings assumes water gets past the cladding and gives it somewhere to go. The cladding sheds most of it. Behind the cladding a continuous drainage plane catches the rest and drains it downward and back out. What matters is not how well the outer surface repels water but how reliably the layer behind it drains.
The drainage plane and the flashings
The drainage plane is the continuous water-resistant layer on the outside of the sheathing: a building wrap, a membrane, or a coating designed for the purpose. It is continuous, or it is not doing its job.
Continuity is broken at every opening and every penetration, and each of those is resolved by a flashing that directs water back out onto the plane below the interruption. The universal rule is shingle lapping: every layer overlaps the one beneath it so that water travelling downward stays on the outside. A single reversed lap at a window head will deliver water into the wall for the life of the building, and it is invisible once the cladding is on.
Which is why window and door openings are where wall failures concentrate. They are the places where a continuous plane has to be interrupted and rebuilt in three dimensions, by a person, on site.
The cavity, and why it earns its keep
A rainscreen is a cladding installed over a deliberate gap, most often battens, so that a ventilated and drained space sits between the cladding and the drainage plane.
The gap does three things. It gives water that gets behind the cladding a clear path down and out rather than a capillary path across to the sheathing. It breaks the capillary connection between wet cladding and the wall. And it allows air movement that dries both faces, which matters because every wall gets wet occasionally and the difference between a durable wall and a rotting one is how quickly it dries.
The cost is battens, some detailing at the top and bottom, and slightly deeper window reveals. Against that, drying capacity is the single most reliable predictor of how long a wall assembly lasts, which makes it one of the better trades available in the envelope.
What changes by climate
Wet and mild climates put a premium on drying, which is the strongest case for a ventilated cavity and for vapour-open materials that let the assembly dry outward.
Hot and humid climates reverse the direction of the vapour drive for much of the year, since the outside is warmer and wetter than the conditioned inside. An assembly detailed for a cold climate, with a vapour retarder on the inside face, can trap moisture in that situation. This is the clearest example of why vapour control is climate-specific rather than a product to be added.
Cold climates concentrate on keeping interior moisture out of the assembly, which is an air sealing problem more than a vapour problem, and on making sure whatever does get in can dry somewhere.
Freeze-thaw cycling adds a separate mechanism: water held in a porous material expands when it freezes. Materials and details that tolerate that are a specific requirement rather than a general preference.
Note
Water-resistive barrier and flashing requirements are set by the adopted building code and vary by jurisdiction and edition. Confirm what your jurisdiction requires, and confirm that the manufacturer instructions for the specific products are being followed, because the code commonly requires that too.
What to look at while it is being built
Three things, all visible for a few days and none visible afterwards.
Look at the laps on the drainage plane, from below, and confirm each upper layer sits over the one under it. Look at every window opening before the cladding goes on and confirm the sill flashing turns up at the ends and the head flashing runs out over the face of the plane. And look at the bottom of the cavity to confirm there is a way out, rather than a sealed detail that collects water at the base of the wall.
A builder who has detailed this well will be happy to walk you through it. That conversation is worth more than any specification, and it applies the same principle every other sheet on the contents sheet returns to: continuity is what performs, not products.
If the answer to any of the three is a shrug, the useful move is not an argument, it is a photograph. Walls are covered quickly and the record of what was behind the cladding disappears with them. A homeowner who photographs each elevation before the cladding goes on has an asset that costs nothing and is worth a great deal in the event that something later goes wrong, because it converts a dispute about workmanship into a question of fact.
Questions
- Do I need a rainscreen cavity everywhere?
- Not universally, and some claddings are more forgiving than others. In wet climates, on tall walls, and behind absorptive claddings it is close to essential. In dry climates with generous overhangs the case is weaker, though the drying capacity is still an asset.
- Which cladding material lasts longest?
- The question is less useful than it sounds, because the detailing behind the cladding predicts durability better than the material does. A well-detailed wall in a modest material outlasts a badly detailed one in an expensive material, reliably.
Sources
- 1International Code Council. International Code Adoptions. Accessed 2026-08-29