Short definition
Trap evaporation is the slow drying of a fixture’s trap seal when the fixture isn’t used. A standard 2-inch trap evaporates roughly 6 mm per day in dry indoor air, so an unused trap dries fully in about 10 days under typical conditions. The result: sewer gas enters the home through the empty trap. Washington vacation cabins, snowbird homes, and unused basement floor drains are the usual victims.
What it is
Every trap holds about 2 to 4 inches of standing water that blocks sewer gas from entering the home. When the fixture isn’t used, that water slowly evaporates into the room. The rate depends on indoor air humidity, temperature, and the trap’s surface area, but published rates put a 50 mm (2-inch) trap seal at roughly:
- 6 mm/day in dry indoor air
- 8–10 days to dry a sink or lavatory trap fully
- 2–3 weeks for a floor drain (smaller water volume but cooler air)
In Washington’s moderate Puget Sound climate, vacation cabins typically lose all their trap seals within 10 to 14 days of being closed up. East-side homes (drier indoor air) can dry faster.
Why it matters to a homeowner
If you own a cabin in the Cascades, on the Olympic Peninsula, or in the San Juans, trap evaporation is the most reliable “unexpected return-to-cabin” plumbing surprise. The diagnostic is unmistakable: every drain in the cabin smells like sewer the moment you walk in. The fix is just as reliable: pour a quart of water into every trap and floor drain. Within a few hours the smell is gone.
The same applies to:
- A guest bathroom that only sees use twice a year.
- A basement floor drain in a finished basement no one uses.
- A snowbird home shut up for 3 to 5 months.
- A fixture connected to an HVAC condensate primer that stops running in winter.
When you’ll encounter this term
- Returning to a vacation cabin after 2+ weeks and finding sewer-gas smell throughout.
- Basement floor drain “smells in summer” — same mechanism, slower.
- Spring return to a snowbird home — every trap dry.
- New build or remodel where one bathroom sees almost no use.
Common variants / not the same as
- Evaporation vs. siphonage. Evaporation is static (no flow needed). Siphonage is dynamic (during flow).
- Evaporation vs. capillary action. Capillary action is something — usually hair or string — draped over the trap weir wicking water out. Evaporation is pure surface evaporation.
- Evaporation vs. wavering out. Wavering is wind-driven oscillation that walks water over the trap weir. Evaporation is purely static.
Washington note
WA rural cabins (Cascades, Olympic Peninsula, San Juan Islands, Methow Valley) sit empty for weeks at a time during the off-season. Standard prep before leaving for more than two weeks:
- Pour mineral oil into every trap. A tablespoon of mineral oil floats on top of the trap water and forms a vapor cap that slows evaporation dramatically — typically 6+ months of protection from a single dose.
- Trap-primer kits are a more permanent solution: a small line tees off a nearby supply pipe and trickles water into the trap on a schedule. Standard on commercial floor drains, available for residential.
- Float-ball drain kits work similarly, sealing mechanically when the trap dries.
For an unused floor drain in an occupied home, just pour a quart of water in once a month.