Leaving a Home Empty for a Month

Going home for the summer, travelling for work, or away for Tet — a month is long enough for a Ho Chi Minh City apartment to change materially, and people are consistently surprised by what they find on their return.

Three things happen, and none of them are mysterious. Water evaporates, waste ferments, and nothing gets disturbed. Everything else follows from those.

One: every water trap dries out

Floor drains, shower gullies, the trap under a basin, the washing machine standpipe. Each holds a small volume of water that seals your apartment from the building drainage, and in this heat that water evaporates within roughly two to three weeks of disuse.

The result is an apartment with several open pipes into the drainage stack. American cockroaches use them routinely, the smell arrives at the same time, and both start while you are still away.

Two: whatever is in the bin completes its cycle

A small amount of food waste left in a kitchen bin is a minor thing on the day you leave and a fly generation by the time you land. Fruit flies complete a cycle in about a week here; a month is four generations. The same applies to a bag of rubbish left on a service balcony, a fruit bowl, and the last few centimetres in a wine bottle.

A technician treating an empty room with a wooden wardrobe in a home.

Three: nothing is disturbed for a month

This is the least visible of the three and the most significant. Normal life — walking, cleaning, opening cupboards, moving furniture — constantly interrupts anything that is establishing. An empty apartment removes all of it, and a population that would have been noticed at a small size in week two is discovered at a much larger size on your return.

It is also why fleas emerge in numbers on the day you get back rather than while you were away. Pupae wait for warmth, vibration and carbon dioxide, and you are all three walking through the door.

The ten-minute list before you go

Fill every drain and trap, and add a spoon of cooking oil to the ones in rooms you never use — the oil floats and slows evaporation considerably.
Empty every bin and take the bag out of the building, not onto the balcony.
Remove fruit, bread and anything open; move opened dry goods into sealed containers.
Empty balcony saucers and the air-conditioner tray, and wipe them.
Run the dishwasher and washing machine empty, then leave both doors open.

A school garden with large trees treated for mosquitoes.

The humidity question

A closed apartment with no air-conditioning sits at ambient humidity, which through the rainy season is above 80 per cent. That is enough for mould on leather, on wooden furniture and inside wardrobes, and mould damage is more common than insect damage in this situation.

Leaving wardrobe doors ajar and putting a dehumidifier box in each one costs very little. Leaving an air-conditioner running on dehumidify mode for a month costs a great deal and is rarely justified unless you are storing something genuinely valuable.

What to ask someone to do while you are away

If a friend, a housekeeper or a neighbour can visit once, the single most useful thing is not watering plants — it is running water down every drain for thirty seconds. That one action removes the largest of the three problems entirely. Opening the windows for an hour and walking through the rooms deals with a good part of the third.

What to do on the day you return

Open everything, run every tap and shower for a minute, and look at the bathroom and kitchen floor before you unpack. If there is a smell, it is a dry trap and it will clear within a day of filling it. If there are large cockroaches in the bathroom, that is the same cause rather than an infestation, and it usually needs nothing beyond restoring the seals. Small pale cockroaches in the kitchen are a different matter and worth acting on quickly.

Common questions

Does the cooking oil trick really work?

Yes, and it is worth doing for any drain in a room you will not use. A thin layer of oil on the surface of the trap water slows evaporation substantially, so a trap that would dry in two weeks lasts most of a month. Use a spoonful, not a cupful, and run water through normally when you return.

Should I leave the air-conditioning on while away?

Rarely worth it for pest reasons, since cooler air discourages very little and the running cost over a month is significant. It is worth it if you are leaving leather goods, a piano, artwork or a wardrobe of wool suits behind, and in that case a dehumidifier in the room is cheaper than air-conditioning the whole apartment.

Is it worth having a treatment before a long trip?

Only if there is an existing problem, in which case yes and the empty period actually helps, because bait works undisturbed for a month with no competing food. As a precaution in a property with no current issue, it is money spent on nothing — the ten-minute list above does far more.