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VALLEY  
The internal angle formed by the intersection of two sloping roof planes.
VAPOR BARRIER  
(1) A sheet material designed to restrict the passage of water vapor through a roof or wall. In the roofing industry, a vapor retarder should have a perm rating of 0.5 or less; (2) Roof component designed to obstruct water vapor flow through a roof or wall. See VAPOR RETARDER.
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 at any given temperature exerted by a vapor that is in equilibrium with its liquid or solid form.
VAPOR-PRESSURE GRADIENT  
A graph, analogous to a temperature gradient, indicating the changes in water vapor pressure at various cross-sectional planes through a roof or wall system.
VAPOR RETARDER  
(1) A sheet material designed to restrict the passage of water vapor through a roof or wall. In the roofing industry, a vapor retarder should have a perm rating of 0.5 or less; (2) Roof component designed to obstruct water vapor flow through a roof or wall; (3) Any material used to prevent the passage of water vapor; (4) Material installed to impede or restrict the passage of water vapor through a roof assembly. See VAPOR BARRIER.
VENEER  
(1) A single wythe of masonry for facing purposes, may not be structurally connected; (2) Any of the thing layers of wood glued together to form plywood.
VENT  
(1) An opening designed to convey air, heat, water vapor or other gas from inside a building or a building component to the atmosphere, thereby relieving vapor pressure; (2) Any outlet for air that protrudes through the roof deck such as a pipe or stack; (3) Any device installed on the roof, gable or soffit for the purpose of ventilating the underside of the deck.
VENTILATION SHORT CIRCUIT  
(As it related 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 shor 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.
VENT SLEEVE  
See COLLAR.
VERMICULITE  
An aggregate used in lightweight insulating concrete, formed by the heating and consequent expansion of a micaceous mineral or mica rock.
VISCOELASTIC  
Characterized by changing mechanical behavior, from nearly elastic at low temperature to plastic, like a viscous fluid, at high temperature.
VISCOSITY  
(1) Index of a fluid's internal resistance to flow, measured in centistokes (cSt) for bitumens. (Water has a viscosity of roughly 1 cSt; light cooking oil 100 cSt.); (2) The resistance of a material to flow under stress. For bitumen, measured in centipose. See VISCOUS.
VISCOUS  
Resistant to flow under stress. Viscous materials are usually cohesive, and have a sticky consistency.
VOID  
An open space or break in consistency.
VOLATILITY  
The tendency of a solid or liquid material to pass into the vapor state at a given temperature.
VULCANIZATION  
Any of various processes by which natural or synthetic rubber or other polymeric materials may be cured or otherwise treated (i.e. exposed to chemicals, heat or pressure) to render them non thermoplastic, and which improve their elastic and physical properties.
VULCANIZE  
The change from a thermoplastic to a thermoset; a physiochemical reaction resulting in the cross linking of a polymer. Synthetic rubber materials are an example of a material which can be vulcanized.
   
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