Editor’s Note
The difference between a DBV system and a pressure-equalized rainscreen is significant, but it is often glossed over in practice.
As buildings get taller, wind pressures increase, and water penetration becomes harder to control. If we expect our walls to resist that, we need to understand how pressure is managed behind the cladding.
This is a closer look at how PER systems actually work, and why the details cannot be ignored.
In modern building science, the term rainscreen is often used as a catch-all for any assembly with a cavity. However, there is a fundamental functional divide between a Drained and Back-Ventilated (DBV) system and a true Pressure-Equalized Rainscreen (PER).
While both are superior to traditional face-sealed enclosures, the physics governing them and the detailing required to make them work are worlds apart.
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The Core Principle: Neutralizing the Force
For water to breach a facade, three conditions must exist: water on the surface, an opening for it to enter, and a force to drive it through.
While gravity and surface tension are important, wind-induced air pressure differential is the most relentless force.
A drained rainscreen concedes the first two points and focuses on managing water once it gets in. A pressure-equalized system is an active defense designed to eliminate the driving force itself by balancing the air pressure in the cavity with the wind pressure on the exterior. When these pressures match, wind has no force to push water through joints, no matter how large the openings.
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The Danger of the Large Interconnected Cavity
Most architects unknowingly design DBV systems because they utilize a single, continuous cavity behind the cladding. While this works for drainage and convective drying, it makes pressure equalization physically impossible.
Wind pressure is never uniform across a building face. It is typically highest in the center and reaches negative pressures (suction) at the corners and parapets. If your cavity is interconnected, air will rush laterally from high-pressure zones to low-pressure zones. This phenomenon, known as wind-washing, prevents the cavity from ever reaching the pressure necessary to push back against the wind.
In an interconnected cavity, the pressure differential remains active. This means that rainwater on the facade is still being actively sucked toward the water-resistive barrier (WRB). If there is even a minor discontinuity or fastener penetration in that barrier, the pressure differential will funnel water deep into the structural wall.
The key detail
Drained and Back Ventilated Rainscreen System (DBV)
Pressure Equalized Rainscreen System (PER)
Did you catch that? The key is the compartmentalization of the cavity behind the cladding, both horizontally and vertically, while still allowing for drainage and ventilation. Rather than treating the cavity as one continuous air space, breaking it into smaller pressure zones helps limit air movement, reducing the risk of water being driven deeper into the assembly.
Why You’re Probably Designing a DBV System…
Most residential and many commercial projects utilize DBV systems because they are robust, flexible, and tolerant of construction variability
If your design relies on natural convection (the “chimney effect”) to dry the assembly and assumes the water-resistive barrier (WRB) is the primary weather protection, you are designing a DBV system.
True pressure equalization requires a level of design discipline and engineering that goes far beyond a simple drainage mat or furring strips.
The Technical Requirements for True PER
To achieve a functioning pressure-equalized assembly, your design must meet three strict criteria:
Meticulous Compartmentalization: You cannot equalize pressure in a continuous, interconnected cavity. You must divide the cavity into small, sealed chambers both horizontally and vertically. These compartments must be smaller at building corners and parapets where pressure gradients are steepest.
Air Barrier Rigidity: For the cavity pressure to rise instantly to match a wind gust, the air barrier must be stiff. If you use a flexible air barrier, such as loose polyethylene film, it will deflect or pump under wind loads. This increases the cavity volume and creates a pressure lag that allows water to be sucked in during the delay.
Perfectly Airtight Cladding: Materials like cedar shingles or traditional plank siding are generally incompatible with PER because they are not airtight enough to allow for calculated pressure transfer. True PER typically requires engineered panels that can be fabricated into airtight trays with integrated baffles to control airflow.
Conclusion
Specifying a pressure-equalized system without providing the necessary detailing for compartmentalization and air barrier rigidity is a recipe for failure. Without these features, you have a well-intentioned drained system being stressed by wind loads it was not detailed to manage.
For most projects, a high-performance DBV system is the most reliable choice because it is tolerant of construction variability. However, if you are designing a high-rise or a building in an extreme wind zone, a true pressure-equalized system is the gold standard, provided you are prepared for the engineering complexity and precise installation it demands.
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From the Architect’s Desk
“Less is only more where more is no good.”
– Frank Lloyd Wright
Until next time,
Bibliography
Benjamin Obdyke. A Guide to Pressure-Equalized and Drained, Back-Ventilated Rainscreen Systems. This source establishes the fundamental functional divide between DBV and PER systems
Rousseau, M.Z., Poirier, G.F., & Brown, W.C. (1998). "Pressure Equalization in Rainscreen Wall Systems." Construction Technology Update No. 17, National Research Council Canada. This document provides the technical framework for neutralizing driving forces and the necessity of compartmentalization
Brown, W.C., Rousseau, M.Z., & Dalgliesh, W.A. (1991). "Field Testing of Pressure-Equalized Rain Screen Walls." In B. Donaldson (Ed.), Exterior Wall Systems: Glass and Concrete Technology, Design, and Construction. ASTM International. This research details the physics of wind pressure gradients across building faces and the failure of poorly vented systems to equalize
Palani, H., Vo, T., Bhandari, M., & Karatas, A. (2025). "A Review of the State-of-the-Art of Rainscreen Cladding Performance in Residential Building Walls." The Proceedings of the 23rd CIB World Building Congress, Vol. 1, Article 232. This review synthesizes current industry standards for moisture management and the distinct roles of ventilation cavities
Ogle, R., & O’Connor, J. "Failure of the Building Envelope: Two Case Studies." Morrison Hershfield & Alberta Public Works. These case studies highlight the critical failures associated with flexible air barriers (such as polyethylene film) and the resulting pressure lag and condensation issues
CEI Materials. TECHNICAL BULLETIN: Pressure Equalized or Drained and Back Ventilated Rainscreen Systems - AAMA 508 or 509? Metal Construction Association. This bulletin explains the requirements for engineered metal composite material (MCM) panels and the differences between testing standards
AAMA (1996). The Rain Screen Principle and Pressure Equalized Wall Design. This foundational text clarifies that the rain screen principle and pressure-equalized design are interdependent but not strictly synonymous
Sto Corp. Drained vs Pressure-Equalized Rainscreens. This source provides a comparative overview of how different systems integrate with continuous insulation and air/water-resistive barriers
BIM Heroes. Drained vs. Pressure-Moderated Rainscreens: A Technical Guide. This guide emphasizes the role of the air cavity and its specific configuration in maintaining a building’s durability
