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Valley
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The internal angle formed by the junction of two sloping sides of a roof.
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Valley Rafter
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A rafter that forms the intersection of an internal roof angle. The valley rafter is normally made of double 2-inch-thick members.
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Valve
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A device to stop, start or regulate the flow of liquid or gas through or from piping.
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Vapor
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The gaseous form of any substance.
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Vapor Retarder (Vapor Barrier)
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A membrane which is placed between the insulation and the roof deck to retard water vapor in the building from entering the insulation and condensing into liquid water. Material used to retard the movement of water vapor into walls and prevent condensation in them. Usually considered as having a perm value of less than 1.0. Applied separately over the warm side of exposed walls or as a part of batt or blanket insulation.
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Varnish
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A thickened preparation of drying oil or drying oil and resin suitable for spreading on surfaces to form continuous, transparent coatings, or for mixing with pigments to make enamels.
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Veneer
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Thin sheets of wood made by rotary cutting or slicing of a log.
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Vent
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A pipe or duct which allows flow of air as an inlet or outlet.
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Vent Pipe
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A vertical pipe of relatively small dimensions which protrudes through a roof to provide for the ventilation of gasses.
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Ventilator
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Device installed on the roof for the purpose of ventilating the interior of the building.
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Venting
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The process of installing roof vents in a roof assembly to relieve vapor pressure; The process of water in the insulation course of the roof assembly evaporating and exiting via the roof vents.
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Vent Stack
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A vertical vent pipe installed for the purpose of providing circulation of air to and from any part of a drainage system.
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Vent System
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In plumbing, a system to provide a flow of air to or from a drainage system or to provide circulation of air within such system to protect traps seals from siphonage and back pressure.
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Vermiculite
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An aggregate somewhat similar to perlite that is used as an aggregate in lightweight roof decks and deck fills. It is formed from mica, a hydrous silicate with the ability of expanding on heating to form lightweight material with insulation quality. Used as bulk insulation and also as aggregate in insulating and acoustical plaster and in insulating concrete.
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Viscosity
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The internal frictional resistance offered by a fluid to change of shape or to the relative motion or flow of its parts.
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Visible Light Transmittance
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The percentage of visible light (390 to 770 nanometers) within the solar spectrum that is transmitted through glass.
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Visual Mock-up
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Small scale demonstration of a finished construction product.
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Voltage
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The driving force behind the flow of electricity somewhat like pressure is in a water pipe.
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Voltmeter
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Measures the voltage flowing through a circuit. The circuit must be closed to allow the voltage to flow.
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