BC Building Envelope Council – Avoiding Double Vapour Barriers: A Fundamental Principle Overshadowed in Some Current Design Practices

BC Building Envelope Council – Avoiding Double Vapour Barriers: A Fundamental Principle Overshadowed in Some Current Design Practices

Avoiding double vapour barriers in building envelope 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.

Date: September 15, 2026
Time: 12 – 2 p.m.
Location: Italian Cultural Centre, 3075 Slocan Street, Vancouver
Cost: $50 (BCBEC Member) / $60 (Non-member) / $25 (Student)
Learning Units (LUs): 1 Core
Register: Click here