An experimental apparatus was built and operated for the measurement of moisture permeability of building materials. The data are for use in resolution of problems associated with moisture buildup in porous building materials. The apparatus is capable of maintaining simultaneous humidity and temperature differences across a test specimen. In contrast with existing experimental methods, the relative humidity on either side of the specimen is controlled without the use of quiescent saturated salt solutions. Forced-air convection at the surface of the specimen is used, resulting in uniform spatial conditions and faster results. Data are obtained for fiberboard sheathing at various temperature and humidity setpoints. The apparatus consists of two environmental chambers between which a wood-based test specimen is sealed. An external forced-air conditioning system using distilled water and molecular sieve desiccant humidifies or dries the chamber air as needed. The moisture transfer rate across the specimen is determined gravimetrically: the desiccant column is weighed to measure its change in mass as a result of moisture diffusion across the specimen. The apparatus is capable of maintaining relative humidities over a range of 5 to 65 percent RH, with a temperature difference across the specimen of up to 20°C. Furthermore, the apparatus is capable of automated relative humidity and temperature control to within ±0.5 percent RH and ±0.5°C of the setpoints, respectively. Test results for fiberboard sheathing subjected to a range of humidity and temperature conditions are presented. Results are compared with the limited data from the literature. Recommendations for improvement of the data measurement methods are included. / Master of Science