Rainscreen exterior wall design

Placeholder page for the supporting reference Rainscreen exterior wall design, part of the Examitect reading list for the ExAC.

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Rainscreen guide at a glance

Here is the at-a-glance summary an Intern Architect can scan before opening the PDF for the first time.

Full titleDesigning Exterior Walls According to the Rainscreen Principle
SeriesConstruction Technology Update No. 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)
Related updatesBuilds on CTU No. 9 (evolution of wall design for rain penetration) and CTU No. 17 (pressure equalization in rainscreen wall systems)
LanguagesEnglish; the NRC Publications Archive also lists the French version
LengthSeven pages, including a single design-approach table and several detail figures
Primary audienceArchitects, building scientists, builders, and code officials
ExAC relevanceSupplementary on Examitect's ExAC study plan 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)
Where to accessNRC Publications Archive, DOI 10.4224/40002833. Free PDF download.

Why rainscreen design matters for the ExAC

The rainscreen principle is the conceptual backbone behind every envelope question the ExAC asks. National Building Code of Canada (NBC) 2020 Part 5 requires that walls control rain penetration, air leakage, and moisture flow. The code does not always spell out how. The rainscreen approach, as presented in this NRC bulletin, is the design logic most Canadian practitioners use to meet those requirements.

Examitect's ExAC study plan lists this guide as a supplementary resource for two categories in Section 2 (Envelope and Environmental Separation) and two categories in Section 3 (Building Science and Systems, and Assemblies and Detailing). When a question asks where a sheathing membrane sits, why a 10 mm cavity is enough behind cement-board cladding, what a drip edge is for, or how to flash a shelf angle, the answer almost always traces back to a paragraph in this update.

It is also the vocabulary source. Terms like first line of defence, second line of defence, capillary break, driving rain wind pressure, and drained and vented cavity show up in ExAC questions without being defined. Reading the guide gets you fluent fast.

ExAC sections this reference supports

Section 1
Section 2
Section 3
Section 4

What the guide is

Designing Exterior Walls According to the Rainscreen Principle is a seven-page technical bulletin from the National Research Council's Institute for Research in Construction, published in December 1999 as Construction Technology Update No. 34. It is one of a series of short articles the NRC produced to translate building-science research into practical design guidance for architects, engineers, and builders.

CTU 34 is not a textbook. It is a focused explanation of how the rainscreen principle, introduced in CTU No. 9, gets applied to actual wall design. The bulletin works through the two lines of defence (cladding and drained cavity), the five forces that drive water through walls, and the detailing decisions that keep the wall dry over its service life. It also references the air-barrier discussion in CTU No. 17, since pressure equalization across the cladding is one of those forces.

It is short enough to read in a single sitting and dense enough to repay a second pass.

Inside the guide, two lines of defence

The bulletin is organized around the two-line concept. Every other section either defines a line of defence, explains how to design it, or explains how to manage the forces moving water across it.

SectionWhat it coversWhere it lands on the ExAC
Environmental Conditions Moisture loads, driving forces, and climate data sources: NBC rainfall data, CSA A440.1 driving rain wind pressure (DRWP), and CSA A370 annual driving rain index (ADRI). Section 2 (Envelope and Environmental Separation, 5.21 environmental separation requirements).
The First Line of Defence Cladding objectives: reduce moisture load, minimize the number and size of holes, manage the driving forces. Includes overhangs, drip edges, joint design, and pressure-equalization references. Section 2 (envelope performance, 5.22); Section 3 (building science, assemblies and detailing).
Managing Forces (Table 1) A single page that lists each driving force (gravity, capillarity, air pressure difference, surface tension, kinetic energy) with the detailing tip that controls it. Section 2 (5.21, 5.22); Section 3 (8.2, 8.3). This page alone is worth memorizing.
Second Line of Defence Intercepting free water with a cavity or waterproof membrane; intercepting bound water with a capillary break; continuity across junctions; flashing details with end dams and drip edges. Section 2 (5.22); Section 3 (assemblies and detailing).
Dissipating Water Drainage cavity depths (25 mm for masonry, 10 mm for most other cases, less than 5 mm holds water by surface tension) and the role of evaporation in favourable versus unfavourable climates. Section 3 (assemblies and detailing).
Overall Moisture Management Connects the rainscreen to the air barrier, vapour control, and heat flow. Reminds the designer that a rainscreen element may also act as a vapour or air barrier and must be located accordingly. Section 2 (5.22); Section 3 (building science).
Summary and References One-page summary of the design principles plus sixteen references back to NRC, CSA, and CMHC publications. Useful for tracing claims back to source documents.

If you have ten minutes, read the Managing Forces table and the Summary. They carry most of the ExAC-relevant content.

Key rainscreen terms every ExAC candidate should know

The guide introduces vocabulary the ExAC reuses without redefining. Learn these early so you spend exam time choosing the answer, not parsing the question.

