the air space 88 the air space 88 allows pressurized vapor to escape from the venting chamber 74 outward into the atmosphere while the rim 86 prevents external environmental elements from entering the venting chamber 74 the pressurized vapor escapes from the chamber 12 through the passage 53 and through the venting chamber 74 to the air space 88 and out through the exhaust part 90 into the atmosphere the venting chamber 74 is partially filled with a peripheral insulation layer 94 to surround the exterior of the pipe 52 and the cap 54 in the preferred embodiment the insulation layer 94 is filled to a level 95 substantially even with the top of the cap 54 the peripheral insulation layer 94 thermally isolates the pipe 52 and cap 54 from heat sources external to the shell 70 in this manner the layer 94 prevents heat transfer along the body of the pipe 52 through the insulation layer 20 to the inner tank 10 optionally the amount of insulation within the layer 94 may be increased or decreased so long as adequate insulation is provided consistent with the present invention optionally a mesh screen 92 may be secured to the inner surface of the casing 72 at an intermediate point 93 thereabout the mesh screen 92 covers the entire venting chamber 74 to support an upper layer of insulation 98 the upper insulation layer 98 may be filled to any desired thickness but is preferably filled to a level even with the upper edge 78 and flange 80 of the casing 72 the thickness of the.