Course Description

This technical CPD takes you inside one of the most misunderstood parts of building physics: how water vapour moves through the building envelope, and where it turns into the liquid water that damages walls. Presented as an in-depth conversation with one of the world’s leading building scientists, it explains the physics without formulas and connects it directly to the walls being built in the UK today.

You will learn why most condensation problems are caused not by vapour diffusing through materials but by humid air leaking through small gaps, why a vapour control layer does nothing to stop air unless it is continuous and sealed, and why the polythene layer that UK walls have relied on for decades can become a liability once summers get hotter and buildings are air conditioned.

Presented by Eugene Korch (façade engineer and IAST Programme Director) in conversation with John Straube (principal at RDH Building Science, associate professor at the University of Waterloo, and author, with Eric Burnett, of Building Science for Building Enclosures), the course draws on decades of North American forensic experience to examine the traditional steel stud wall, the misleadingly named breather membrane, and the brick-faced precast concrete façade that has become the default for tall buildings in London.

Combining the expert interview with supporting visuals, an interactive 3D detail and downloadable materials, this course is designed to give architects, engineers and façade professionals a clear working understanding of vapour, air and condensation, so that moisture risk is designed out at the detailing stage, not discovered behind the plasterboard years later.


Included in Facade Intelligence Professional Membership (FI PRO)

Learning Outcomes

By the end of this course, you will be able to:

  • Explain the difference between vapour diffusion and air leakage, and why air leakage carries far more moisture into walls

  • Explain which way vapour and air flow, and how wind and mechanical ventilation set the direction of air leakage

  • Recognise why a vapour control layer does not stop air unless it is continuous and all its joints are sealed

  • Identify the two conditions condensation needs to cause damage: a cold enough surface and a material that can be harmed

  • Explain how sun on wet absorbent cladding drives vapour inward, and why air conditioning turns the inner layers of a wall into the cold surface

  • Describe the risks of the traditional UK steel stud wall with polythene, including stud corrosion, timber decay and mould

  • Distinguish the air, water and vapour functions of a membrane, and explain why “breather membrane” is a misleading name

  • Apply the different joint rules for vapour, air and water control layers

  • Evaluate alternatives to polythene, including smart vapour control membranes and warm-side insulation strategies, within UK fire safety constraints

  • Understand why a one-dimensional hygrothermal model can show a wall as safe when real air leakage makes it fail

What's Covered

Why Vapour Comes Before Rain

• Why the textbook teaches heat, then vapour, then rain
• How precise our heat, vapour and rain calculations really are
• Why real-world impact runs the opposite way: rain first, then air leakage, then vapour, then heat

How Vapour Moves

• Diffusion through materials versus air movement through gaps
• Why a gap of a millimetre or so is enough to move a damaging amount of moisture
• Why drawings show no holes, but real buildings always have them
• Why most condensation problems are caused by air leakage, not diffusion

Direction of Flow and Where Damage Happens

• Vapour moves from more to less; air moves from high to low pressure
• Wind, extract fans and supply fans as the drivers of air leakage direction
• Why condensation needs both a cold surface and a material that can be damaged
• From a wet brick outer leaf, to a fogged sealed glazing unit, to mould on paper-faced gypsum

The UK Stud Wall Under Summer Heat and Air Conditioning

• How hot cladding surfaces get in direct sun
• Metal, fibre cement and clay brick, and why water storage in the cladding matters
• Inward vapour drive and condensation on the outer face of the polythene
• Corrosion of steel studs, tracks and service boxes, and decay of timber studs
• Why polythene is a double-edged sword

Polythene, Breather Membranes and the Three Control Layers

• The North American history of polythene, and the discovery that it worked because it stopped air
• Separating the air control layer from the vapour control layer
• Why “breather membrane” is a misleading name, and gypsum sheathing as a vapour-open air barrier
• Different joint rules for vapour, air and water control layers

