The following glossary contains terms relevant to discussions of the thermal performance of
building envelopes. Many of the definitions are based on the glossaries of the documents
referenced at the end of this section.
Adhesion - the clinging or sticking together of two surfaces. The state in which two surfaces are
held together by forces at the interface.
Adhesive - a substance capable of holding materials together by surface attachment.
Adhesive Failure - type of failure characterized by pulling the adhesive or sealant loose from the
Aging - the progressive change in the chemical and physical properties of a sealant or adhesive.
Air Barrier (sometimes referred to as Air Retarder) - an assembly or building element that provides
resistance to through-flow of air from inside to outside or vice-versa.
Air Infiltration - air leakage into a building. Conversely, air exfiltration is air leakage out of a
Air Leakage - the passage of uncontrolled air through cracks or openings in the building envelope
or its components because of air pressure differences.
Alligatoring - cracking of a surface into segments so that it resembles the hide of an alligator.
Ambient Temperature - temperature of the air surrounding the object under construction.
As-built - pertaining to the as-constructed state of a finished product relating to size, shape,
materials, and finish regardless of drawings or specifications.
Asphalt - naturally occurring mineral pitch or bitumen.
Back-up - a compressible material used at the base of a joint opening to provide the proper shape
factor in a sealant. This material can also act as a bond-breaker.
Bead - a sealant or compound after application in a joint irrespective of the method of application,
such as caulking bead, glazing bead, and so on.
Bedding Compounds - any material into which another material such as a plate of glass or a panel,
may be embedded for close fit.
Bond-Breaker - thin layer of material such as tape used to prevent the sealant from bonding to the
bottom of the joint.
Bond Durability. a test cycle in ASTM C-920 for measuring the bond strength after repeated
weather and extension cycling.