TermWhat it means in the guide
Rainscreen principleA design approach for controlling rain penetration using two complementary lines of defence: a cladding that sheds most water, and a drained cavity assembly that catches and dissipates the rest.
First line of defenceThe cladding. Its job is to minimize 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.
Second line of defenceThe drained and vented cavity assembly behind the cladding, with an inner boundary that resists water. Intercepts what gets past the cladding and dissipates it to the exterior.
CladdingThe outermost wall layer, in direct contact with the weather. Permeable cladding (brick, stucco, wood) absorbs water; impermeable cladding (glass, metal, vinyl) does not.
CapillarityCapillary suction. The force that draws water into permeable materials and joints narrower than about 5 mm.
Capillary breakA cavity or an impermeable layer in the assembly that stops water from being drawn further inward by capillary action.
Driving rain wind pressure (DRWP)The maximum instantaneous wind pressure that is coincident with rainfall and likely to be exceeded once in five or ten years. Values are 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.
Pressure equalizationReducing the air-pressure difference across the cladding so it cannot drive water inward. Achieved through cavity venting, compartmentation, and a tight inner air barrier.
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 membrane) installed on the outer face of sheathing to form the inner boundary of the second line of defence.
FlashingA waterproof element at horizontal interruptions in the cavity (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.

How the guide compares to other ExAC references

This bulletin is the principle. It belongs next to 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 rainscreen design relates
Designing Exterior Walls (CTU 34)The design logic for controlling rain penetration through two lines of defence.The principle source for envelope questions on the ExAC.
NBC 2020The national model building code: Part 5 sets out the performance and prescriptive rules for environmental separation, including rain control.The code says what the wall must do; CTU 34 says how the rainscreen principle gets you there. The NBC is the primary resource for Section 2; CTU 34 is supplementary.
NECBThe national model energy code for buildings.Different job. NECB drives the insulation and thermal continuity; CTU 34 drives the water and air control layers. Both meet in the assembly drawing.
CHING (Building Construction Illustrated)The illustrated reference for assemblies, materials, and detailing.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 assemblies.Adds the heat-flow layer on top of CTU 34's water and air logic. Both are supplementary on Examitect's ExAC study plan for Section 2 and Section 3.
Canadian Wood-Frame House ConstructionCMHC's detailed guide to Part 9 wood-frame residential assemblies.Shows how the rainscreen principle is applied to typical houses, with chapters on sheathing, claddings, windows, and roofs.
CHOPThe architectural-practice reference: profession, business, project delivery, and contract administration.Different job. CHOP covers the architect's role; CTU 34 covers the wall itself.

How to study the rainscreen guide for the ExAC

  • Read all seven pages in one sitting. The two-line concept is the spine, and breaking up the reading makes the connection between cladding (first line) and cavity (second line) harder to hold.
  • Sketch a wall section by hand, labelling cladding, cavity, inner boundary, sheathing membrane, sheathing, and the air, vapour, and thermal layers. Mark which element handles which line of defence.
  • Memorize the five driving forces and the detailing tip for each: gravity (shingle lap, 10 mm overlap), capillarity (25 mm overlap or a break), air pressure difference (pressure equalization), surface tension (drip edges), and kinetic energy (shielding).
  • Cross-reference the guide with NBC 2020 Part 5, especially Sections 5.6 (Heat, Air and Moisture Transfer) and 5.9 (Wall and Roof Assemblies). The exam tests both the principle and the code provision.
  • Pair the reading with CHING's wall-section illustrations and with the Building Envelope Thermal Bridging Guide. Together they cover water, air, vapour, and heat.
  • Test recall with scenario-based practice questions on envelope failures, cavity sizing, flashing details, and pressure equalization. Reading recognition is not the same as exam recall.

ExAC sections the rainscreen guide supports

Examitect's ExAC study plan lists primary and supplementary resources for each category. Here is where this guide shows up on that plan.

ExAC sectionHow rainscreen design shows up on Examitect's study plan
Section 1
Design and analysis		Not on the primary or supplementary lists. Envelope detailing belongs to Sections 2 and 3.
Section 2
Codes		Supplementary resource under Envelope and Environmental Separation for category 5.21 (environmental separation requirements) and 5.22 (building envelope performance). Sits alongside the Thermal Bridging Guide, Canadian Wood-Frame House Construction, the basement-wall insulation guide, and the windows overview.
Section 3
Sustainability and final project		Supplementary resource under Building Science and Systems (category 8.2) and Assemblies and Detailing (category 8.3). Pairs with CHING, CHOP, and the wall-assembly references.
Section 4
Construction and practice		Not on the primary or supplementary lists. Section 4 leans on CHOP and CCDC documents.

Tips for Intern Architects reading the rainscreen guide

This is a 1999 technical bulletin written for working architects, not a textbook. Here is how to read it efficiently as an Intern Architect under the Internship in Architecture Program (IAP) or its provincial equivalent.

