Permeability refers to the degree to which pore spaces (voids that can be filled by a fluid) in a medium connect to each other, promoting the movement of fluid through that material. Permeability and porosity (the number of void spaces in a medium) 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 assessing how much a material resists the flow of fluids—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 has high permeability.
The porosity and permeability of rocks is important in determining which rocks will make a good reservoir. 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 natural 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 to ensure 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.