Fibrous glass duct systems have inherently low leakage. While air duct leakage is a relative concept and air-tightness not an absolute requirement, ducts should be sufficiently air-tight to ensure quiet, economical performance. Duct leakage also reduces the delivered volumes of air at diffusers and grilles; this must be compensated for by increasing the total quantity of supply air. For details on test results, refer to NAIMA Fibrous Glass Duct Construction Standard, current edition.