Adsorption

adsorption is a surface phenomenon where a something, water is our main concern, is adheres to a surface such as drywall as a function of temperature, humidity, and moisture content. It differs from absorption in that it is a surface layering of the adsorbate, and the adsorbate is not pulled into the volume of the material adsorbing it. This can occur at temperatures below above dew point and can result in wetting of the material.  Adsorption properties of materials such as drywall are defined by isotherms.  Good deep dive discussion here