The TG-21 protocol recommends using a thin sheath for waterproofing an ion chamber used in the calibration of photon and electron beams. A thickness of 0.5 mm is suggested for a material having a composition and density close to that of water. This work investigates the effect on the calibration of photon beams ranging from Co60 to 25 MV, and electron beams ranging from nominal energies of 7-18 MeV, for changes in the thickness of the waterproofing sheath from 0.5 to 5.5 mm. For photon beams, a maximum change of 1.2% was found for the 25-MV x-ray beam. For electron beams, a maximum change of 0.5% was found for 10-MeV electrons. It is concluded that the thickness of the waterproofing sheath is not a very sensitive variable, assuming the thickness is between 0.5 and 2.0 mm.