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Damper: A metal flap that acts as a valve for air flow in a duct, a fuel-burning appliance, or a fireplace.
Damp-proof coating: A thin asphaltic coating applied to the outside of a building foundation wall in order to limit the penetration of moisture through the wall.
Daylighting: Lighting the interior of a building with direct or indirect light from the sun.
DC: See Direct current.
Dead load: The weight of building components and other loads that don't change over time.
Decibel: A measure of the intensity of sound.
Deciduous tree: A tree that drops all its leaves in winter.
Deflection: The amount by which a given point on a structural member moves when the member is placed under load.
Dehumidifier: A machine that removes moisture from the air, usually by condensing it on metal coils that are cooled by a compression cycle.
Dewatering: Lowering the level of water in the soil of a building site in order to keep the excavation dry during foundation work.
Dew point: The temperature at which water will begin to con dense from a given mass of air.
Diffraction: The bending of light or sound waves by their being passed over sharp edges, especially a series of edges whose spacing is similar to the wavelength of the light or sound.
Direct current: A flow of electricity that is constant in polarity and intensity.
Direct gain system: Heating a building by admitting sunlight into the inhabited space.
Dome: A structural form consisting of an arch rotated about its vertical axis. A dome is often a hemisphere or some other portion of a sphere.
Double-envelope building: A building with two independent, complete enclosures, one inside the other, with an air space between.
Double-hung window: A window with two sashes, both of which slide on vertical tracks.
Double-skin facade: A building wall with two layers separated by an air space.
Downspout: A vertical pipe that conducts water runoff from a roof to the ground.
Drainage composite: A thick, highly porous material placed against the outside of a foundation wall so that ground water approaching the wall falls down through the material to drainage pipes at the base of the wall instead of reaching the outside surface of the wall.
Drip: Any building feature that causes water to fall clear of the building at that point rather than run along the surface.
Dual duct system: An air conditioning system in which parallel ducts carry warm air and cool air. In each room of the building, thermostatically controlled dampers regulate the relative amounts of air from each of the two ducts that are mixed to achieve the desired temperature.
Duct: A round or rectangular tube through which air is circulated.
Eave: The lowest edge of a sloping roof.
Echo: A reflection of sound that occurs long enough after the original sound to sound like a separate sound.
Effluent: The supernatant of decomposed sewage.
Elastic modulus: The ratio of stress to deformation in a given material; a measure of the stiffness of a material.
Emittance: A measure of the ability of a material to radiate heat to another body.
Equinox: A position in the earth’s orbit where the north and south poles are equidistant from the sun. There are two equinoxes each year, occurring on or about March 21 and September 21. Day and night are of equal length at all latitudes on these days.
Evaporative cooler: A device in which air is cooled by passing it through a welled pad.
Evaporative cooling: Cooling air by causing it to absorb moisture. The latent heat of vaporization of the water is furnished by the air, which reduces the temperature of the air.
Exit, horizontal: A passage through a self-closing, fire-resistant door to an area of refuge on the same level of a building.
Expansion joint: A linear device that allows a material to expand without damaging itself.
Facade: The principle face of a building.
Factor of safety: The ratio of the stress for which a structure is designed to the ultimate stress of the material of the structure.
Fan room: A space in a building in which fans and other air-conditioning equipment are located.
Fire cut: A beveled cut on the end of a heavy timber beam or girder that allows the member to fall harmlessly out of the wall if the member should burn through during a fire, rather than topple the wall.
Flashing: A continuous strip of metal, plastic, or composite that acts as a barrier to the passage of water.
Flat arch: A masonry arch whose top and bottom edges are straight and level.
Flat-plate collector: A device for gathering heat from the sun by allowing sunlight to impinge directly upon a flat surface from which the heat is removed by circulating air or liquid.
Floating foundation: See Foundation, floating. Fluid-filled structure: A surface that is supported by internal pressure of air or water.
Fluorescent: A device in which an electrical discharge activates a phosphor to generate light.
Flue: A tube for evacuating combustion gases.
Flux: A flow of energy.
Flying buttress: See Buttress, flying.
Formwork: A temporary structure of wood, metal, and /or plastic that gives shape and support to a structure of masonry or concrete until that structure becomes self-supporting.
Foundation, floating: A foundation in which the weight of the soil excavated for the building’s basement(s) is equal to the weight of the building.
Foundation, mat: A single spread footing that is as large in horizontal extent as the building that it supports.
Funicular: Having the shape that would be taken by a flexible rope or chain that supports a given set of weights, or having the inversion of that shape.
Fusible link: A connector made of a special alloy with a very low melting point, used for the automatic control of fire safety devices.