Moisture Flow Drives Water Induced Problems

Originally published by the following source: Applied Building Technology GroupAugust 15, 2018

by Timothy Ahrenholz with contributions by Kirk Grundahl, P.E., Larry Wainright and Sean Shields

The hardest part of building a resilient wall is managing moisture. If water gets inside the wall, all sorts of problems arise, including mold, rust, and structural degradation, depending on the materials used. It would be simple enough if moisture was only coming from one direction: just put a water resistive barrier (WRB) layer facing that direction and be done!  But moisture comes from both sides.  In the winter, warm, moist air on the inside heads towards the cool, dry air outside, and the opposite occurs in the summer.  The most effective solution is to block moisture on one face of the wall, and promote drying through the other face.  There are plenty of resources available at that get into the technical aspects of this approach, collected on the water vapor control topical library page

In the video below, Joe Lstiburek gives a quick overview of moisture transmission. For more information and videos please visit



Here are other related videos:

  1. Fear Building Envelopes No More with This Website & Videos
  2. Thermodynamics Simplified Heat Flows from Warm to Cold

For relevant research reports and general assistance on this topic, please visit these pages:

  1. FSC Wall Calculator
  2. Steel Wall Calculator
  3. Assessment of Moisture Control & Insulation Requirements in Vermont’s Final Draft 2015 Residential Building Energy Standard (RBES) and Handbook
  4. Assessment of Water Vapor Control Methods for Modern Insulated Light-Frame Wall Assemblies
  5. Model Moisture Control Guidelines for Light-Frame Walls: A Building Code Supplement for Builders, Designers, and Building Officials
  6. Water-Resistive Barriers: Assuring Consistent Assembly Water-Penetration Resistance
  7. Installing Windows with Foam Sheathing on a Wood-Frame Wall
  8. The Importance of Integrating Flashing and the Water Resistive Barrier in the Exterior Wall Systems of Residential Buildings
  9. Window Installation Details for Effective Sealing Practice
  10. Window Sill Details for Effective Drainage of Water
  11. Assessment of Water Vapor Control Methods
  12. Water-Resistive Barriers: Assuring Consistent Assembly Water Penetration Resistance
  13. Water, Water Everywhere and Not a Drop to Drink: Practical Building Science to Keep Walls Dry
  14. Window Installation: Pan Flashing and Air-Sealing to Prevent Water Intrusion
  15. Tolerable Moisture Content of Materials - ASHRAE 160