When a drain is used regularly, the water inside the P-trap is constantly refreshed. But if a sink or shower remains unused for days or weeks, especially in warm or dry conditions, that water can slowly evaporate. Once the water level drops too low, the protective seal weakens and odors or insects may begin entering the home. Although covering the drain with paper and a glass may seem helpful, it does not create a reliable airtight seal, and the paper can absorb moisture and become ineffective.
A better approach is to run water through every sink, shower, bathtub, and floor drain before leaving so each P-trap is full. For extra protection, use built-in drain stoppers or tightly fitting rubber or silicone covers. Some homeowners also add a layer of plastic wrap beneath a stopper to create a stronger temporary seal over drains that may remain unused for an extended period.
For longer absences, a small amount of mineral oil can sometimes be poured into the drain after the water has filled the P-trap. Because the oil floats on top, it can slow evaporation and help the water seal last longer. The viral paper-and-glass trick is based on the right idea but is not the most dependable solution. Properly filling P-traps and sealing drains with suitable covers are much more reliable ways to reduce odors and unwanted pests while you are away.
