Often more than one of type of fibrous glass insulation treatment may be specified in a single HVAC duct system. For example, in a variable air volume system, sheet metal ducts with fibrous glass duct liner may be used on the high pressure side if design static pressure exceeds the 2 in. w.g. (500 Pa) limits of the fibrous glass duct system; the fibrous glass duct system may be used on the low pressure side; and insulated flexible ducts may be used as run-outs from trunk ducts to diffusers.