Brick-Faced Precast Concrete

• Why precast concrete is airtight and fairly vapour-tight
• Moisture trapped between the precast and the polythene
• Where the significant air leaks really are, at slab heads and wall bases
• Why the risk rises with timber studs and bio-based insulation
• Why a one-dimensional model can show the wall as safe
• Finding hidden mould: skirtings, smells, corroded carpet strips and swollen MDF

What to Do Instead of Polythene

• Making the inner lining continuously airtight through partitions, floors and services
• Smart and adaptive vapour control membranes, and the resilience they buy
• North American practice: closed-cell foam or taped rigid board against the concrete
• How much of the insulation needs to sit on the warm side of the condensing surface
• UK fire safety constraints on insulation in external walls

Course Schedule

    1. Course Overview

    2. Course Objectives

    3. How to Navigate the Learning Materials

    1. Introduction

    1. Interview with John Straube

    2. Further Reading Recommendations

    1. Knowledge Check

    1. Certificate: Download & Share

About this course

  • Practice test questions
  • CPD Certificate: 1 hour structured CPD / 1 Learning Unit
  • FREE for FI Pro Members

Your Instructors

Principal, RDH Building Science; Associate Professor, University of Waterloo John Straube

John is one of the best-known building scientists in North America. He is a principal at RDH Building Science, where he leads forensic investigations, hygrothermal research and field monitoring of wall assemblies, and an associate professor jointly appointed to the Department of Civil and Environmental Engineering and the School of Architecture at the University of Waterloo. With Eric Burnett he wrote the textbook Building Science for Building Enclosures, now in its second edition; he is also the author of High Performance Enclosures and has received a lifetime achievement award for building science education. He brings decades of forensic experience with failed walls, from leaking towers to mouldy precast façades, to the questions UK designers face today.

Programme Director, IAST Eugene Korch

Eugene is a façade engineer and educator with extensive experience in building envelope design and technical specification. Through the Institute for Architectural Science and Technology (IAST) and Facade Intelligence, he delivers CPD training to architects and construction professionals across the UK. His teaching approach emphasizes practical application and accessibility, making complex technical subjects understandable for design professionals.

Format: Self-paced online CPD, with the interview with John Straube, supporting visuals, an interactive 3D detail of a precast concrete façade, and downloadable materials
Duration: Approximately 45 minutes of video
CPD Points: 1 hour structured CPD / 1 Learning Unit
Access: 12 months from enrolment
Certificate: CPD certificate issued on passing the multiple-choice test
Prerequisites: None — a basic familiarity with external wall build-ups will help
Materials: Downloadable resources and reference guides included

This course is designed for:

Architects who want to understand how their wall build-ups manage moisture
Architectural Technologists detailing membranes, linings and junctions
Envelope Designers and Façade Engineers specifying vapour and air control layers
Façade Specialist Contractors and installers whose workmanship decides real-world airtightness
Building Envelope Consultants investigating damp, mould and condensation complaints
Building Physics and Sustainability Consultants running hygrothermal assessments
Precast Concrete Specialists working on brick-faced and insulated precast façades
Building Control Professionals reviewing moisture and fire safety in external walls
Project Managers and Design Managers responsible for façade risk and long-term durability

Taught by John Straube, who wrote one of the most widely used building science textbooks with Eric Burnett
Grounded in decades of North American forensic investigations, applied directly to walls being built in the UK
Explains the physics of vapour and air without formulas, using clear visuals and real failure evidence
Includes an interactive 3D detail of a precast concrete façade panel and its joints, showing the layers, vapour drive and condensation risk
Challenges long-standing habits, from the default polythene layer to the “breather membrane” label
Practical about what to specify instead, how to detail airtightness, and how to check existing buildings for hidden problems
Relevant both to new design and to the existing stock of steel stud and precast façades

Included with IAST FI PRO Membership

This course is included as part of your IAST FI PRO Membership at no additional cost.

Enrol on the Course

Join the Facade Intelligence Professional Membership (FI PRO)

Membership fees start from £320 per year

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