Composting covers are considered to be an affordable yet very effective means of improving moisture control when it comes to the composting process. In many cases, the composting covers are manufactured using synthetic fibers that provide a lightweight but very durable blanket which is capable of shedding up to 100% of the moisture off from the compost windrow while at the same time allowing for gases like oxygen and carbon dioxide to flow freely. The same type of fabric is often used for other purposes like as grain bunker tarps for instance. This permeability can be observed if you take a piece of the fabric used as composting cover and hold it up against the light, where you should see minuscule holes that allow gases to pass through.
With regards to water-shedding ability, this is mostly achieved through gravity and the free-flowing properties of water. When water and moisture, either brought about by rain or dew formation, accumulates in the composting cover, it naturally slides down the sides and to the edge of the composting cover due to gravitational forces. The same principle applies when tarps are used as hay tarps covers. But aside from keeping water off, composting covers also perform a crucial function of keeping the composting pile from drying off completely by reducing moisture loss. This is especially true and a crucial factor to be considered in hot and dry places.
There is also the issue of composting odours, which most of the time can be really unpleasant and often smells as odorous as the one you smell in pig shelters. There are but a few researches that studies and analyses just how composting covers can help with odour control. However, there are many intelligent guesses as to how this works. For one, it is believed that the odour is incapable of escaping through the composting cover because they are trapped and held in the fibers of the composting covers. Another plausible explanation is that the odours may be present, but they don’t cause too much of a bother because they are not being blown by the wind since the composting covers have wind-resistant properties. Lastly, composting covers help to protect the windrows from excessive moisture, which sometimes lead to anaerobic activities which can cause bad odours to develop.