This chart explains how moisture behaves in the air as temperatures rise and fall—rainstorms, flooding, and glaciers in wall cavities are distinct possibilities.
Vapor barriers cause a lot more harm than good in many parts of North America. Understanding vapor barriers begins with knowing what it means to be one
Thin foil can block massive amounts of heat transfer, but it needs a massive temperature difference to be effective. So, insulation sort of makes radiant barriers irrelevant.