Leaving a second home empty without coming back to mold
An empty house grows mold when nobody is there to notice the humidity climbing. Keep indoor relative humidity under 60 percent the whole time you are away, make sure something is actively removing moisture, shut off or monitor the water, and have the numbers sent to your phone. Mold needs sustained dampness; take that away and a closed house stays clean.
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Why empty houses are the worst case
In a lived-in house, people run the AC, open windows, use exhaust fans and notice a musty smell. In a closed house none of that happens. A small leak, a clogged AC drain line or a thermostat turned off to save money can hold a house at mold-friendly humidity for weeks before anyone walks in. The damage you come back to is usually not from one event; it is from weeks of quiet dampness.
The one number that matters: humidity
Keep indoor relative humidity between 40 and 60 percent. Sustained humidity above about 70 percent gives mold what it needs to grow. In a closed house the danger is not a hot day; it is the steady, invisible climb in humidity when nothing is removing moisture.
Before you leave
- In humid climates, do not turn the AC off. Air conditioning removes moisture as it cools. Turning it off to save money is how many coastal and southern homes grow mold over a summer. Ask your HVAC contractor what setting keeps your house under 60 percent humidity while it is empty.
- Add dehumidification where it is needed. Basements, crawlspaces and closed-off rooms may need a dehumidifier with a hose to a drain, so it runs for months without anyone emptying a bucket.
- Deal with the water. Shut off the main supply if nothing needs it, or fit leak sensors under sinks, the water heater, the washer and the AC air handler.
- Clear the AC condensate drain line before you go. A clogged line is one of the most common sources of water damage in an empty house.
- Leave interior doors and closets open so conditioned air reaches every room.
- Empty the fridge and prop the door if you switch it off, or it will grow its own mold.
While you are away: have the house tell you
A connected humidity and temperature monitor that sends an alert to your phone is the cheapest insurance a second home can have. Set the alert at 60 percent humidity. If it fires, you have days to fix a problem that would otherwise be found months later. Put one sensor in the main living area and one in the dampest space: the basement, the crawlspace or the room over it.
When you come back
- Smell first, before you open windows. A musty smell is the earliest warning.
- Check the monitor history, not just today's reading.
- Look under sinks, around the water heater, at the AC air handler and its drain pan, and at ceilings below bathrooms.
- If something smells or looks wrong, do not scrub it and repaint. Find the moisture source first; see why mold keeps coming back.
More than one property?
For households with several homes, Keystone puts one expert in charge of all of them: how each house handles moisture, what to set before each season, and what to do when a reading goes wrong. It starts with a free call.
Education, not a professional inspection of your property. Humidity guidance from our humidity guide.