A showcase provides an isolated space for the exhibition and preservation of cultural relics. The tightness of a showcase affects the control of the micro-environment within the showcase. Good tightness can reduce permeation of the external pollutants, buffer humidity fluctuations, improve the effects of passive conditioning agents such as humidity controls and absorbent materials, and increase the effectiveness of purification within one power cycle. Therefore, tightness is one of the most important factors to evaluate for a showcase. To measure the exchange rate between air inside and outside of a showcase, a method to detect tightness of showcase using tracer gases was developed. Carbon dioxide and ethylene as tracer gases in terms of convenience, safety, and data quality were evaluated. The results showed that the values of the two tracer gases had a certain correlation and both tracer gases can be used to detect the tightness of a showcase. Detection with carbon dioxide was more convenient, because one can use a portable instrument that records data in real time, but because carbon dioxide is denser than air, it sank to the bottom of showcases. Relative humidity was also found to impact the detection results when carbon dioxide was used as the tracer gas. Carbon dioxide as the tracer gas was used to measure the tightness of a number of showcases. The experiments showed that the air exchange rates of different showcases varied from 0.1 times a day to 37 times a day and that the use of carbon dioxide as a tracer gas provides a simple method to evaluate the tightness of showcases.