Textiles maintain wearer comfort by allowing evaporated sweat to permeate through, providing thermal management and keeping skin dry. For single layers, resistance to mass transport is relatively straightforward. However, when textiles are layered, water vapor transport becomes more complex because diffusing molecules must traverse interstitial spaces between layers. Interstitial mass transport resistances of significant magnitude can reduce rates of water vapor transport through layered textile stacks. The prevailing textile mass transport resistance interrogation method is ASTM F1868: “Standard Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate.” Four improvements to ASTM F1868 are recommended: (1) gravimetric mass transport measurement, (2) evaluating transport using the Stefan flow model, (3) correct accounting for apparatus mass transport resistances, and (4) recognizing and measuring interstitial mass transport resistances. These improvements were implemented and evaluated by running tests using Southern Mills Defender™ 750 fabric, the calibration standard used for ASTM F1868, on a new gravimetric experimental apparatus. For a single layer of calibration fabric, the gravimetric approach is consistent with the prescribed result from ASTM F1868; however, for stacks of two or more calibration fabric layers, the gravimetric approach does not agree with the prescribed ASTM F1868 result due to interstitial mass transport resistance between fabric layers.