Construction · Lesson 10 of 13Advanced8 min read

Moisture, Mold and Building Science

A lot of what goes wrong with houses comes back to water. This lesson explains where it comes from, the four ways it moves, why cold surfaces sweat, and how good details keep wood dry and mold away.

Where the water comes from

University of Minnesota Extension sorts the moisture in a house into three sources: liquid water from rain or groundwater; moisture made inside the house by cooking, bathing, unvented clothes dryers and humidifiers; and humid outdoor air that gets in and condenses on cooler surfaces. Plumbing leaks add a fourth, and new construction adds a fifth: fresh concrete gives off about 0.2 gallons of water per square foot of wall and 0.1 gallons per square foot of floor as it dries.

The amounts can be big. A 1-inch rain drops about 1,250 gallons on the roof of a 2,000-square-foot house.

Four ways water moves

Bulk water flows: rain, snowmelt, groundwater, leaks. Gravity drives it, so the fix is to direct it away and lap every layer so water runs out over the one below.
Capillary water wicks through tiny pores, up into concrete or along wood, even uphill. Gravel and plastic sheeting under a slab are capillary breaks. In soil, capillary water can climb a few inches in gravel and as much as 12 to 20 feet in clay.
Air-carried moisture rides along with air leaking through gaps. An air barrier stops it.
Vapor diffusion is water vapor moving slowly through materials. Vapor retarders slow it.

Each one needs its own control: drainage and flashing for bulk water, capillary breaks, air barriers, and vapor retarders. Sources: DOE Building Science Education Solution Center; EPA (“The key to mold control is moisture control”).

Condensation and the dew point

Air can hold only so much water vapor, and warm air can hold much more than cold air. Relative humidity is how much vapor the air holds compared with the most it could hold at that temperature. Cool that air without adding or removing any water and its relative humidity rises, until at the dew point it reaches 100 percent. Any surface colder than the dew point collects condensation.

That’s why windows fog and drip on cold nights, why cold-water pipes sweat in summer, and why basement walls get damp when warm, humid summer air comes in and meets the cool concrete. University of Minnesota Extension notes that people often think that last one is a leak when it’s really condensation.

Pick a scenario or set your own room temperature, humidity and surface temperature. The curve is how much water vapor air can hold at each temperature, from OpenStax’s table. The room air slides left at constant moisture until it hits the curve: that’s the dew point.

Mold

EPA’s first rule: “The key to mold control is moisture control.” Mold can grow on almost anything that stays damp, and a musty smell is sometimes the only clue. EPA’s advice:

Dry water-damaged areas and items within 24 to 48 hours to prevent mold growth, and fix plumbing leaks and other water problems as soon as possible.
Keep indoor relative humidity below 60 percent, ideally between 30 and 50 percent.
Reduce condensation: insulate cold surfaces such as cold-water pipes, vent clothes dryers, stoves and other moisture-producing appliances outdoors, and run bathroom and kitchen fans.
Clean and repair gutters, and slope the ground away from the foundation.
If the moldy area is less than about 10 square feet, many homeowners can clean it up themselves following EPA’s guidance. Clean up the mold and fix the water problem; cleaning alone lets it come back.
Absorbent materials such as ceiling tiles and carpet may have to be thrown away if they get moldy. Don’t paint or caulk over mold.
Don’t run the heating and cooling system if you know or suspect it’s contaminated with mold.

Mold can cause allergic reactions in sensitive people, irritate the eyes, skin, nose, throat and lungs, and trigger asthma attacks in people with asthma who are allergic to it.

Why houses rot

Wood is food for decay fungi when it stays wet. Wood kept below about 20 percent moisture content is less susceptible to fungi and insects, and dry indoor wood sits far below that (Lesson 4). Rot starts where wood stays wet: behind siding with no drainage plane, under a window with no sill flashing, below a roof-wall joint with no kick-out flashing, on a sill plate over a damp foundation, or on crawlspace joists above bare soil. Water intrusion around windows and doors often isn’t discovered until damage has already happened inside the wall.

Where water causes trouble, and the detail that prevents it
Problem	The detail that prevents it
Rain behind the siding	A lapped weather-resistive barrier and flashing: a drainage plane.
Leaks under windows	Sill flashing first, sides next, head last; don’t seal the bottom flange.
Rot below a roof-wall joint	Step flashing and a kick-out flashing that steers water into the gutter.
A wet basement after rain	Gutters, downspout extensions and soil sloped away from the house; drain tile.
A damp slab or musty crawlspace	A capillary break: gravel and sealed plastic sheeting.
Ice dams and attic moisture	Air sealing the ceiling, insulation, ventilation, fans vented outdoors.
Condensation inside walls	Air sealing, a vapor retarder on the warm side where the code calls for one, and a warmer sheathing.
Mold in bathrooms	An exhaust fan vented outdoors, used during and after showers.

You canYou can

Run bathroom and kitchen fans, and make sure the dryer vents outdoors.
Keep an eye on indoor humidity with an inexpensive hygrometer, and aim for 30 to 50 percent.
Fix drips fast, and dry anything wet within 24 to 48 hours.
Know where to shut off the water (Being an Adult 101).

StopCall a pro for big water and big mold

For heavy water damage or mold over about 10 square feet, EPA points to its professional remediation guidance. If you hire a contractor, EPA says to make sure they have experience cleaning up mold. If you have health concerns, talk to a health professional before you start any cleanup.

Check yourself

Tap an answer to check it, or open “Show the answer.”

According to EPA, what is the key to mold control?
Show the answer
Moisture control. “The key to mold control is moisture control.” Fix the water problem, or the mold comes back.
EPA says to dry water-damaged areas within…
Show the answer
24 to 48 hours. Within 24 to 48 hours, to prevent mold growth.
What indoor relative humidity does EPA suggest?
Show the answer
Below 60%, ideally 30 to 50%. Below 60 percent, ideally between 30 and 50 percent.
The dew point is the temperature at which…
Show the answer
air cooled at constant moisture reaches 100% relative humidity. Below the dew point, vapor condenses into liquid water on a surface.
A basement wall is damp in humid summer weather, even with no rain. The likely cause is…
Show the answer
humid air condensing on the cool wall. University of Minnesota Extension says this is often condensation, not a leak.

Go deeper

U.S. EPA: A Brief Guide to Mold, Moisture and Your Home EPA’s Brief Guide to Mold, Moisture and Your Home.
University of Minnesota Extension: Moisture in Basements: Causes and Solutions University of Minnesota Extension on basement moisture.
OpenStax, College Physics 2e, 13.6 Humidity, Evaporation, and Boiling OpenStax on humidity, dew point and evaporation.
U.S. DOE Building Science Education Solution Center: Building Science Intro: Moisture Flow DOE building science: how moisture moves.
Lesson 11: Troubleshooting a House by Symptom Next: troubleshooting a house by symptom.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how houses are built in general and follows U.S. sources; the local building code, the building department and the manufacturer’s instructions are the final word.