Because air can still pass around the edges, the setup may offer little real protection against the gases or pests it is supposed to block. The source of those problems lies deeper in the plumbing, not at the surface of the drain.
The P-trap: the curved pipe that actually does the work
Every sink, shower, and bathtub is connected to a curved section of pipe known as a P-trap. Its shape allows it to hold a small reservoir of water at all times, and that water acts as a physical barrier between the interior of the home and the sewer system below.
As long as the trap stays filled, sewer gases and insects cannot travel up through the drain. The problem begins when a home sits empty for an extended period: that water slowly evaporates. Once the trap dries out, the barrier disappears entirely, and unpleasant smells — along with insects — can enter freely through the drain opening.
How the P-trap works
A P-trap is the curved section of pipe found beneath every sink, shower, and bathtub. Its bend retains a small amount of water at all times, creating a seal that prevents sewer gases and insects from rising into the home through the drain. The system works passively — but only as long as that water does not evaporate.
10 to 20 seconds of running water is all it takes before you leave
The straightforward fix is to run water through every sink, shower, and bathtub for about 10 to 20 seconds before leaving. This refills each P-trap and restores the water barrier. Toilets should also be flushed as part of the same routine.
Less-used drains in basements and laundry rooms deserve particular attention, since they tend to dry out more quickly than drains in daily use. For longer trips, properly fitted rubber or silicone drain covers can provide an additional barrier, and built-in stoppers can help slow evaporation further.
