Guide

Humidity, Dew Point and Why Mold Grows Indoors

Relative humidity is relative to temperature, and that is the whole point

Warm air holds more water vapor than cold air. Relative humidity describes how full the air is compared to how much it could hold at that temperature. Cool the same air without removing any water and its relative humidity rises, because the capacity fell while the content stayed the same.

That is why a room can read a comfortable 50 percent in the middle and still grow mold behind a sofa on an exterior wall. The air touching that cold surface is at a lower temperature, so its relative humidity is far higher than the meter in the middle of the room reports. Mold responds to the conditions at the surface, not to the room average.

Dew point in plain terms

The dew point is the temperature at which air becomes fully saturated and water starts coming out of it. Any surface colder than the dew point of the air touching it will collect water. This explains the condensation on a glass of iced tea, the frost on a windowpane, and the wet spot behind a bookcase.

In practice it gives you two levers and only two: lower the moisture content of the air, or raise the temperature of the cold surface. Everything that works is one of those. A dehumidifier is the first. Insulation on a cold wall, and letting air circulate to a closet or a corner, is the second.

The Adams County seasonal pattern

Summer, roughly June through September

This is the season that produces most of the calls. Outdoor dew points here regularly sit in the sixties, and basements stay in the sixties too. Humid air entering a cool basement, through an open window, an unsealed rim joist or just a door left ajar, arrives at a surface at or below its dew point. Opening basement windows on a humid day makes it worse, not better, which is the single most common well-intentioned mistake.

Winter, roughly December through March

The direction reverses. Indoor air is now the humid side, and the cold surfaces are exterior walls, window frames, unheated closets and the attic roof deck. This is where attic condensation and window-frame growth come from. The fix is usually air sealing and ventilation rather than a dehumidifier.

Spring and fall

The awkward stretch is the swing season when it is humid outside but not yet hot enough to run the air conditioning. The dehumidification that comes free with cooling is not happening, and indoor humidity drifts upward with nothing to check it. A standalone dehumidifier covers this gap.

Where the water in your indoor air comes from

• Showers and baths, which is why a working exhaust fan vented outside matters more than any other single fixture
• Cooking, especially boiling and simmering, which a range hood vented outdoors handles
• A clothes dryer whose vent has come loose inside the house
• Evaporation from bare soil in a crawlspace or an unsealed basement floor
• Firewood stored indoors, and a lot of houseplants in a small room
• The occupants themselves, since people release moisture continuously just by being there

Measuring it costs about fifteen dollars

Buy two or three digital hygrometers and put them where you suspect a problem: the basement, the worst bedroom, and a main living area for comparison. Read them over a couple of weeks across different weather. You will learn more about your house from that than from any inspection, and it tells you whether you have a humidity problem, a localized cold-surface problem, or an actual leak.

Signs you are running too humid: condensation on the inside of windows, a musty smell that comes and goes with the weather, doors that stick in summer, cold water pipes that drip, and laundry that takes a long time to dry indoors.

What to do about it

1. Put a dehumidifier in the basement, set it near 45 percent, and run a hose to a drain so it never shuts off full
2. Verify every bathroom fan actually moves air and actually terminates outside
3. Keep basement windows shut on humid days and run the dehumidifier instead
4. Insulate cold water pipes so they stop condensing
5. Leave a gap behind furniture on exterior walls, and leave closet doors ajar in winter
6. Extend downspouts and grade soil away from the foundation, since less water outside means less vapor inside
7. Seal crawlspace soil with a vapor barrier

More detail on each of these is on preventing mold, basement mold and crawlspace mold.

Common questions

What humidity level stops mold?

The EPA recommends keeping indoor relative humidity between 30 and 50 percent. Sustained readings above roughly 60 percent are where problems become common, though what matters is the humidity right at the cold surface, not the average in the middle of the room.

Why is my house humid when the AC is running?

Most often the air conditioner is oversized. It cools quickly and shuts off before it has run long enough to pull moisture out, leaving the house cool and clammy. An inexpensive hygrometer will tell you within a day whether that is what you have.

Do I need a whole-house dehumidifier?

Most Adams County homes do not. A portable unit in the basement, correctly sized and set around 45 percent with a drain hose so nobody has to empty it, handles the majority of cases. A whole-house unit earns its cost when the problem is spread across the building rather than concentrated below grade.