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Valley
the internal angle formed by the intersection of two sloping roof planes.

Vapor migration
the movement of water vapor from a region of high vapor pressure to a region of lower vapor pressure.

Vapor pressure
the pressure exerted by a vapor from a region of high vapor pressure to a region of lower vapor pressure.

Vapor retarder
a layer(s) of material or a laminate used to appreciably reduce the flow of water vapor into a roof assembly.

Veneer
(1) a single wythe of masonry for facing purposes that may not be structurally connected; (2) any of the thin layers of wood glued together to form plywood.

Vent
an opening designed to convey air, heat, water vapor or gas from inside a building or a building component to the atmosphere.

Ventilation Short Circuit
(as it relates to a passive ventilation system where the system is designed for air flow between intake and exhaust vents) a ventilation short circuit occurs when air is introduced into the ventilation system from an area higher than the intake vent thereby minimizing or defeating the effectiveness of the intake vent. One example can be a gable vent in a soffit-to-ridge ventilation system. Air intake from the gable vent can short circuit the stack-effect draw of air through the soffit vents, and interrupt the thorough venting of the roof cavity.

Ventilator
an accessory that is designed to allow for the passage of air.

Verge of popcorn texture
in SPF roofing, the verge of popcorn surface texture is the roughest texture suitable for receiving the protective coating on a sprayed polyurethane foam roof. The surface is acceptable for receiving a protective coating only because of the relatively cured valleys. However, the surface is considered undesirable because of the additional amount of coating material required to protect the surface properly.

Vermiculite
an aggregate used in lightweight insulating concrete, formed by heating and expanding of a micaceous material.

Viscosity
the resistance of a material to flow under stress. For bitumen, measured in centipoise. (see Viscous.)

Viscous
resistant to flow under stress.

Void
an open space or break in consistency.

Volatile organic compounds (VOC)
means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participate in atmospheric photchemcial reactions.

Volatile
a relative term expressing the tendency to form vapor.

Vulcanization
an irreversible process during which a rubber compound, through a change in its chemical structure (for example, cross-linking), becomes less plastic and more resistant to swelling by organic liquids and elastic properties are conferred, improved, or extended over a greater range of temperature.