There is a published number, and it does not come from a cleaning company: the EPA’s moisture guidance says to keep indoor relative humidity below 60% — ideally between 30% and 50%. In Miami the realistic goal is the top of that band: under 60%, all year. The 30% end belongs to drier climates than ours.
That is the whole answer to the question as asked. The useful part is what the band means in this county — where the outdoor air spends a good part of the year wetter than the band’s ceiling, the A/C is the only thing holding the line, and the standard national advice (“just ventilate”) runs exactly backwards from June to November. Below: why the number is hard here, how to read a hygrometer without fooling yourself, the honest answer on dehumidifiers, and what a cleaning visit does — and does not — change.
Where the number comes from — and why we quote it instead of authoring it
We clean houses; we do not write moisture guidance. The below-60%, ideally-30–50% band is the EPA’s published advice, it is public, and it is the same number every Florida page on the subject repeats, because there is no other honest source. So we repeat it too, with the author attached. What we can add is not a better number — it is what the band looks like from inside Miami homes every week: which houses hold it, which drift, and what the drift leaves on surfaces.
Why Miami makes an easy number hard
For a good part of the wet season — and hurricane season with it, June through November — the outdoor air in Miami-Dade sits above the band all on its own. That flips the standard advice. In a dry climate you lower indoor humidity by opening windows; here, June to November, the open window is how the moisture gets in. The houses that hold the band through a Miami summer are the ones that stay closed and cooled — which is also why those same months are the months of shut houses, shutters up, and everything the musty-smell conversation is about.
And there is one more local twist: the calendar of empty houses. Seasonal homes closed for the summer, units waiting between guests, apartments being shown but not lived in. A closed, unconditioned house in a Miami summer drifts up on its own — no leak, nothing broken. It is why seasonal owners here leave the thermostat on auto rather than off when they go: a humidity decision wearing a temperature dial.
“The A/C is the dehumidifier most Miami homes already own — for the hours it runs. The band gets lost in the other hours.”
When the band gets lost in a Miami house
- The summer the house sits closed — A seasonal home with the system off, or a unit sitting empty between guests, drifts on its own through a Miami wet season — no leak, nothing broken, just still air with nothing drying it. This is the single most common way the band gets lost.
- The shutter week — Storm prep seals a house tighter than any other week of the year, and it lands in the wettest months. If the power goes out with it, the drift is faster still. That is calendar, not carelessness — worth knowing about, not worth guilt.
- The mild stretch in winter — A December or January week that never gets warm enough for the A/C to cycle is a week in which nothing is drying the air. Comfortable for people — and a pause in the only dehumidifying most homes have.
- The rooms the air skips — A guest room kept shut with its vents closed, an interior closet, a bathroom whose exhaust fan never runs long enough. The whole-house number can look fine while one room lives above the band.
- The wet habits — Laundry dried indoors, long showers with the fan off, rugs that came in damp from the patio. Each one is small; in August, in a closed house, they stack.
How to use a hygrometer without fooling yourself
A hygrometer costs about as much as lunch and answers more than this article can — but only if you read it as a pattern instead of a verdict. Put it in the room you are worried about, not in the hallway. Note the reading at the same times for a week: after showers, during rain, after the house sat closed all day. A stray 65% on a rainy afternoon is weather; a room that never comes back under the band is a pattern. The band is your reference line — the pattern tells you when the moisture arrives, which is the only thing you can actually act on.
And if what brought you here was a smell rather than a number, start with the other guide: why a clean house still smells musty. A smell has its own checking order, and it starts at the A/C return, not at a gauge.
What the damp hours leave on the house — the part that is ours
Humidity sets the pace of buildup here. Our own deep-clean page says it in one line: most Miami homes benefit from a deep clean two to three times a year — humidity, salt air, and year-round pollen mean buildup happens faster here than in drier climates. In practice that is film on grout and caulk that a quick wipe does not lift, dust that sticks to sills and slider tracks instead of lifting off, and the dinginess a damp, closed season leaves on baseboards, vent covers, and door frames.
The visit built for that reset is the deep clean: a 4–6 hour visit, 2-person team, 42 items verified by photo, starting at $375. We arrive with everything — vacuums, mops, and eco-friendly products by default, with fragrance-free options on request. Those figures are the service page’s, quoted here so you never have to take a blog’s word for a price.
What a cleaning visit will not change
Two things, said without hedging. First: the number on the hygrometer. A cleaning removes what damp air left on surfaces; it does not take moisture out of the air, and a company promising otherwise is selling you another trade’s work. Second: anything past cleaning. Our Terms draw the line in their own words: we reserve the right to refuse or stop service where conditions are unsafe for our team — such as mold, infestation, biohazards, or abusive behavior — and if we stop early for safety, a partial refund applies for the unworked portion. If what you are looking at is visible growth rather than a reading, which call goes first is its own question, and we wrote it down: cleaning service or remediation company.
Quick answers
What humidity does mold actually grow at?
We are not going to hand you a growth threshold — we are a cleaning company, not a laboratory, and a number like that would be us inventing authority we do not have. The number we will repeat is the EPA’s published band: keep indoor humidity below 60%, ideally 30–50% — guidance that exists precisely so the question never gets tested in your house. And if where you live is past prevention, a cleaning visit is not remediation; that is a different trade.
Is staying under 60% even realistic in Florida?
In a lived-in, air-conditioned Miami home, yes — the A/C is drying the air every hour it runs, which is most hours from May to October. The band does not get a Florida discount: the humid outdoors is the reason closed windows and cooling matter here, not a reason to move the goal. Where it genuinely gets hard is the closed, unconditioned house — the season the thermostat is off is the season the reading climbs.
Do I need a dehumidifier in Florida if I already have A/C?
While the A/C runs it is already dehumidifying; the honest question is what your readings do in the hours it does not run — mild weeks, closed rooms, a house set warm for a season away. We do not sell, size, or install equipment, so we will not answer with a product: take a week of readings, and if one room never comes back under the band, show that pattern to an HVAC professional. Sizing is their calculation, and it is a better conversation than any general article can have with you.
Will a deep clean bring my humidity reading down?
No — and we would rather say so than let you buy the wrong fix. A deep clean changes what the damp months left ON the house: the film on grout and caulk, the stuck dust on sills, tracks and vent covers, the dinginess of a closed season. The number on the hygrometer is a moisture question — the A/C, the habits, the building. If the film or the smell is back within days of a visit, the source is still wet, and the answer stopped being a cleaning one.
The short version
Keep a Florida house under 60% relative humidity — ideally inside the EPA’s published 30–50% band — and expect the A/C to be the thing doing it. Read patterns, not single readings. Closed and unconditioned is how a Miami house drifts; lived-in and cooled is how it holds. A cleaning cannot move the number — we say so plainly — but it does take back off the house what the damp season left on it.
This guide is part of the JAS operating playbook — written to put an attributed number in public where the industry usually puts a guess.