Pressure Equalisation: Why a Facade Lets Air In Deliberately

Why it matters on a project

This is the single most misunderstood idea in facade design, and the misunderstanding is expensive.

To most people a wall that keeps rain out should be sealed. So when someone sees open joints on a rainscreen, or finds water inside a curtain wall's drainage channels, the instinct is to seal it — and sealing it is precisely what causes the leak.

The three conditions water needs

Condition	Sealed approach	Pressure-equalised approach
Water at the joint	Cannot be prevented	Cannot be prevented
An opening	Removed by sealing	Accepted, then drained
A force to drive it	Still present	Removed

A sealed facade depends on removing the opening, and keeping it removed for the building's life — through thermal movement, building sway, UV, and workmanship that varies across thousands of metres of joint.

A pressure-equalised facade removes the driving force instead. Wind pressure on the outside is matched inside the cavity, so there is no push. The joint can be imperfect and the wall still stays dry.

What makes it work in practice

• Adequate vent openings so the cavity can equalise quickly as wind gusts change, not minutes later.
• Compartmentation so each part of the cavity equalises against the pressure immediately outside it, rather than air flowing sideways from a high-pressure corner to a low-pressure middle.
• A drainage path for the water that does get in, and weep holes that stay clear.
• An air barrier on the inner leaf, because equalisation depends on the cavity being sealed on the inside, not the outside. This is the part most often missed.

Design pressures come from IS 875 (Part 3) for the specific site and height, and water penetration performance is proven by test, commonly to ASTM E331, with EN 13830 as the curtain walling product standard.

Common mistakes

• Sealing weep holes because they look like a defect. This converts a drained wall into a water trap.
• Filling the cavity with insulation not intended for it, removing the ventilated space entirely.
• Omitting compartment closures — the cavity then behaves as one volume and equalisation fails at the corners, where pressures are highest.
• A leaky inner leaf. If the inside is not airtight, the cavity never equalises and the outer joints are left doing work they were not designed for.
• Treating a trace of water in drainage channels as a failure. It is the system working.
• Painting over vents during maintenance or refurbishment, years later.

Standards referenced

EN 13830 as the curtain walling product standard; water penetration testing commonly to ASTM E331; wind loading to IS 875 (Part 3). Design pressures, vent areas and compartmentation for a specific building must be established by the project's facade engineer.