Supporting Reference

Rainscreen Exterior Wall Design for the ExAC

Designing Exterior Walls According to the Rainscreen Principle is the National Research Council's guide to how Canadian buildings keep rain out. This NRC Construction Technology Update sits on Examitect's study plan as a supplementary reference for Section 2 (Codes, Envelope and Environmental Separation) and Section 3 (Sustainability and Final Project, Building Science and Assemblies).

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Rainscreen exterior wall design overview

Rainscreen guide at a glance

Full titleDesigning Exterior Walls According to the Rainscreen Principle
SeriesConstruction Technology Update No. 34 (CTU 34)
AuthorsW.C. Brown, G.A. Chown, G.F. Poirier, M.Z. Rousseau
PublisherInstitute for Research in Construction (IRC), National Research Council of Canada (NRC)
PublishedDecember 1999 (ISSN 1206-1220)
LengthSeven pages with tables and design-detail figures
LanguagesEnglish; French version available from NRC Publications Archive
Primary audienceArchitects, builders, engineers, and code officials
Where to accessFree PDF download from NRC Publications Archive (DOI 10.4224/40002833)

Why rainscreen design matters for the ExAC

The rainscreen principle is the conceptual backbone behind every envelope question on the ExAC. National Building Code of Canada (NBC) 2020 Part 5 requires that walls control rain penetration, air leakage, and moisture flow, but doesn't always specify how. Rainscreen design, as explained in this NRC bulletin, is the logic most Canadian architects use to meet those requirements.

Examitect's study plan lists this guide as supplementary for Section 2 (Envelope and Environmental Separation, categories 5.21 and 5.22) and Section 3 (Building Science and Systems 8.2, Assemblies and Detailing 8.3). When the exam asks why a cavity is 10 mm deep, how a sheathing membrane works, what flashing does, or how pressure equalization controls driving forces, the answer traces back to this bulletin.

How to study rainscreen design for the ExAC

  • Read all seven pages in one sitting so the two-line concept stays connected
  • Sketch a wall section by hand, labelling cladding, cavity, inner boundary, and marking which line of defence each plays
  • Memorise the five driving forces (gravity, capillarity, air pressure difference, surface tension, kinetic energy) and the detailing tip for each (Table 1, page 3)
  • Cross-reference the guide with NBC 2020 Part 5, Sections 5.6 and 5.9, so you know both principle and code provision
  • Pair the reading with CHING's wall-section drawings and the Building Envelope Thermal Bridging Guide to cover water, air, vapour, and heat together
  • Test yourself with scenario-based questions on cavity sizing, flashing, pressure equalization, and envelope failures

Inside the guide: two lines of defence

The bulletin organises around the two-line concept. Each section either defines a line of defence, explains how to design it, or shows how to manage the forces moving water across it.

SectionWhat it coversWhere it lands on the ExAC
Environmental Conditions Moisture loads and driving forces. Climate data sources: NBC rainfall, CSA A440.1 driving rain wind pressure (DRWP), CSA A370 annual driving rain index (ADRI). Section 2, Envelope and Environmental Separation (5.21 environmental separation requirements)
The First Line of Defence Cladding design to reduce moisture load, minimise holes, and manage driving forces. Includes overhangs, drip edges, and joint detailing. Section 2 (5.22 envelope performance); Section 3 (building science, assemblies and detailing)
Managing Forces (Table 1) The five driving forces, what each does, and the detailing move that controls it (gravity, capillarity, air pressure, surface tension, kinetic energy). Section 2 (5.21, 5.22); Section 3 (8.2, 8.3). This page alone repays memorisation.
Second Line of Defence Intercepting free water with a cavity or waterproof membrane; intercepting bound water with a capillary break; flashing details with end dams and drip edges. Section 2 (5.22); Section 3 (assemblies and detailing, 8.3)
Dissipating Water Cavity depth (25 mm for masonry, 10 mm for most others, under 5 mm holds water by surface tension). Role of evaporation in different climates. Section 3 (assemblies and detailing)
Overall Moisture Management How the rainscreen layers (cladding, cavity, inner boundary) work with the air barrier and vapour control. Design implications for layer placement. Section 2 (5.22); Section 3 (building science)

Key rainscreen terms every ExAC candidate should know

The guide introduces vocabulary the ExAC reuses without redefining. Learning these early saves exam time parsing questions instead of choosing answers.

