When Sunshine Drives Moisture into Walls
Energy Nerd: How inward solar vapor drive happens, and avoiding the problems it creates.
Builders first started worrying about vapor diffusion in 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. The article, “Preventing Condensation in Insulated Structures,” states, “A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.” Rogers assumed that the main source of condensation on sheathing was the result of the diffusion of water vapor through the wall and ceiling plaster in wintertime.
Rogers’ recommendation to use interior vapor barriers, which was eventually incorporated into most model building codes, became established dogma for over 40 years. But eventually, building scientists discovered that interior vapor barriers were causing more problems than they were solving. Interior vapor barriers are rarely necessary, since wintertime vapor diffusion doesn’t typically lead to problems in walls or ceilings. What Rogers didn’t know back in 1938 is that the main way that sheathing gets damp in the winter is actually via air leakage, not vapor diffusion. But a different phenomenon — summertime vapor diffusion — has become the far more serious matter.