Insertion loss tests were conducted for three different thicknesses of fiberglass pipe insulation and five different jacketings using a proposed test standard. Results indicate that the procedure is a feasible method for testing pipe lagging. The insertion loss increased with increasing thickness of insulation. A significant increase resulted at all test frequencies for the insulation without jacketing and with the aluminum jacketing. The increases at the higher frequencies due to increased insulation thickness diminished with the use of heavier jacketing. When the insulation thickness was held constant at 2 in., each doubling of the weight of the jacketing resulted in increases in insertion loss at all frequencies. For each thicker insulation, these increases due to heavier jacketings were restricted to fewer test frequencies. There was good agreement between the 100‐Hz and 1/3‐oct test data for the 2‐ and 3‐in. pipe insulation with a slight preference for the 100‐Hz data measured at the lower end of each 1/3 oct.