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Unbuild It | Episode 163: Back to Basics: Vapor Control Layers Explained
It is truly sad that vapor is so misunderstood in buildings. To quote Steve: "It's just not the big bad monster everyone in our industry thinks it is. Vapor is not a liability; it is an asset." In most buildings and most climates, it is drying assemblies by way of vapor diffusion that keeps them safe if--or more accurately--when they get wet. To be fair, the code language requiring interior vapor retarders on the warm-in-winter side of building assemblies for many many years was wrong, confusing, and not based on building science.
If focusing on vapor means how it contributes to drying--not wetting--then we need to know the vapor permeability of EACH AND EVERY layer in any building assembly. So, where do we get that info? For generic materials, go to the BSC Building Materials Property Table (see resource below). For specific building materials, go to manufacturer tech support and demand vapor permeability standardized test results for every material they sell you.
Finally: building professionals and homeowners need to know the moisture content of the air in their buildings and many buildings don't have built-in hygrometers as part of their thermostats. Measuring humidity electronically is not easy so accurate hygrometers can't be had for a Jackson; more like a C-note...
PETE'S RESOURCES:
- Water in Buildings: An Architect's Guide to Moisture and Mold - https://www.wiley.com/en-us/shop/general-introductory-architecture/water-in-buildings-an-architect's-guide-to-moisture-and-mold
- "Vapor Profiles Help Predict Whether a Wall Can Dry" - https://www.greenbuildingadvisor.com/article/vapor-profiles-help-predict-whether-a-wall-can-dry
- ICC Digital Codes R702.7 Vapor retarders - https://codes.iccsafe.org/s/IRC2021P2/part-iii-building-planning-and-construction/IRC2021P2-Pt03-Ch07-SecR702.7 NOTE: NOT required in wall assemblies in Climate Zones 1 - 3.
- BSC Building Material Property Table - https://buildingscience.com/documents/information-sheets/building-materials-property-table
-"Measuring (and Understanding) Humidity" - https://www.greenbuildingadvisor.com/article/measuring-and-understanding-humidity
- NOTE: For building science geeks: "ASTM E96: Explaining Water Vapor Transmission Rate Testing" - https://blog.ansi.org/ansi/explaining-water-vapor-testing-astm-e96/
- BuildingScienceAdvisor - https://bsa.ornl.gov.
Huge thanks to our episode sponsor, Diamond Kote. Learn more at: https://diamondkotesiding.com/
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Unbuild It | Episode 162: Back to Basics: Air Control Layers Explained
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Unbuild It | Episode 159: 3Qs: Climate Zone 7 T&G Ceiling, Aging in Place Design, and Alternative PreFab Foundations
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This Q&A episode tackles listener questions on ZIP sheathing vs. Smart Vapor Retarder for cold-climate ceilings, zero-threshold exterior doors and Universal Design, and innovative wood-based alternatives to concrete foundations. The discussion wraps up with a look at below-grade SIP foundation systems and plenty of valuable resources for deeper exploration.
Unbuild It | Episode 158: The Doctor is In - Retired building scientist and pulmonary physician Nathan Yost
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Steve and Pete welcome Nathan Yost to discuss Multiple Chemical Sensitivity (MCS), exploring how building materials, indoor environments, and evolving medical understanding can impact those living with sensitivities. Nathan shares insights from years of working with clients, while Steve recounts recent projects where MCS dramatically influenced design decisions, specifications, and material choices—including one project in a surprising "dead zone."
