Measurements of relative humidity and temperature were made on the internal environment of a packing case used for shipping kimono objects between Japan and the United States. The moisture content of the silk fibers was calculated to increase by 1% on the outbound journey while decreasing by 1.5% on the return flight based on measurements of temperature and relative humidity inside the packing case. Actual ratios of relative humidity variation during the two flights were 0.9 and 1.5. If the container were hermetically sealed, this ratio should, theoretically, have worked out to 0.3. The lack of agreement between theory and practice indicates that the container was not entirely airtight. Moreover, the airtightness was further compromised on the return flight, causing even greater change in the moisture content.