The moisture content of building structures is almost independent of air humidity

The moisture content of building structures is almost independent of air humidity because the main sources of moisture in walls, floors, and ceilings are construction defects, inadequate waterproofing, leaks, and capillary rise, rather than indoor air humidity.

For example, two neighboring apartments in the same building may have identical air temperature and humidity conditions, yet one has dry walls while the other is covered in mold.

This is because air humidity is evenly distributed throughout the space, whereas the condition of the structures depends on their materials, construction quality, thermal bridges, and local sources of moisture. If one apartment has thermal bridges, condensation will occur there, regardless of the overall humidity in the building.

Hidden leaks or damp soil beneath the building can also cause moisture to rise through the walls without affecting neighboring units. This is why addressing moisture and mold issues should start with diagnosing the building structures rather than simply trying to reduce air humidity.