The problem with crawl spaces ventilated by outdoor air is that they get
too little heat during the winter, resulting in them staying cold during
most of the summer.
When the warm outdoor air enters the crawl space during the summer, it cools down fast and increases the humidity in the crawl space or in a worse case scenario condensates.
In such an environment the risk is big for micro-organisms to develop.
If more damp enters the crawl space, e.g. via water leaks, damp that comes from the ground, walls and others, the risk increases for micro-organisms to develop there.
There is also a risk that the micro-organisms produce smells that might spread to the living areas.
The fact is that almost every third house that has a crawl space has
problems with damp and mould. The problem arises when warm air comes
into the cold space and condensates.
Mould, bacteria and rot fungus affects the indoor environment negatively and can cause damages that are costly to repair, since this kind of damage is usually not covered by traditional home insurance policies.
How does it looking under your floors?
A modern crawl space is today seen as an unsafe construction since
they easily get damp problems. The problem is a result of seasonal changes
and occur primary in unheated sections of the house such as crawl spaces,
garrets and cold storages areas.
The moisture content can periodically be so high, that bacteria, mould and other micro organisms begins to develop there. When the humidity is over 75% they thrive very well.
The yearly average for relative humidity outdoors is ca 80-85%. Therefore it is important to check the humidity in the crawl space so it does not become damp damaged, leading to costly renovations.