The First Way
There are four main ways that moisture enters a common crawl space. The first and most obvious way is through the open dirt. As the ground outside the home is saturate with rain, the moisture level in a crawl space raises rapidly. The warmer the air, the more moisture it can hold. This means the most devastating months are the warm ones. This excess moisture condensates to cooler surfaces like water lines and the foundation walls. It also settles on the ground when the temperature falls, gets absorbed by the wood structure and any insulation that is installed in the crawl space. The building code in nearly every area of the country requires a vapor retarder with a perm rating of less that 1 to be installed on the floor of the crawl space. There are no requirements to how it should be installed, just that it must be. This leaves a wide gap in the final performance of a vapor retarder. If anyone wants to find recommendations on how to install; there are many opinions on the internet. It is not really a mystery if you know what it is suppose to do. For the sake of conversation, let’s say we go down to the local hardware and buy some plastic and install it in the crawl space according to the local building code. The problem of moisture entering the crawl space from the ground is fixed! Right? Well not entirely, any gaps in the plastic (at the seams, around supports and where it meets the wall) will still allow moisture to escape. It will continue to allow this to happen until the air, the wood structure and the concrete foundation are saturated. To make it more complicated, when the temperature falls in the evening the excess moisture leaves the air and settles like dew on top of the plastic only to evaporate again when the warmer air returns. Then the next time your plumber has to go down there to fix a frozen water line the plastic will probably not be in the same place as it was when you installed it causing your efforts to be wasted. The other common issue is when the crawl space floods, water gets on top of the plastic and can not be absorbed by the earth. This of course leaves a breading ground for bacteria and mosquitoes while introducing a new long lasting supply of moisture.