Permeability refers to the degree to which pore spaces - small pockets of air and empty space - in some material connect to each other, promoting the movement of fluid through that material. Permeability and porosity - the number of small pockets of air in a material - are closely connected, however a material may be highly porous but have few channels connecting these pores, leading to a low permeability. In a permeable material, holes, pores, and cracks are lined so that fluid is able to flow through the material. The permeability of a material is determined by seeing how much resistance to the flow of fluid exists, if it takes a lot of pressure to squeeze fluid through the material it has low permeability. Conversely if the fluid travels through easily it is high permeability.
The porosity and permeability of rocks is important in determining what rocks will make good reservoir rocks. A rock that is both porous and permeable would make a good reservoir rock as it allows oil and gas to move up through the pores in the rock closer to the surface where it can be extracted. The greater the permeability, the easier it is to extract oil from the rock. Rocks such as sandstone have a very high porosity and permeability and make a productive oil or gas well. Looking at the permeability of rocks is one way that geologists can determine where a good location for an oil well is.
Understanding the permeability of different building materials is vital in ensuring that there is no water or vapour intrusion into the building envelope, which could promote rotting of materials or mould growth. In general, permeability is an important consideration for interior wall finishes. In hot and humid climates the permeability of the interior finish of a wall should be much higher than that of the exterior finish of the same wall. This difference in permeability will allow moisture that enters the wall to migrate to the inside of the building where the moisture will be removed by the air conditioning system. The reverse is true for cold climates; the exterior finish should have a higher permeability than the interior finish.