During the high humidity season, the temps on the main/upper levels of our homes reflect both warmer outside temp and normal living activities (our bodies, use of various electrical appliances, computers, stoves, etc.) and generally ranges in the mid 70’s to mid 80’s (depending on weather and A/C use). On these upper levels, the high seasonal humidity besides condensation on uninsulated toilet tanks or cold water pipes, only create discomforts, unless mitigated by the use of A/C.
Basement/crawl space units are affected by the surrounding ground temperatures, which at an average basement depth (approx. 6-8 ft.) In the northern 2/3 of the US, generally averages from 52F to 72F (13C to 22C) with the lower values occurring late winter/early spring and the higher value reached during late summer (ground temp for southern Canada are slightly lower). This ground temperature effect keeps the basement/crawl space areas, especially the wall and floor surfaces, much cooler, than the rest of our homes. This can, during the high humidity season, especially at the onset of the high humidity season May/June when ground temperatures are still cool, result in the air immediately adjacent to these cooler surfaces, being cooled to its dew point temp. When this occurs, condensation results at these points as often evident on cold basement floors/walls, cold water pipes, A/C ducts, etc. and especially where there is poor air circulation, trapping the cooled air and cooling it even more, as between contents and walls, underneath items, in corners, under stairs, etc. Condensation can be so slight that it may not even be visible at first. With vapor pressure being approximately the same throughout the home, any humidity that condenses out of the air on colder basement/crawl space surfaces is continuously replaced, thus a continuous condensation cycle occurs, unless these cooler basement surface temperatures are increased to above the dew point temp of the air or the humidity is decreased sufficiently to increase the dew point temp of the air above these surface temp.
As basement/crawl space materials, such as concrete, cinder block, drywall, framing, etc. and many basement/crawl space contents are porous, the condensation moisture is absorbed into these materials even to the point that they become saturated by moisture. As a matter of interest, concrete that becomes saturated in this manner can take several months or more to dry out completely.
Also, being located in ground, basement/crawl space can be a source of moisture/humidity themselves, IE dirt floors in crawlspaces, foundation cracks (even minute), porous materials, including cinder blocks and even concrete (unless well sealed from the outside). Cinder block/Concrete can permit moisture to pass through from the outside. Concrete basement/crawl space walls/floors become wet, setting up a wicking effect causing a continuous flow of moisture from the water source to the wall/floor surface. These surfaces have to be dried (or sealed on outside) to stop this effect. As well, other basement/crawl space moisture/humidity sources, such as open sump pump pits, poorly sealed dryer ducts, minute leaks in drain or water pipes, etc. can contribute humidity and are often overlooked.
All these potential moisture/humidity sources contribute humidity to the basement/crawl space air and can serve to keep basement/crawl space RH at elevated levels, with a potential for more condensation, as well as contributing humidity back to the entire home.
The primary problem with high humidity/condensation/moisture conditions in a basement/crawl space, besides creating an uncomfortable, unsightly and hard to keep clean environment, is that it fosters mold growth. Mold spores are forever present in the air, both outside and in our homes and these are at elevated levels during seasonal hot/humid weather. Mold growth will spread rapidly if a source of moisture is present. There are hundreds of different types of mold and mold growth may sometimes be difficult to visibly detect, however, is usually evident by the musty odors (gas produced by various molds) we smell in the basement/crawl space. Mold, besides being a potential health hazard, can also create very rapid structural and aesthetic damage to basement/crawl space framing, flooring, drywall and other finishing, as well as contents.
A high humidity environment is also conducive to growth of other biological bacterial pollutants, as well creating an environment that fosters the growth of many insect such as dust mites, termites, etc.