During water-absorption tests to determine the liquid-water transfer properties of porous building materials, the side surfaces of a specimen are covered with a vapor-impermeable material, such as wrap, adhesive, resin, or tape. Such variety of sealing materials is considered a cause of errors in the water-absorption test results, but the significance of these errors for various combinations of building and sealing materials remains unclear. This study investigated the influence of the sealing method on the water-absorption test results of building materials by analyzing the effects of multiple potential error factors and their dependency on material characteristics to realize more reliable testing methods. Water-absorption tests were conducted using three types of building materials and various sealing methods: plastic wrap, three types of resin, and aluminum tape. The results indicated that multidimensional water inflow near the water surface when a nonadhesive sealing material is used on a specimen and air entrapment caused by air-impermeable side sealing did not significantly influence the accuracy of the measurements. Moreover, decreases in water absorption caused by resin penetration into the specimen could not be negligible, and depend on the resin curing time and viscosity and material characteristics. Plastic wrap and tape may be the safest choices for sealing building materials as long as they do not touch the water surface, which can cause capillary uptake between the specimen and sealing material.