People look for a leak because that is the intuitive model: water gets in, mold grows. It is often right and it is frequently not.
The other mechanism is ambient humidity, and in this climate it explains a great deal.
The number that matters
Fungal growth on common building materials and furnishings is supported at surface conditions corresponding to roughly 60% relative humidity and above, sustained.
Below about 50%, growth on ordinary surfaces essentially stops. Between 50 and 60 is a grey band. Above 60 for weeks at a time, and it will grow — on drywall paper, on upholstery, on leather, on stored clothing, in closets, on the inside face of exterior walls.
No leak required. No water event. Just air.
Why buildings here sit above it
The outdoor baseline is high. Central Texas carries a great deal of atmospheric moisture for most of the year. Every time a door opens, humid air comes in.
Air conditioning is the only dehumidifier most houses have, and it dehumidifies only as a side effect of running.
Systems here are frequently oversized. A larger unit felt safer to whoever specified it. An oversized system cools the air quickly, satisfies the thermostat, and shuts off — before it has run long enough to remove much moisture. This is called short cycling, and the result is a house that is cold and clammy.
Modern envelopes are tight. Good for energy, and it means moisture generated inside has fewer paths out.
Occupancy adds moisture. Showers, cooking, laundry, and people. In a unit occupied above its design density — very common in this town's rental stock — the moisture load is several times what the ventilation assumed.
The buildings this affects most
Houses that are cool but feel sticky
The classic oversized-system signature. The thermostat says 74 and the house feels unpleasant. That is high relative humidity, not temperature.
Buildings that stand empty
The single most reliable version of this problem. A closed building with the air conditioning off holds interior conditions that track the outdoors, and that is enough on its own.
Applies to vacant houses, inherited property, houses between tenants over summer, weekend cabins, and rooms or wings of commercial buildings taken out of service.
We are called to properties every year where nothing leaked at all. The building sat closed through a summer.
Closets and corners on exterior walls
A closed closet on an exterior wall has no air movement, a colder surface, and often clothing pressed against the wall. Same for a corner behind a wardrobe or a sofa pushed tight.
Crawl spaces and unconditioned attics
Where the moisture arrives from outside by design. Foundation vents in this climate deliver humid air to cool surfaces rather than removing moisture.
How to find out whether this is you
Buy a hygrometer. They cost very little.
Put it in a hallway, away from a bathroom or kitchen, and read it over a week — not once. Ambient conditions change through the day and a single reading tells you little.
Consistently above 60%: you have a building or mechanical problem, and growth in several rooms is explained without any leak. Looking for a leak will waste your time.
Between 50 and 60%: borderline. Worth improving, and probably not the whole story.
Below 50%: ambient humidity is not your problem. Go and find the water.
What to do about it
Do not simply turn the thermostat down. Colder air holds less moisture, so relative humidity rises and surfaces get colder — you can make condensation worse rather than better.
Get the system checked for sizing. A correctly sized unit runs longer at lower capacity and dehumidifies properly. Replacing an oversized system with a smaller one is counterintuitive and often correct.
Consider a variable-speed or two-stage system if you are replacing equipment. Longer run times at partial capacity is exactly what removes moisture.
A whole-house dehumidifier tied into the return handles it directly rather than as a side effect. In this climate it is a reasonable thing to own.
Run bathroom and kitchen extract fans properly. Twenty to thirty minutes after use, not just during. A timer switch costs very little and changes it permanently.
Seal the crawl space if you have one, with a ground vapour barrier and closed vents, rather than ventilating it.
Keep unoccupied buildings conditioned. A modest thermostat setting, or a humidistat-controlled dehumidifier plumbed to a drain. It costs a fraction of one remediation.
Move furniture off exterior walls and leave closet doors ajar. Free, and it works.
Why this matters for diagnosis
The practical value of understanding this is that it tells you which kind of problem you have before you spend money.
Growth in one location, with a clear boundary, near a plumbing fixture or below a roof penetration, is a leak. Go and find it.
Growth in several rooms at once, on furniture and in closets, in a house with no history of water intrusion, is ambient humidity. No amount of leak-hunting will resolve it, and remediating it without fixing the humidity means doing it again next year.
See HVAC and duct mold and crawl space mold removal.
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