How Walls Dry: The Ventilation Science Behind Siding That Lasts in BC

Ask why a wall failed and the answer almost never starts with the siding. It starts with water that got in, which is normal, and then could not get out, which is fatal. Drying is the whole game on this coast: the Lower Mainland's walls spend eight months absorbing moisture from outside and inside at once, and the difference between a fifty-year wall and a rot repair is the speed at which it sheds and dries. This guide explains the science under every recommendation this series makes, part of our siding guide for BC buildings.

Where wall moisture actually comes from

Rain is the famous source and not the only one. Bulk water arrives as wind-driven rain penetrating cladding joints and details, more than intuition suggests: coastal design assumes some penetration and plans for its management rather than pretending at perfection. Vapour arrives from both directions: interior humidity, cooking, showering, breathing, in a heated home pushes outward through assemblies all winter, while summer sun on rain-wetted cladding drives moisture inward, the reversal that surprises people. Capillary water wicks upward from grade contact and across touching surfaces, the physics behind clearance rules. Built-in moisture comes with construction itself, wet lumber, curing concrete, rained-on framing, and needs somewhere to go in a building's first years.

Add the microclimate multipliers, canopy shade that halves drying days, reflective water and pavement that concentrate sun-driven vapour pushes, wind exposure that raises penetration on one face of the same house, and two identical assemblies a street apart can live different moisture lives. Four sources, one implication: a wall's moisture budget is never zero. Assemblies succeed by drying capacity, not by sealing fantasy, the principle CMHC's coastal research spent decades quantifying and the leaky-condo era taught this region at catastrophic tuition.

The drying engine: how a vented cavity works

The rainscreen cavity, the gap behind cladding that this series keeps returning to, is best understood as a drying engine with three mechanisms working in series.

Drainage handles bulk water: whatever penetrates the cladding meets the cavity's back face, the weather-resistive barrier, and gravity takes it down to flashings that direct it out. This is fast-response moisture management, storm water gone in minutes, and it requires only that the path stay open: unblocked cavity, functioning weeps, flashings lapped so water moves outward at every level.

Ventilation handles the slower loads: air enters at the cavity's base, warms against the building, rises, and exits at the top, carrying evaporated moisture continuously. This stack-effect airflow is gentle and relentless, the mechanism that dries the cavity's surfaces between storms and pulls stored moisture out of claddings like wood. It requires intake and exhaust, open detailing at bottom and top, screened against pests but never sealed, which is why a caulk gun aimed at a cavity opening is the most dangerous tool on a maintenance truck.

Capillary breaking is the gap itself: with no continuous contact between cladding and barrier, water cannot wick inward across the assembly, and the barrier plane stays decoupled from the wet cladding. Even the modest cavities behind strapped installs deliver it; deeper cavities improve airflow on top.

The engine's output is measured in drying rate, and the coastal design goal is simple: drying capacity greater than wetting load, across the worst winter the wall will meet.

Vapour: the direction problem

Bulk water thinks in gravity; vapour thinks in gradients, moving from warm-humid toward cool-dry, which on this coast means outward in winter and inward on sun-after-rain days. Assemblies manage it with layered permeability, and the working principle is directional: a wall should become more vapour-open toward the side it must dry to, here, predominantly outward. Interior vapour control on the warm side, sheathing and barriers chosen for permeance, and a cladding system that does not seal the exterior face tight complete the gradient.

The failure pattern worth knowing is the trap: impermeable layers on both faces, vapour-tight interior plus vapour-tight exterior, leaves moisture that enters no exit, and assemblies built or renovated into that condition accumulate until something rots. It is the science behind why certain retrofit shortcuts, sealing coats over old stucco, foam layered carelessly over damp-prone walls, backfire, and why insulation retrofits are designed as assemblies rather than shopping trips. When we add exterior insulation during a re-clad, permeance is one of the selection criteria doing quiet work in the background, mineral wool's vapour-openness being one of its coastal virtues.

Why some walls stay wet: the usual suspects

Translating science into the failures we actually open up. Closed drying paths: cavities blocked by mortar droppings or insulation stuffed where air should move, weeps painted or caulked shut, landscaping burying the wall base, each converting a drying engine into a reservoir. No cavity at all: the pre-rainscreen assemblies of the region's older stock, cladding tight to paper, where every wetting event races slow drying, and the race is lost a little more each winter, the deep reason older-home re-sides so often become assembly conversations. Reservoir claddings in sun: moisture-storing materials that release inward under solar drive, manageable with the right barrier strategy, damaging without it. Interior humidity unmanaged: houses generating more vapour than their ventilation removes, loading assemblies from the living side, why bath fans and fresh-air strategies are wall-preservation tools wearing comfort costumes. Thermal bypass wetting: air leaks carrying humid interior air into cool assembly corners where it condenses, the air-sealing question that couples energy work to moisture work throughout our Step Code insulation guide.

Every one of these is diagnosable, and most are correctable short of reconstruction, which is the practical point: walls staying wet is a mechanism, not a mystery, and mechanism yields to method, the assessment method this series documents.

