Beyond the Sealant: why pressure equalization is the real hero of facade design

Every facade eventually leaks — that's the starting assumption a good designer makes, not a failure they're trying to prevent. Gaskets take a compression set. Sealant joints crack under thermal cycling. Gasoline-grade EPDM hardens with UV exposure over a decade of service. If your water strategy depends on every linear metre of seal staying perfect for thirty years, it was never a strategy — it was a hope.

The alternative is to stop fighting water with a single barrier and instead manage it the way nature already does: with pressure. A facade designed around the rain screen principle and air pressure equalization assumes water will get past the first line of defense, and gives it nowhere to go but back out.

A rain screen facade is built in two decoupled layers rather than one solid barrier:

Between these two layers is a narrow, ventilated, drained cavity. That cavity is the entire mechanism. It's what lets air — and therefore pressure — move freely behind the outer screen, and it's the drainage path that carries away anything that does get through.

The section below is a redrawn, cleaned-up version of a typical unitized mullion detail — the same family of joint shown in most manufacturer shop drawings. Every labeled part plays a specific role in either shedding water outright or managing the pressure in the cavity behind it.

Three things are doing the real work in this joint. The exterior pressure plate and gasket break the bulk of wind-driven rain. The narrow equalized cavity behind it lets air move freely so pressure can't build up and force water further inward. And the EPDM air seal — the true barrier — sits behind all of it, protected from direct weather and doing a much easier job as a result.

Water doesn't need a hole to get into a building — it needs a path and a force. Remove either one and the leak stops. In facade design there are four forces that can move water through an otherwise closed joint, and pressure equalization is aimed squarely at the largest of them.

• Gravity — water runs downward along any surface until it finds a drainage path. Easily managed with slope and weeps.
• Kinetic energy — wind-driven droplets with enough momentum to punch through a small gap. Managed by baffles and labyrinth geometry, not tight seals.
• Surface tension / capillary action — water drawn into narrow gaps against gravity. Managed with capillary breaks — small air gaps too wide for surface tension to bridge.
• Air pressure differential — the dominant force in a storm. Wind striking a facade creates a positive pressure on the outside face relative to the inside. That pressure difference will push water through almost any imperfection it can find, however small.

Of the four, pressure differential is the one that turns a minor manufacturing tolerance into an active leak path. It's also the one you can design out entirely — not by sealing harder, but by equalizing the pressure on both sides of the outer screen.

The mechanism is simple once it's drawn as three chambers rather than a section detail. Wind loads the exterior face with a positive pressure, Po. If the cavity behind the rain screen is vented to the exterior — through weeps sized correctly — and sealed against the interior, its internal pressure, Pc, rises to match Po almost instantly. With no pressure differential across the outer screen, there is no force left to drive water through it. Any water that does cross the outer screen by gravity or kinetic energy simply drains down the cavity and out through the same weeps.

Two conditions have to be true for this to work, and both are decisions made on the shop drawing rather than in the field:

1. The cavity must be vented to the exterior — sized weep holes or open joints, generous enough that air can flow in and out fast enough to track changing wind pressure in real time.
2. The cavity must be sealed from the interior — the air seal has to be genuinely airtight, or interior conditioned air will pressurize or depressurize the cavity independently and defeat the equalization.

A pressure-equalized cavity only equalizes over a limited run. If the cavity is continuous around an entire building corner or full floor height, wind pressure at one location can push air — and water — sideways to a completely different part of the facade before it equalizes, defeating the whole mechanism. The fix is to break the cavity into short, sealed compartments, typically bounded by the mullion and transom grid itself.

This is why the mullion and transom intersections in a curtain wall grid are treated as critical seal points, not just structural connections — they are the compartment walls of the drainage system as much as they are load-bearing members.

Before a mullion detail goes from theoretical to battle-tested, verify these on the section:

• 01Cavity continuityConfirm the equalized cavity is unbroken along its intended run and properly compartmentalized at every floor line and corner.
• 02Weep sizing and placementWeeps sized for the expected rainfall and wind pressure for the project's location, placed at the low point of every compartment.
• 03Air seal continuityTrace the interior air seal line around the entire section — it must form a continuous, unbroken loop with no gaps at corners or splices.
• 04Capillary breaksConfirm a minimum air gap at every point two surfaces come close enough for surface tension to draw water inward.
• 05Drainage path to exteriorFollow the path water takes after it crosses the outer screen — it should reach a weep by gravity alone, with no dams, ledges, or reverse slopes.
• 06Thermal break integrityVerify the thermal isolation strip is not bridged by a fastener, shim, or sealant that would create a condensation path alongside the water path.

A gasket is a maintenance item. A pressure-equalized, compartmentalized, well-drained cavity is a system — and systems degrade gracefully instead of failing catastrophically. When a junior designer asks "what happens when this seal eventually fails," the answer on a well-detailed facade should always be the same: nothing happens, because the seal was never the only thing standing between the building and the weather.