V
| vapour control barrier | a vapour control layer is defined as a material that substantially reduces the water vapour transfer through a building element into which it is incorporated i.e. polythene sheet materials or foil backed plasterboard. vapour control layers are sometimes required on the warm side of the insulation, to reduce the possible risk of interstitial condensation within the construction element.. |
| vermiculite | a hydrous silicate of magnesium or iron which is expanded by heating to produce a lightweight fire resistant insulation. |
| volatile organic compound (voc) | a carbon-containing compound that vaporises at a relatively low temperature, generally below 40°c. vocs can contaminate water, and in the atmosphere can react with other gases in the presence of sunlight to form ozone or other photochemical oxidants. they can generate ozone, a major factor in the formation of photochemical smog. |