TermWhat it means
Rainscreen principleA design approach using two complementary lines of defence: a cladding that sheds most water, and a drained cavity that intercepts and dissipates the rest.
First line of defenceThe cladding. Minimises rainwater reaching the interior by reducing the moisture load, limiting holes, and managing driving forces.
Second line of defenceThe drained and vented cavity assembly. Intercepts water that gets past the cladding and dissipates it back to the exterior.
CapillarityCapillary suction. The force that draws water into permeable materials and into joints narrower than about 5 mm.
Capillary breakA cavity or an impermeable material placed to stop water being drawn further inward by capillary action.
Driving rain wind pressure (DRWP)The maximum instantaneous wind pressure coincident with rainfall, likely to be exceeded once in five or ten years. Tabulated in CSA Special Publication A440.1.
Annual driving rain index (ADRI)The product of annual rainfall and mean annual wind speed at a site. Used to gauge rain climate severity. Tabulated in CSA Standard A370.
Drained and vented cavityThe space behind the cladding that catches free water, drains it to flashing, and lets the assembly dry by vapour diffusion and airflow.
Sheathing membraneA breather-type membrane (asphalt-impregnated paper or polymeric) on the outer face of sheathing to form the inner boundary of the second line of defence.
FlashingA waterproof element at horizontal interruptions (windows, shelf angles, wall-roof junctions) that collects water from inside the cavity and directs it back to the exterior. Must have a drip edge and end dams.

Tips for Intern Architects reading the rainscreen guide

This is a 1999 technical bulletin written for practising architects, not a textbook. Here is how to read it efficiently as an Intern Architect preparing for the ExAC.

Tip 1, focus on Table 1 (Managing Forces). One page carries the entire detailing logic of a rainscreen wall: each driving force, what it does, and the design move that controls it. Most ExAC envelope questions test one row of that table.

Tip 2, memorise the cavity depth numbers. Twenty-five millimetres behind brick veneer (required by CSA). Ten millimetres for most other cladding. Less than five millimetres and the cavity holds water by surface tension, so the inner boundary needs more water resistance. These numbers appear on the exam.

Tip 3, understand the BC coastal context. The overhang research (Figure 1) comes from the mid-1990s leaky-condo investigation in British Columbia. Knowing why this document exists helps you understand the principles instead of just memorising them.

Tip 4, check the code references for updates. The bulletin cites NBC 1995, CSA A440-98, and CSA A371-94. The principles still hold, but the specific clauses have moved. Cross-reference NBC 2020 Part 5 and current CSA editions so you don't memorise a stale clause number.

Tip 5, link the two lines to the air barrier. The bulletin assumes you've read CTU No. 17 on pressure equalization. The air barrier is what makes the first line of defence work by removing the pressure difference that drives water inward. The exam tests these three concepts (cladding, cavity, air barrier) together more often than separately.

Tip 6, ask a senior architect to walk you through a wall section. Pull a recent project drawing and ask which layer does what. Tracing water from rain to the exterior across each detail locks in the rainscreen principle faster than re-reading the bulletin a fourth time.

Common ExAC scenarios where the rainscreen principle is the answer

These question shapes appear across ExAC sittings. When you see one, ask: which line of defence and which driving force is being tested?

  • A brick-veneer wall is leaking at the third floor of a six-storey building in a high-wind coastal climate. What cavity depth, flashing detail, and drainage arrangement should the design specify?
  • A window jamb seal has failed and water is staining the gypsum board inside. Which line of defence should have caught the water, and how should the flashing have been detailed?
  • A wood-frame house is clad with stucco directly over asphalt-impregnated paper with no air gap. In what climates is this acceptable, and what design measures (overhangs, drip edges) reduce the risk?
  • A curtain wall mullion is accumulating water at its windward edge under high wind. Which driving force is at work, and what design response does the bulletin recommend?
  • A shelf angle on a steel-framed multi-storey building has no flashing. The architect must specify a remediation detail: where should the flashing extend, what slope, and what minimum upstand?
  • A horizontal cladding detail is collecting water in a trough condition. What slope and drainage features should be added so it stops acting as a reservoir?
  • A small project in a coastal climate cannot rely on evaporation to dry the cladding. What changes to the second line of defence should the architect specify?

How rainscreen design compares to other ExAC references

This bulletin is the principle. It belongs alongside the prescriptive code, the illustrated assembly book, and the thermal guide that together form Examitect's envelope and building science reading list.

