Ballast: A material, such as minimum nominal #1-1/2 inch size or #4, or alternatively, #3, #24, #2 or #1—as specified in ASTM D 448—smooth river stone, crushed stone, standard precast concrete pavers or interlocking, beveled, doweled or contoured fit lightweight concrete pavers, which employs its mass and the force of gravity to hold a roof membrane system in place.
Base Flashing: Plies or strips of roof membrane material used to seal a roof at the horizontal-to-vertical intersections, such as at a roof-to-wall juncture. Membrane base flashing covers the edge of the field membrane. (See Flashing and Membrane Base Flashing.)
Baseline: A series of visual and corresponding thermal infrared images taken at a specific time. This baseline may or may not contain thermal infrared images that have exceptions.
Bitumen: (1) A class of amorphous, black or dark-colored, (solid, semi-solid or viscous) cementitious substances, natural or manufactured, composed principally of high-molecular-weight hydrocarbons, soluble in carbon disulfide, and found in asphalts, tars, pitches and asphaltites. (2) A generic term used to denote any material composed principally of bitumen, typically asphalt or coal tar.
Blackbody: A theoretical, perfect emitter and absorber of thermal radiation. It emits radiant energy at each wavelength at the maximum rate possible for its temperature and absorbs all incident radiation.
Blackbody Simulator: A device with an emittance close to 1.00 that can be heated or cooled to a known, stable temperature.
Blister: An enclosed pocket of air, which may be mixed with water or solvent vapor, trapped between impermeable layers of felt or membrane, or between the membrane and substrate.
Boiling Point: Temperature at which the vapor pressure of a liquid is equal to the 3 pressure bearing on its surface.
British Thermal Unit (BTU): The heat energy required to raise the temperature of one pound of water one degree Fahrenheit at a specified temperature (as 39°F).
Building Envelope: The exterior surfaces of a building, including walls and roof that separate conditioned from unconditioned spaces.
Built-up Roof (BUR): A continuous, semi-flexible roof membrane, consisting of multiple plies of saturated felts, coated felts, fabrics or mats assembled in place with alternate layers of bitumen, and surfaced with mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet.
Butyl Rubber: A synthetic elastomer based on isobutylene and a minor amount of isoprene. It can be vulcanized and features low permeability to gases and water vapor. Butyl rubber is manufactured into various sheet goods, blended with other rubber materials, and is often used to make sealant.
