Glossary of Green Building: V

Vapour barrier
Vapour check
Vapour control layer (VCL)
Visible transmittance (Tvis)
Volatile organic compounds (VOCs)
Vapour barrier
Vapour permeable membrane
Vapour pressure

 

Vapour barrier

 

A vapour control layer material that retards the transfer of the warm, potentially moisture-laden air from an internal space, penetrating the external fabric and coming into contact with cooler elements within the construction. The different membranes used as VCLs are distinguished by their abilities to retard moisture under different conditions defined by BS5350 as a series of ‘Classes’. The term ‘vapour barrier’ (rather than ‘vapour check’) usually refers to a membrane providing the most resistance to water vapour.

Vapour check

A vapour control layer material that retards the transfer of the warm, potentially moisture-laden air from an internal space, penetrating the external fabric and coming into contact with cooler elements within the construction. The different membranes used as VCLs are distinguished by their abilities to retard moisture under different conditions defined by BS5350 as a series of ‘Classes’. The term ‘vapour check’ (rather than ‘vapour barrier’) usually refers to a membrane providing the least resistance to water vapour.

Vapour control layer (VCL)

A feature of lightweight construction, a VCL prevents moisture vapour from penetrating and condensing amidst cooler, water-vulnerable, components such as insulation and timber structure. Usually in the form of a water-vapour proof sheet or membrane, the VCL is located on the ‘warm’ or room side of the construction element.

Visible transmittance (Tvis)

The fraction of visible light transmitted through a window.

Volatile organic compounds (VOCs)

Organic chemicals that easily vaporize at room temperature. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects.

Vapour barrier

A material, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor elements.

Vapour permeable membrane

A membrane, usually manufactured from single ply or laminated spun bound polypropylene or polyethylene, that prevents liquid water from entering the building fabric whilst enabling water vapour to diffuse. ( see also: Airtightness and intelligent membranes)

Vapour pressure

The more moisture present in the air the greater the contribution of water vapour to the total pressure of the air

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