How to close a South Florida home for the summer without coming back to mold
The advice people give each other at the clubhouse — set it to 80, shut off the water, see you in November — is the single most reliable way to produce a mold problem in this market. Here is what actually works, and why.
Seasonal housing, Palm Beach County. Five months closed is the whole problem.
- 55%Relative humidity to hold while the house is empty. Not a temperature target — a humidity target.
- 5 monthsTypical May-to-October vacancy, which is also the hottest and wettest part of the year.
- Tens vs thousandsWhat holding humidity costs each month against what remediating the result costs. We are not going to invent a figure for your power bill — it depends on the equipment, the house and the rate you pay — but the two numbers are not in the same range, and that is the whole argument.
Every year between Thanksgiving and January we take a run of calls that all start the same way: the owner opened the door for the first time since spring, and the smell hit them before they got their bag inside. Almost none of those houses had a leak. They had five months of unmanaged humidity.
This page is the whole answer, and it is deliberately written so you can act on it without calling anybody.
One number to set expectations before the detail. The EPA puts indoor humidity below 60 per cent, ideally between 30 and 50. The figure recommended below for a closed-up Florida house is 55 per cent, which sits above that ideal band on purpose. Chasing 45 in August means an air conditioner running almost continuously in an empty building. And the failure we actually get called to is never the house held at 55 — it is the house left at 80 degrees with no dehumidification at all. Under 60 is the line that matters. Lower is better if your equipment can hold it without running itself into the ground.
Why “set it to 80” fails
An air conditioner removes moisture as a side effect of running. The coil sits below the dew point, water condenses on it, and that water leaves through the drain. No run time, no dehumidification — it is that direct.
Set the thermostat to 80°F in a closed South Florida house in July and the system satisfies that setpoint almost immediately, then sits idle for hours. Total run time collapses. Meanwhile moisture keeps entering through the slab, through the envelope, and through every humid air exchange, with nothing removing it. Interior humidity climbs into the seventies and stays there.
At sustained relative humidity above about 60%, mold does not need a leak, a flood, or a plumbing failure. Ordinary house dust on ordinary surfaces is enough. That is the entire mechanism, and it is why the houses we open in November are usually bone dry and still smell.
The three strategies that work
In order of what we would recommend, not in order of cost.
- 1. A thermostat with a true humidity setpointNot “dry mode” and not a temperature setting you hope produces dehumidification. A thermostat that reads humidity and calls for cooling to hold it — typically around 55% — with the temperature allowed to drift into the high 70s. Many modern thermostats can do this and most owners have never turned it on.
- 2. A standalone whole-house dehumidifierA ducted or free-standing unit with a hard-piped drain, set to 55%, running independently of the air conditioning. This is the most reliable option because it does not depend on your AC being correctly sized, and it keeps working if the compressor fails in August.
- 3. A humidistat wired to the air handlerThe older, cheaper version of option one: a mechanical or digital humidistat that overrides the thermostat and calls for cooling when humidity rises. Common in Florida condos and still perfectly effective.
- What is not a strategyTurning the system off entirely. Setting the thermostat to 80 and nothing else. Leaving buckets of desiccant in the closets. Asking a neighbor to “run the AC now and then.” None of these hold a humidity ceiling for five months.
The pre-departure checklist
Roughly an hour of work, most of it the week before you leave.
- Service the air conditioning before you go, not after Coil condition, drain pan, condensate line, and the float safety switch. The system is about to run unattended through the worst five months of the year, and a clogged drain line in July is how a ceiling comes down in a house nobody is in. How to checkAsk specifically whether the safety switch was tested, not just whether the line was cleared.
- Put in a fresh filter A new filter on the way out, sized correctly and seated without gaps at the edges. Filter bypass is how dust reaches a wet coil, and a wet coil with a dust mat on it is the most common growth we find.
- Set a humidity target, not just a temperature Around 55% RH, with the temperature allowed to sit at 78 to 80°F. If your equipment cannot hold a humidity setpoint, this is the thing to fix before you leave — it matters more than any other item on this list. How to checkAfter you set it, watch the reading for a full day before you travel. It should settle, not climb.