ReferenceWhat it's forHow it relates to rainscreen
Rainscreen Exterior Wall Design (CTU No. 34)The supplementary NRC Construction Technology Update that explains the rainscreen principle: the cavity, the drainage plane, pressure equalization, and how each layer manages water in an exterior wall assembly.The supplementary reference Examitect's ExAC study plan cites for Section 3 building science questions on rain control and envelope design. Pairs with NBC Part 5 environmental separation provisions.
NBC 2020The national model building code. Part 5 sets the performance and prescriptive requirements for environmental separation and rain control.NBC says what the wall must do; CTU 34 says how the rainscreen principle gets you there. Primary reference for Section 2; rainscreen is supplementary.
NECBThe national model energy code for buildings.Different job. NECB drives insulation and thermal continuity; CTU 34 drives water and air control. Both meet in the assembly drawing.
CHINGIllustrated assemblies, materials, and detailing across the construction process.CHING shows the wall section; CTU 34 explains why each layer is where it is. Read them together for Section 3.
Building Envelope Thermal Bridging GuideQuantified thermal performance and detailing for Canadian wall assemblies.Adds the heat-flow layer on top of CTU 34's water and air logic. Both are supplementary for Sections 2 and 3.
Canadian Wood-Frame House Construction (CMHC)Detailed guide to residential wood-frame assemblies under the National Building Code Part 9.Shows how the rainscreen principle applies to typical residential houses, with chapters on sheathing, claddings, windows, and roofs.

How Examitect reinforces rainscreen design

Reading the bulletin is half the work. The other half is recognising the content under exam pressure. Examitect's question bank includes scenario-based questions on envelope failures, cavity sizing, flashing detail, driving forces, and pressure equalization, tied back to NBC 2020 Part 5 and this NRC bulletin. Each answer explanation points to the specific principle or page, so you can re-read just what you need.

You also get full-length mock exams timed to ExAC pacing, study notes for every Section 2 and Section 3 category, and the flexibility to focus on whichever section you need most. Try a few sample questions first, then check pricing when you're ready for the full question bank.

FAQ

Rainscreen FAQ

It is Construction Technology Update No. 34, a seven-page technical bulletin published in December 1999 by the Institute for Research in Construction (IRC) at the National Research Council of Canada (NRC). It explains how to apply the rainscreen principle to exterior wall design through two lines of defence: a cladding that sheds most rainwater, and a drained cavity assembly that catches and dissipates what gets past.

No. It appears as a supplementary reference on Examitect's ExAC study plan under Section 2 (Codes, Envelope and Environmental Separation) and Section 3 (Sustainability and Final Project, Building Science and Systems and Assemblies and Detailing).

The first line of defence is the cladding, which minimises how much rainwater reaches the wall interior by reducing the moisture load, limiting the number and size of holes, and managing the forces that drive water inward. The second line of defence is a drained and vented cavity assembly that intercepts any water that gets past the cladding and dissipates it back to the exterior through drainage and evaporation.

CSA masonry standards require a 25 mm cavity behind brick veneer. For most other wall types, the guide states that a 10 mm cavity provides sufficient drainage while accommodating typical construction practices. Cavities smaller than 5 mm retain water by surface tension, so the inner boundary must offer increased water resistance if the cavity is that shallow.

Driving rain wind pressure. It is the maximum instantaneous wind pressure coincident with rainfall that is likely to be exceeded once in five or ten years. DRWP values for Canadian locations are tabulated in CSA Special Publication A440.1 and are used to size overhangs and predict where wind-driven rain will concentrate on a building.

Gravity (moves water down the face), capillarity (draws water into permeable materials and small openings), air pressure difference (drives water toward lower pressure), surface tension (causes water to cling to the underside of horizontal surfaces, which is why drip edges are needed), and kinetic energy (propels raindrops into unprotected holes). Table 1 in the guide provides detailing strategies to control each force.

The rainscreen principles described in the guide remain standard practice in Canada, but some code references have been updated. The NBC has moved from 1995 to 2020, CSA A440 has been revised, and the masonry standard has evolved. Read the guide for design principles and detailing logic, then cross-reference current NBC 2020 Part 5 and current CSA standards for specific code clause numbers.

Pressure equalization is one strategy within the rainscreen approach. It reduces the air pressure difference across the cladding so it cannot drive water inward. This is achieved through cavity venting, compartmentation, and a tight inner air barrier. NRC Construction Technology Update No. 17 covers pressure-equalized rainscreen systems in detail.

Keep exploring

Other ExAC reference books

Rainscreen design is one of several supporting references on Examitect's study plan. Continue with the rest.

CHING

Building Construction Illustrated. The visual reference for assemblies, materials, and detailing.

NBC 2020

National Building Code of Canada. The primary code reference for Section 2 (Codes).

NECB

National Energy Code of Canada for Buildings. Energy performance requirements for buildings.

Thermal Bridging Guide

Canadian thermal performance reference for envelope details and assemblies.

Wood-Frame House Construction

CMHC guide to residential assemblies and detailing under NBC Part 9.

CHOP

Canadian Handbook of Practice. The practice spine of the ExAC, primary across most of Sections 1, 3, and all of 4.

RSMeans

RSMeans Cost Data. North American construction cost data, used for Section 1 Cost Management questions.

Yardsticks

Hanscomb Yardsticks for Costing. Canadian elemental cost data by building type and region, for Section 1 Cost Management.

All supporting references

Browse the full reading list of supplementary references across the ExAC.