Avoiding Double Vapour Barriers: A Fundamental Principle Overshadowed in Some Current Design Practices
September 3, 2026 (7:30 AM - 12:00 PM) (PDT)
Description
CPD - 4 credits (TBC)
Course Overview:
Avoiding double vapour barriers in building assemblies is among the foundational principles of building science. This guidance was reinforced for decades through the education and research conducted by the Division of Building Research at the National Research Council of Canada throughout the 1960s, 70s, and 80s, and captured in publications such as the Canadian Building Digests and reference texts like Building Science for a Cold Climate. This fundamental has long been embedded in Canadian Building Code provisions and remains central to durable, predictable building‑envelope performance.
While this topic received significant attention historically, recent years have seen the introduction of various assemblies—often configured to meet energy‑efficiency targets—that create double‑vapour‑barrier conditions. In many cases, these configurations have led to performance failures, compromised long‑term durability, and nuisance conditions for end users. Forensic reviews confirm that these outcomes are not attributed solely to construction error; rather, in many cases, the underlying issues stem from design decisions.
This session revisits the principle of avoiding double vapour barriers by examining its historical context, highlighting why this “old wisdom” remains crucial for long‑term durability, and reviewing how it should continue to inform design decisions. Using case studies supported by photos, videos, field documentation, and mobile roof and wall assemblies, the session will illustrate where design choices have disregarded this principle and will provide practical, reliable takeaways to support performance, durability, and end‑user experience.
Agenda:
7:30 AM - 8:00 AM: Breakfast and Networking
8:00 AM - 12:00 PM: Course
Speaker:
Hamid Heidarali is a Building Science specialist at HDB with a strong passion for high performance buildings that minimize their impact on the environment, while providing superior experience for occupants. He has been providing Building Science engineering on a wide range of projects in Canada over the past two decades. Hamid taught building envelope courses to Masters students at British Columbia Institute of Technology (BCIT) from 2015 till 2020, and offers holistic building envelope workshops for the construction professionals on a regular basis.
Sponsors
Session Sponsor
Master Group
5888 Trapp Avenue
Burnaby, BC V3N 5G4 Canada
5888 Trapp Avenue
Burnaby, BC V3N 5G4 Canada
September 3, 2026 (7:30 AM - 12:00 PM)
(PDT)
7:30 AM - 12:00 PM Pre-registration is required.
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