From our experience, we know that when a glass of cold water (or another refrigerated item) is taken outside when it is warm, or placed on the table in the house at room temperature, condensation forms on the outside of the glass. As we were taught in science class, this is not because water is seeping through the walls of the glass, or because of some leak. It is the result of the water vapor in the air around the glass changing into liquid form. The water vapor changes state when it cools as a result of being in contact with the surface of the cold glass. This same simple phenomenon can occur in structures and can create moisture damage and promote mold and fungal growth.
Ideally, the building envelope of a structure creates an insulated barrier between the outdoor and indoor environments. Consequently, there is a gradual temperature gradient from the outside surface of a wall to the interior surface of the wall (or window, etc) when there is a temperature difference between the outdoor and indoor environments. Each surface then is closer to thermal equilibrium with the corresponding environment with which it is in contact. However, in some situations, the insulating properties have inadequacies that allow the interior surfaces' temperature to trend toward equivalence with the exterior environment. If the interior surface temperature drops below the dew point for the given relative humidity level and atmospheric pressure, water will leave the air and condense on the wall surface.
In the image above, moisture has condensed on the interior surface of the window and is dripping down onto the window sill. Notice the fungal growth that is occurring on the sill and around the edge of the window.
In this photograph, moisture along with its dripping pattern can be seen on the wall to the right. Although the presence of moisture here is obvious and could be detected both visually and manually, moisture meter testing and infrared thermography can be used to investigate the area of influence as well as the pattern of exposure. (So as to differentiate between condensation and some kind of leak, etc.) Additionally, areas, where the insulation is inadequate, can be identified.
The infrared image above shows an area of low temperature located on an exterior wall. Below is shown the same area and the moisture damage and fungal growth that have resulted subsequent to the forming of condensation on the wall.
Indoor relative humidity can also be assessed with a hygrometer or psychrometer as the higher the relative humidity, the more readily condensation can form.