Economical fabrication of waterproof/breathable substrates has many potential applications such as clothing or improved medical dressing. In this work, a facile and reproducible fabrication method was developed to render the Janus property to arbitrary porous substrates. First, a hydrophobic surface was obtained by depositing a fluoropolymer, poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl methacrylate) (PHFDMA), on various porous substrates such as polyester fabric, nylon mesh, and filter paper. With a one-step vapor-phase deposition process, termed as initiated chemical vapor deposition (iCVD), a conformal coating of hydrophobic PHFDMA polymer film was achieved on both faces of the porous substrate. Since the hydrophobic perfluoroalkyl functionality is tethered on PHFDMA via hydrolyzable ester functionality, the hydrophobic functionality on PHFDMA was readily released by hydrolysis reaction. Here, by simply floating the PHFDMA-coated substrates on KOH(aq) solution, only the face of the PHFDMA-coated substrate in contact with the KOH(aq) solution became hydrophilic by the conversion of the fluoroalkyl ester group in the PHFDMA to hydrophilic carboxylic acid functionality. The hydrophilized face was able to easily absorb water, showing a contact angle of less than 37°. However, the top side of the PHFDMA-coated substrate was unaffected by the exposure to KOH(aq) solution and remained hydrophobic. Moreover, the carboxylated surface was further functionalized with aminated polystyrene beads. The porous Janus substrates fabricated using this method can be applied to various kinds of clothing such as pants and shirts, something that the lamination process for Gore-tex has not allowed.