Tip 1, read the Managing Forces table twice. Table 1 on page 3 packs the entire detailing logic of a rainscreen wall onto a single page: each force, what it does, and the design move that controls it. Most ExAC envelope questions test one row of that table.

Tip 2, treat the cavity depth numbers as load-bearing facts. Twenty-five millimetres behind brick veneer (required by CSA masonry standards). Ten millimetres for most other cladding types. Less than five millimetres and the cavity holds water by surface tension, so the inner boundary needs more water resistance. These numbers come up.

Tip 3, anchor the reading to the BC coastal context. The bulletin's overhang research (Figure 1) is from BC's lower mainland during the leaky-condo investigation in the mid-1990s. Knowing why this document exists helps you internalize the principles instead of just memorizing them.

Tip 4, check that the code references are current. The bulletin cites NBC 1995, CSA A440-98, and CSA A371-94. The principles still hold, but the specific clauses have moved. When studying for the 2026 ExAC, cross-reference NBC 2020 Part 5 and the current CSA editions so you do not memorize a stale clause number.

Tip 5, link the two lines of defence to the air barrier. The bulletin assumes you have 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 ExAC tests these three concepts (cladding, cavity, air barrier) together more often than it tests them separately.

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

Tip 7, write your own one-page summary. Compress the seven pages of the bulletin into one page of your own notes, organized around the two lines of defence and the five driving forces. Many candidates say this one habit moved them faster than any other.

Common ExAC scenarios where the rainscreen principle is the answer

These question shapes appear across ExAC sittings. When you see one, your first instinct should be to 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 on the BC coast. What cavity depth, flashing detail, and weep arrangement should the design call for?
  • 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 furred cavity. 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 in a tall building. Which driving force is at work, and what design response does the bulletin recommend?
  • A shelf angle on a steel-framed multi-storey wall has no flashing. The architect of record must specify a remediation detail: where should the flashing extend, what slope, and what minimum upstand?
  • A flat 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?

Each of these traces back to one of the two lines of defence and one of the five driving forces. Practising on scenarios in this form is the fastest way to convert the bulletin into exam recall.

How Examitect reinforces the rainscreen guide

Reading the bulletin is half the work. The other half is recognizing the content under pressure on a timed exam. Examitect's question bank includes scenario-based questions on envelope failures, cavity sizing, flashing, and pressure equalization, tied back to NBC 2020 Part 5 and to 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 that mirror ExAC pacing and free study notes for every category. Try a few sample questions first, then check pricing when you want the full bank.

Rainscreen and ExAC FAQ

It is Construction Technology Update No. 34, a seven-page technical article published in December 1999 by the Institute for Research in Construction at the National Research Council of Canada. 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 is listed as a supplementary resource on Examitect's ExAC study plan. It appears under Section 2 (Envelope and Environmental Separation) for the categories that ask candidates to understand environmental separation requirements and building envelope performance, and under Section 3 (Building Science and Systems, and Assemblies and Detailing).

Sections 2 and 3. Section 2 questions about environmental separation and envelope performance under NBC 2020 Part 5 often use rainscreen vocabulary. Section 3 questions about building science, assemblies, and detailing test the same first-and-second-line concepts in the context of specific wall assemblies.

The first line of defence is the cladding, which minimizes how much rainwater enters the wall 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 years or once in 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.

The rainscreen principles described in the guide are still standard practice in Canada, but some of the codes and standards it references have been updated. The NBC has moved from 1995 to 2020, CSA A440 has been revised, and the masonry standard has moved through several editions. Read the guide for principles and detailing logic, then check current NBC 2020 Part 5 and current CSA standards for specific numbers.

A rainscreen wall uses two lines of defence to control rain penetration. A pressure-equalized rainscreen, discussed in NRC Construction Technology Update No. 17, is one variation: the cavity is vented and compartmented so that air pressure on the inside of the cladding matches the outside, removing the pressure difference that drives water through holes. Pressure equalization is one of several ways to manage the forces that move water through a rainscreen.

Keep exploring

Other ExAC reference books.

The rainscreen bulletin is one of several supplementary references on Examitect's ExAC study plan. Continue with the rest.

CHING

Building Construction Illustrated. The visual reference for assemblies, materials, and detailing across Sections 1 and 3.

NBC 2020

National Building Code of Canada. The primary reference for every category in ExAC Section 2, including envelope and environmental separation.

NECB

National Energy Code of Canada for Buildings. Supports the envelope and energy questions in Section 2.

Thermal Bridging Guide

The Canadian thermal-performance reference for envelope details. Pairs directly with the rainscreen principles.

Wood-Frame House Construction

CMHC's detailed guide to Part 9 residential assemblies, including rainscreen detailing for typical houses.

Supporting references

The extended reading list of supplementary references that show up across the ExAC reading list.