Buildings, even homes, are no longer simple structures. They have become complex; consisting of interdependent systems. Consider just the building envelope: The walls are composed of a number of components to form a "wall assembly". This wall assembly (outer finished surfaces, stucco, brick, block, steel studs, insulation, interior wallboard, wallpaper, paint) supports the structure and keeps out the elements, protecting the interior components of the system. The roof assembly is designed to repel water; again the purpose of which is to protect the interior of the structure.
Within the building envelope are other systems. For example, the plumbing and electrical systems. The mechanical system is another - it provides the cooling, heating, dehumidification or humidification of the interior spaces. It also provides fresh air for the occupants to breathe. All of these systems are part of a much larger system we call a "building". If any of these fail, the entire building system is affected. So we must address moisture management from this perspective, that is, view the building as a system.
When we find mold growth within a building we know there is a failure in our building system, perhaps multiple failures. For example, we may find a roof or window improperly flashed allowing water to enter the wall assemblies - a part of the moisture management system has failed. Another example would be a mechanical system that failed to properly dehumidify the makeup air, causing moisture levels to rise, encouraging mold growth. Leaks in a plumbing system. An exhaust system that fails to exhaust moisture from showers or dishwashers or other moisture producing activities as required; again a part of our moisture management system has failed.
Sometimes we build for failure. "Value Engineering" is a common term. In many cases it means "buy cheap now, pay big bucks later". Cutting corners to save money up front affects the building system in the long term. A friend uses the term "self-composting" buildings. My wife calls them S.T.R.D. "Soon To Rot Down". Either one means a building that is literally decomposing due to water intrusion. The building envelope leaks water so badly that the components rot away. Mechanical systems totally inadequate to deal with moisture loads allow moisture levels to rise out of control within the building space and components of the structure, resulting in their failure.
As bad as this scenario is, add uncontrolled mold growth within the building envelope and now your "value engineered building" just became very expensive. It looked good on paper, but it failed to perform in the real world. Really then, moisture management is paramount. Without the ability to properly manage moisture, (in liquid or vapor form) a building will fail.
Does that mean that a building must be water tight? For the answer check out part 3 of "Is Mold the Problem" the page is entitled "All Buildings Leak"