LAITANCE: A layer of weak non-durable material containing cement and fines, brought by bleeding water to the top of over-wet concrete. Laitance may be detected by scraping the concrete surface with a putty knife; if a quantity of loose powdery material is observed or easily removed, excessive laitance may be considered to be present.
LATEX: A COLLOIDAL DISPERSION of a polymer or elastomer in water that coalesces into a film upon evaporation of the water. (See EMULSION; COLLOIDAL DISPERSION)
LCA: See LIFE-CYCLE ASSESSMENT.
LEADER HEAD: See CONDUCTOR HEAD.
LEADERSHIP IN ENERGY AND ENVIRONMENTAL DESIGN (LEED): Commonly referred to as LEED, it is a prominent green building rating system, developed by the United States Green Building Council (USGBC), that provides a suite of standards for environmentally sustainable construction. Energy efficiency, through proper application of insulation and air sealing technologies, plays an important role in the LEED evaluation process.
LEED: See LEADERSHIP IN ENERGY AND ENVIRONMENTAL DESIGN.
LIFE CYCLE ANALYSIS: See LIFE-CYCLE ASSESSMENT.
LIFE-CYCLE ASSESSMENT (LIFE-CYCLE ANALYSIS, LCA): An accounting and evaluation of the environmental aspects and potential impacts of materials, products, assemblies, or buildings throughout their life, from raw material acquisition through manufacturing, construction, use, operation, demolition, and disposal.
LIFE-CYCLE INVENTORY (LIFE-CYCLE INVENTORY ANALYSIS, LCI, LCIA): The identification and quantification of energy, resource usage, and environmental emissions for a particular product, process, or activity.LIFT: The sprayed polyurethane foam resulting from tying together the perimeter of adjacent passes of foam in a specific area, as shown in Figure A below. A lift is defined by its thickness. Multiple lifts over the same area may be needed to achieve the final foam thickness. For example, a one inch lift of foam can be installed over a 20 x 20 ft area, and then a second lift of foam 1.5 inch thick can be installed to the same area to create a final installed thickness of 2.5 inches. For small areas, such as between framing members, a lift of foam is synonymous with a pass of foam per Figure B below.
LIFT LINE: See KNIT LINE.
LIVE LOAD: The weight on a roof of temporary, mobile loads, such as foot traffic, roofing equipment, water, snow, and ice.
LOW-DENSITY SPF (OPEN-CELL SPF, ocSPF): A type of SPF expanded with reactive blowing agents to yield a semi-rigid cellular structure and a density between 8–22 kg/m3 (0.4 and 1.4 lb/ft3).
LOW PRESSURE SPF: See SPF, LOW PRESSURE.
LOW-TEMPERATURE FLEXIBILITY: The ability of a membrane or another material to remain flexible (resist cracking when flexed), after the membrane or material has been cooled to a low temperature.
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