In this episode of If Walls Could Talk, W&C Editor John Wyatt talks with Tom Harris of Tom Harris PUR Consulting. In this podcast, the two tackle one of the most technical and misunderstood topics in insulation—vapor barriers.

Vapor Barriers Explained: Clearing the Air on Moisture, Codes and Spray Foam

Tom Harris demystifies one of the most misunderstood aspects of insulation—vapor barriers. Harris explains the crucial difference between air and vapor barriers, the science of moisture movement, and the importance of proper vapor control by climate zone, offering builders and homeowners practical insights on preventing condensation, mold, and costly building failures.

Harris begins by clarifying the critical distinction between air barriers and vapor barriers, and why both are required for true code compliance. While spray foam is often marketed as a solution that “does it all,” Harris emphasizes that installers and contractors must understand how moisture really moves in a building. “Moisture moves in two main ways—by diffusion through materials, and by air leakage,” he explains. “Stopping air movement doesn’t necessarily mean you’ve stopped vapor diffusion.”

The discussion dives into ASTM testing, perm ratings, and how materials are classified: vapor impermeable (0.1 perm or less), semi-impermeable (0.1–1.0), semi-permeable (1.0–10), and vapor permeable (greater than 10). Harris warns that misclassifying insulation can lead to condensation within assemblies, especially in colder climates where the vapor drive moves from the warm interior to the cold exterior.

“Codes divide the U.S. into climate zones for a reason,” Harris notes. In zones 5 and above, permeable and semi-permeable insulations like unfaced fiberglass, cellulose, rockwool, or open-cell foam require additional vapor protection—typically a polyethylene sheet installed on the warm side. By contrast, closed-cell spray foam is semi-impermeable and can provide sufficient vapor resistance on its own in any climate zone.

Wyatt presses for practical implications, and Harris provides clear takeaways: builders must not only choose the right insulation but also ensure that vapor barriers are properly placed and climate-appropriate. He also introduces listeners to advanced tools like hygrothermal modeling, which simulate how assemblies handle heat, air, and moisture over time. Programs such as WUFI or HAM can help contractors predict condensation risks and use the results as a sales tool to demonstrate code compliance and long-term performance.

For homeowners, contractors, and designers alike, this episode sheds light on why vapor control isn’t just a code requirement—it’s essential to preventing moisture damage, mold, and costly failures. As Harris puts it, “We don’t want our insulation getting wet. When that happens, everything else in the assembly is at risk.”

With clarity and technical expertise, Tom's 20th appearance with us offers practical guidance for anyone working with spray foam or managing moisture in buildings. Next month, Wyatt and Harris will continue the conversation with a focus on air barriers.