Bathrooms are often hotspots for cockroach problems — not just because of general cleanliness, but because the environment there tends to offer exactly what roaches look for: moisture, warmth, and hidden entry points. Even a bathroom that’s cleaned frequently can remain vulnerable if plumbing systems remain accessible. Roaches are able to move through drains and plumbing lines, bypassing visible surfaces entirely, because they can flatten their bodies and squeeze through surprisingly small openings. One reason drains — especially shower drains — are attractive entryways is that they often stay damp. Shower drains rarely fully dry out because of frequent water use; the constant moisture makes the drain and adjacent plumbing an inviting environment. Also, plumbing frequently links different parts of a building — bathrooms, kitchens, neighbouring units — so a roach infestation in one area can spread through these “hidden highways.” Traditional drain covers often don’t stop this because their holes are designed to catch hair and debris, not to block determined pests.

Because of the shortcomings of standard covers, a low-tech solution — like the balloon trick you describe — can make sense. The idea is to modify a simple elastic balloon into a stretchy seal around the drain cover, tightening the opening so that while water still drains, the passage becomes too narrow for cockroaches to crawl through. You essentially create a more restrictive barrier without blocking drainage or resorting to harsh chemicals. In theory this works because cockroaches, even flattened, still need a minimum amount of space to pass. By reducing that space, the balloon acts as a deterrent. Though I didn’t find specific scientific studies evaluating the “balloon method,” the basic principle aligns with expert advice: physically blocking or narrowing the drain opening is one recommended strategy to prevent roaches from entering via drains.

But relying solely on a balloon (or any single barrier) is rarely enough for long-term protection. Experts consistently emphasize combining multiple preventive practices. For example: cleaning drains to remove organic debris (hair, soap scum, soap residue), which otherwise attracts and sustains roaches; ensuring drains and pipes stay dry; and sealing any gaps or cracks around pipes, walls, and floors — especially where plumbing enters the structure. These openings often serve as alternate entry points.

Another critical factor is moisture control. Roaches are drawn to damp environments because water is essential for their survival. Therefore, reducing humidity — by improving ventilation, wiping down surfaces after showering, fixing leaks, and ensuring there’s no standing water — makes the bathroom less hospitable. Keeping pipes insulated or dry (especially those that tend to “sweat” from condensation) can also help because condensation can sustain roaches even when there’s no actual leak.

Finally, a comprehensive defense involves sealing hidden entry routes and maintaining good hygiene. That means using silicone caulk, expanding foam or steel wool — where appropriate — around pipe penetrations, behind sinks, and beneath cabinets. It also means using drain covers or mesh screens with sufficiently small openings, keeping drains and floor traps clean, flushing rarely used drains occasionally (so that built-in water “traps” remain effective), and avoiding accumulation of food debris or organic matter that might attract roaches. When combined with an elastic-seal method like the balloon trick plus moisture management and regular cleaning, you create a multi-layered barrier that significantly reduces the chances of roaches entering through drains — and improves overall bathroom hygiene and comfort.