# Pressure Equalisation: Why a Facade Lets Air In Deliberately

> Pressure equalisation keeps water out of a facade by allowing air into the cavity behind the outer skin, so the pressure inside roughly matches the pressure outside. With no pressure difference across the joint, wind has nothing to push water through, even though the outer surface is not sealed.

## 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.

## Frequently asked questions

### Water needs three things to cross a joint — what are they?

Water present at the joint, an opening for it to pass through, and a force to move it. Sealing tries to remove the opening, which is difficult to achieve perfectly and impossible to keep perfect for decades. Pressure equalisation removes the force instead, which is far more robust — the opening can exist, and water still has no reason to travel inward.

### Is the cavity meant to get wet?

A little, yes, and that is normal rather than a defect. Some water will always cross an open joint by gravity, surface tension or momentum. The design accepts that and gives it a drainage path back out, which is why weep holes and drainage channels exist. A facade showing a trace of water in its drainage channels is working as intended.

### What breaks pressure equalisation?

Blocking the cavity. Sealing weep holes, filling the cavity with insulation that was not designed for it, or omitting the compartment closures that keep the cavity divided. Once the cavity cannot equalise, the joint is left holding back a pressure difference it was never designed to resist, and water is driven inward.

### Why does the cavity need compartments?

Because pressure varies across a facade — corners and upper levels see very different pressure from the middle of an elevation. If the cavity is one continuous volume, air flows within it from high-pressure to low-pressure zones, which creates exactly the pressure difference the design was meant to remove. Closures divide the cavity so each compartment equalises with the pressure immediately outside it.

### Is this the same as a drained and ventilated system?

They are closely related and the terms are often used loosely. Drained and ventilated describes a system that lets water in, drains it and lets the cavity dry. Pressure equalised describes one designed so the cavity pressure actively tracks the external pressure, which needs adequate vent area and proper compartmentation. Every pressure-equalised facade is drained and ventilated; not every drained facade is genuinely pressure equalised.

## Sources

- [EN 13830](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Curtain walling — product standard
- [ASTM E331](https://www.astm.org/products-services/standards-and-publications.html) — Water penetration of exterior windows and curtain walls
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads on buildings and structures

---

Source: https://hagerstone.com/glossary/pressure-equalisation