Why do I have moisture in my walls?

The inside of a wall filled with entrapped moisture

I just received a question that was too complex to answer quickly, and so interesting that it deserves an entire post. Unfortunately, it represents a situation that occurs far too often.

To summarize, the question came from a reader who opened a small hole in their wall and found condensation on the vapor barrier at the inner surface of the wall. The question is, what could cause this?

Here are some more clues:

  • The moisture was observed during the winter
  • The hole was cut in a south wall
  • The home has 3″ foam board sealed to the exterior wall
  • A 2×4 framed wall was built inside this wall and insulated with Roxul
  • A 6-mil poly vapor barrier was then applied just behind the inner sheet rock
  • They did not measure high moisture towards the outer wall
  • The wall was built during a humid summer

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