- Open the house up internally Interior doors open, closet doors open, and furniture pulled a few inches off exterior walls. Air has to reach the surfaces that go first, and closets on exterior corners are almost always the first thing to grow.
- Take the organic material out of the closets Cardboard boxes, stacks of paper, leather goods, and anything upholstered that is stored against an exterior wall. These are food sources sitting in the least ventilated part of the house.
- Deal with the water main deliberately Shutting the main off protects you from a supply-line failure, which is a real risk. But if you have an irrigation system, an ice maker, or a leak-detection valve, decide consciously rather than flipping one handle on the way out. A whole-house leak detection shutoff is worth the money on a seasonal property.
- Empty and prop the refrigerator, or leave it running A closed, unpowered, empty refrigerator grows mold on its own door gasket in this climate. Either leave it running with the doors closed, or empty it, clean it, and prop both doors open.
- Close the blinds on the sun side Solar gain through west and south glass drives the cooling load and shortens equipment life. It is a small thing that costs nothing.
- Arrange a real monthly check, and tell them what to look at Somebody physically inside the house once a month. Not a drive-by. Give them three things: read the humidity, look at the ceiling around every air conditioning register, and put a hand on the floor at the base of exterior walls and in the AC closet. How to checkAsk them to text you the humidity reading each visit. A trend is worth far more than a single number.
Coming back in the fall
If you open the door and the house smells musty, do not turn the air conditioning on. That is the one instruction on this page we would ask you to take literally.
A system that has grown something over the summer is a distribution network. Running the blower pushes it out of the air handler and into every room in the house before anyone has looked at anything, and it turns a contained equipment problem into a whole-house one. We have seen the difference between a one-day job and a one-week job come down to whether somebody flipped the thermostat on before calling.
Instead: open the windows if the weather allows it, get a portable dehumidifier running, and have the air handler, drain pan, and plenum looked at before the system goes back into service.
What this costs versus what it prevents
Holding 55% relative humidity in a closed house for five months costs meaningfully less than most owners assume — the equipment runs in longer, lower-load cycles rather than short hard ones, and you are cooling an empty building with the blinds shut.
The comparison that matters is not electricity against zero. It is electricity against remediating an air handler, replacing a colonized duct board plenum, and cleaning a duct system, in a unit you cannot use until it is finished, during the season you actually came down here for.
What people ask about this
What temperature should I leave my Florida house at for the summer?
The temperature is the wrong dial to be thinking about. Set a humidity target of around 55% and let the temperature land wherever it needs to — usually 78 to 80°F. If your thermostat cannot hold a humidity setpoint, a temperature around 76 to 78 will produce more run time and better dehumidification than 80, but it is a workaround rather than a solution.
Is it cheaper to just turn the AC off completely?
It is cheaper in electricity and considerably more expensive in every other way. An unconditioned closed house in a South Florida summer will sit near outdoor humidity for months. This is the single most common cause of the seasonal mold calls we take, and remediation costs multiples of a summer of run time.
Do I need a dehumidifier if my AC is new?
Maybe not — a correctly sized modern system with a working humidity setpoint can hold the house on its own. Where a dehumidifier earns its keep is on oversized equipment that short-cycles, on zoned systems where a wing rarely calls for cooling, and as insurance on a property you cannot check for months. It also keeps working if the compressor fails while you are away, which the air conditioner obviously does not.
Should I have someone check the house while I am gone?
Yes, physically inside, monthly. The value is not that they will spot mold — by the time it is visible you already have a job. The value is the humidity reading. A house that reads 52% in June and 68% in August has told you something is wrong while it is still cheap to fix.
The house already smells and I just got back. What now?
Follow the steps above before you touch the thermostat — the order matters more than the speed. Beyond that, this is the most common call we take in December, and it is usually very fixable: a coil and plenum that have grown something over a closed-up summer are a contained equipment problem. What decides how big the job gets is whether the blower has already run. If it has, the same growth is now distributed through every room it serves, and we are pricing a house instead of a machine.
The communities this comes up in
Seasonal occupancy is concentrated in a handful of places down here, and these are the pages that go into the housing stock in detail.
Opening the house back up and not sure what you are smelling?
Leave the system off at the thermostat and call before you run it. We can usually tell from a description whether what you are describing is the coil or the house.