The Truth About the “Upside-Down Glass” Hack
Let’s get right to the bottom of this. The short answer? You do not need to cover your drains with a glass, a bag, or a shower cap.
Here is the fascinating kitchen-and-plumbing science behind why:
The Magic of the “P-Trap”
If you look under your sink, you’ll see a pipe that bends down and then curves back up in a “U” shape before going into the wall. That is called a P-trap. Its entire job is to hold a small pool of water.
That little pool of water acts as a physical, liquid seal. It is what blocks heavy, smelly sewer gases from floating up the pipe and into your bathroom or kitchen. It also blocks drain flies and bugs.
Why the Glass Hack Fails
Placing an upside-down glass or a plastic bag over the drain does absolutely nothing to stop sewer gas, because it doesn’t create an airtight seal. Gases will simply find a way around the edges. Furthermore, if you use a plastic bag and a rubber band, you’re just creating a messy, sticky trap for hair and gunk that you’ll have to clean when you get home.
The only time sewer gas escapes is if the water in that P-trap evaporates. But unless you are leaving your house for six months with the heat blasting, the water in your P-trap will not evaporate during a standard vacation!