It’s no secret among the building science community, but everyone should know.
Prior to the 2012 International Residential Code, International Building Code, and the International Energy Conservation Code, there was condensation during cool months on the inner face of OSB sheathing nailed to lumber framing to form the structural wall.
This happens because that surface is cooler than the air inside the structure. Google dew point calculator and have some fun seeing how that works.
However, there was no air barrier requirement, so, in most cases, the wetting that occurred was dried out by significant air movement through the wall assembly.
CONTROLLING MOISTURE FLOW:
ACTION AT THE SURFACES
Above and above right, illustration of condensation in wall detail.
The 2012 codes changed all that by requiring thicker batt insulation (thus making the OSB colder) and air barriers (that restrict drying) for energy efficiency. (In 2005, the federal government issued a report indicating that in a middle-latitude city like St. Louis, an air barrier would save 40% on natural gas and 25% on electricity. “Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use,” National Institute of Standards & Technology.)
In an article for Walls & Ceilings, CertainTeed’s Luca Hamilton explained that “once these walls get wet, they are staying wet.”
Here is an article from the OSB industry on the subject of external insulation: “Wood Structural Panel & Foam Insulation Systems.” https://www.apawood.org/Data/Sites/1/documents/technicalresearch/wood-structural-panel-and-foam-insulation-systems.pdf
Here is one from the foam insulation industry: “Assessment of Water Vapor Control Methods for Modern Insulated Light-Frame Wall Assemblies.” www.appliedbuildingtech.com/rr/1410-03
Paul Grahovac, LEED AP
Air Barrier Association of America, National Concrete Masonry Association’s Air Barrier Task Force, ASTM Committee on Performance of Buildings, RCI Industry Advisory Council, and Building Enclosure Integration Committee of the Building Enclosure Technology and Environmental Council (BETEC) of the National Institute of Building Sciences.