Sustainable Building Glossary

Vapour permeability: Vapour permeability is related to the pore structure of a material or of a set of materials in the case of a wall, floor or roof build up and the size and weight of the gaseous water molecule. The permeability of each material is dependent on the vapour resistivity of the materials and can be measured as the vapour resistance to moisture movement or given as the s-value.

Vapour resistance: The vapour resistance as a construction property is measured as G which is vapour resistivity r x thickness [m], measured as GNs/kg or MNs/g. Thus a paint may have a high r value, but because it is only microns thick on a wall, it may have a low G value. Conversely concrete blocks may have a relatively low r value but a high G value because of being 100 mm wide. According to European Standards and on the continent the vapour resistance is measured as s-value, which is vapour resistivity μ x thickness [m].¹⁷

Vapour resistivity: The vapour resistivity is usually measured as r in¹⁸ GNs kgm^-1 or¹⁹MNs ^-1. r is a material quality, not dependant on size, thickness, shape etc. Many bulk building materials, such as lightweight concrete, bricks, mineral plasters or plasterboard are around 50 MNs ^-1. Most solid timber is around 200 MNs/gm. Plastic membranes can be up to 10,000 MNs ^-1. Still air is 5 MNs ^-1. According to European Standards and on the continent the vapour resistivity is given as μ²⁰(relative to the resistivity of still air).

Ventilation: Ventilation is the intentional movement of air from the outside of a building to the inside (as opposed to infiltration, which is unplanned movement of air). When people or animals are present in buildings, ventilation air is necessary to provide acceptable indoor air quality and to protect building fabric from high levels of moisture. In a common building there is natural ventilation that occurs when air in a space is changed with outdoor air without the use of mechanical systems. Natural ventilation is most often assured through operable windows but it can also be achieved through temperature and pressure differences between spaces. However, to achieve air tightness in low energy buildings according to the relevant standards and/or to reduce energy use in low energy houses such as Passivhaus the ventilation is/has to be mechanically performed with suitable control systems.