How much moisture does mold actually need?

The number that matters

Sustained relative humidity above roughly 60% is enough for growth on many indoor materials. It does not take standing water, a flood or a burst pipe. A bathroom without working extraction, a cold surface that condenses, or an air conditioner that cools without removing moisture will do it.

Growth is fastest in warm conditions — broadly the same range people find comfortable. That is the awkward part: the conditions we keep our homes in are reasonable growing conditions.

The EPA's recommendation is to keep indoor relative humidity below 60%, and ideally between 30% and 50%. A hygrometer costs very little and will tell you which side of that line your rooms sit on.

Why Florida is a harder case

Outdoor humidity here is high for most of the year, so bringing in outside air does not dry a room the way it might elsewhere. Air conditioning is doing double duty as the main dehumidifier, and a system that is oversized cools the room quickly and shuts off before it has removed much moisture.

Add heavy seasonal rain, wind-driven water at windows and doors, and the ordinary consequences of storm season, and there are plenty of routes for water to get in and not leave.

Where it usually starts

Under and behind fittings

Sinks, dishwashers and washing machines, where a slow leak has somewhere to hide.

Wall cavities

Around windows and doors, where wind-driven rain finds a way past a seal.

Bathrooms without extraction

Or with extraction that vents into the roof space rather than outside.

Air handler closets

Blocked condensate drains and long-term condensation on cold surfaces.

Under flooring

Where a spill or a slab moisture issue has no route to dry.

Roof and attic

After storm damage that was not obvious from the ground.

What to do about it

Deal with the water first. Cleaning growth off a surface while the source is still active means doing it again. This is the single most common reason mold comes back.

Ventilate the wet rooms. Extraction in bathrooms and kitchens, actually vented outside rather than into a roof space.

Watch the humidity, not just the walls. A hygrometer tells you about a problem before there is anything to see.

Do not seal a damp material in. Painting or covering over damp drywall traps the moisture rather than removing it.

Health, honestly

What we can and cannot tell you

Damp buildings are worth dealing with on their own merits: they damage materials and they smell. The EPA and CDC report that some people experience symptoms in damp, moldy buildings, and that people with asthma or allergies may be more sensitive. How any individual is affected is a medical question.

Common questions

Questions we are asked

What humidity level should I keep indoors?

The EPA recommends below 60% relative humidity, ideally 30-50%. A cheap hygrometer will tell you where you are.

Will a dehumidifier fix it?

It helps with a general humidity problem. It will not fix a leak, and running one next to an active water source is treating the symptom.

There is no smell and nothing visible. Is that a good sign?

It is a reasonable sign, but growth inside a wall cavity can be neither visible nor obvious. If your humidity readings are consistently high, that is worth investigating.

Does air conditioning make it better or worse?

Better, when it is sized correctly and the condensate drains properly. An oversized system can cool a room without removing much moisture, and a blocked drain adds water.

Keep reading

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Florida licensed

Mold Assessor MRSA3298 and Mold Remediator MRSR3880, both held by Mauricio Gutierrez. Either number can be checked on the state register.

Phone: (754) 703-8317
Email: moldandenvironmental@gmail.com