U-Value (U-Factor)
Heat transmission in unit time through unit area of a material or construction and the
boundary air films, induced by unit temperature difference between the environments
on each side. Units of U and Btu/h•ft2. A lower U-value, means less heat flow that
occurs through an assembly from the warm side to the cooler side.
UBC
Uniform Building Code.
Underlayment
A secondary waterproofing sheet material installed between the substrate and the roof
panels, usually used in hydrokinetic roof construction. Some types may be selfadhering.
Uplift
1) See “Wind Uplift” 2) Upward force at a column base caused by applied building
loads or building geometry.
Urban Heat Island
A built environment wherein the large proportion of dark surfaces such as asphalt
paving and dark roofs absorb solar radiation and radiate the heat back into the
atmosphere causing higher ambient temperatures and higher pollution levels.