Air barrier materials – Building materials that are designed and constructed to provide the principal plane of airtightness through an environmental separator, which has an air permeance rate no greater than 0.02 L/(s•m2) at a pressure difference of 75 Pa when tested in accordance with ASTM E 2178.
Air barrier accessories – Products designated to maintain air tightness between air barrier materials, assemblies and components, to fasten them to the structure of the building, or both (e.g., sealants, tapes, backer rods, transition membranes, nails/washers, ties, clips, staples, strapping, primers) and which has an air permeance rate no greater than 0.02 L/(s•m2) at a pressure difference of 75 Pa when tested in accordance with ASTM E 2178.
Air barrier components – Pre-manufactured elements such as windows, doors and service elements that are installed in the environmental separator and sealed by air barrier accessories and which have an air leakage rate no greater than 0.20 L/(s•m2) at a pressure difference of 75 Pa when tested in accordance with ASTM E 2357.
Air barrier assemblies – Combinations of air barrier materials and air barrier accessories that are designated and designed within the environmental separator to act as a continuous barrier to the movement of air through the environmental separator and which has an air leakage rate no greater than 0.20 L/(s•m2) at a pressure difference of 75 Pa when tested in accordance with ASTM E 2357.
This is because water fits through narrower pores than the other main constituents of air, oxygen(O2) and nitrogen[citation needed].