What this means for your siding decisions

The science compresses into buyer's rules. Buy the cavity before the cladding: any material over a drained, vented, capillary-broken assembly outperforms any material without one, which is why our quotes discuss what is behind the boards before what colour they are. Respect the openings: bottoms and tops of walls, weeps, and vents are functional, maintenance means keeping them clear, never sealing them, the autumn walk-around's most valuable minute. Match material to exposure honestly: tight modern claddings on well-designed cavities, forgiving claddings where assemblies are inherited and imperfect, wood where its storage behaviour will be maintained, per the material personalities this series profiles. Treat humidity and ventilation indoors as envelope protection. And when moisture symptoms appear, staining, musty rooms, paint that will not hold, read them as drying-deficit announcements and diagnose the mechanism before buying any product, because the product was never the problem.

The coast's numbers: what the wetting load actually is

Feel for the magnitudes helps the science stick. Metro Vancouver's rain arrives overwhelmingly between October and April, at annual totals that vary threefold across the region, roughly a metre on the driest delta flats, well past two metres against the North Shore mountains, delivered substantially as wind-driven events that load walls, not just roofs. During those months, outdoor relative humidity parks high enough that evaporation is slow work, and shaded elevations can hold surface dampness for weeks between genuinely drying days. Meanwhile a household's interior generates litres of vapour daily, pushed outward through every assembly by winter's temperature and pressure differences. Sum it honestly and a coastal wall's winter is months of net wetting, punctuated by drying opportunities the assembly must be built to exploit: the cavity airflow that works every hour, the sun-and-breeze days whose value depends on paths being open, the spring recovery that returns the wall to baseline before the next cycle. This is why the drying-capacity framing beats the sealing framing everywhere in this guide: on a coast where the wetting side of the ledger is guaranteed, the design and maintenance work all lives on the drying side, and why the same house in Kamloops forgives assemblies that Vancouver retires early. The regional climate patterns that shape repair caseloads are this ledger, mapped.

Reading a wet wall in the field

The science becomes practical in diagnosis, so here is the assessor's sequence on a wall with moisture symptoms. Outside first, reading the water's story top-down: roofline and gutter condition, because concentrated roof water mimics wall failure one storey below; staining patterns, whose shape points at sources, V-shapes below a point say penetration, horizontal bands at course lines say wicking, bottom-metre gradients say splash or rising damp; and the drying-path audit, weeps, cavity openings, clearances, sealed-shut sins. Then the probe-and-meter pass at the usual suspects, turning suspicion into readings. Inside, the matching wall's story: staining at outlets and window returns, musty baseboards, the condensation calendar (winter mornings say interior humidity; storm-following says penetration). The synthesis names the mechanism, wetting exceeding drying, and locates it: a delivery problem (flashing, detail, exposure), a drying problem (blocked or missing paths), or a load problem (interior humidity, thermal bypass). Only then does anyone talk products, because the repair is the mechanism's correction, and the repair series' method, open, inspect, correct cause, rebuild, is this science wearing work gloves.

The renovation traps

Three well-meaning renovations manufacture drying deficits often enough to deserve warnings. The sealing instinct: owners fighting drafts or moisture caulk every gap they can reach, including cavity bottoms, weeps and vent openings, treating the drying engine's intakes as leaks; the wall tightens, the moisture stays, and two winters later the "fixed" wall is the wet one. The wrap-and-cover reflex: new cladding installed directly over old, no cavity, sometimes over an old impermeable layer, stacking reservoirs and halving permeability in one move, the reason we decline over-cladding even when it prices attractively. And the insulation afterthought: interior insulation or vapour-tight finishes added to walls that previously survived by leaking heat, older assemblies often dried on wasted energy, and retrofits that remove the energy waste without adding drying capacity change the wall's physics uninvited, the reason envelope retrofits get designed as assemblies with permeance on the criteria list. The common thread: every renovation touching a wall's layers is a moisture-physics decision, whether or not anyone made it on purpose.

Why new walls are better, and what to steal from them

Current BC construction builds drying in by default, and renovators can steal the playbook. Modern assemblies arrive with drained-and-vented cavities as standard practice, flashing systems at every level, permeance-planned layers, and increasingly, exterior insulation keeping sheathing warm and condensation-free, the Step Code direction that improves moisture robustness as a side effect of energy targets. The renovation translation: when any project opens a wall, rebuild that section to the current playbook, cavity, flashings, permeance, rather than reproducing the original era's assembly, which is how older homes upgrade one elevation at a time without ever booking a whole-envelope project. It is also the quiet argument inside every re-side quote we write: the cladding is the visible deliverable, but the assembly modernization underneath is where a 2026 project buys its next fifty dry years.

Key takeaways

• Walls are never dry by exclusion, only by drying: healthy assemblies shed and release moisture faster than four sources deliver it.
• The vented cavity is a three-part engine, drainage, ventilation, capillary break, and its openings are the engine's intake and exhaust: keep them open forever.
• Vapour is directional: assemblies should open toward their drying side, and double-tight traps are how renovations manufacture rot.
• Wet walls have mechanisms, not mysteries: blocked paths, missing cavities, reservoir effects, interior humidity and air leaks cover nearly every case.
• Buy the assembly first, the cladding second, and maintain the drying paths like the machinery they are.

If this guide has one sentence worth remembering at the hardware store, it is this: nothing you can buy in a caulk aisle improves a wall's drying, and half of it can end a wall's drying, so before sealing anything on an exterior, name the water you are stopping and the path you might be closing.

If a wall of yours is showing drying-deficit symptoms, or you want the assembly conversation before your re-side rather than after, book a free assessment or call 778-358-2885.