Why the same advice reverses across the country
Nearly every disagreement about humidity advice on the internet is two people in different climates who are both right. "Open the windows" is correct and free in Minneapolis in January. It is actively harmful in Houston in July, when the outdoor air carries more water than the indoor air and every hour of open window adds moisture you then pay to remove.
The rule that resolves it is to compare dew points, not percentages. Outdoor air at 85 °F and 60% RH has a dew point around 70 °F; indoor air at 72 °F and 50% has a dew point around 52 °F. The outdoor air reads lower in percentage terms and carries far more water. Ventilating with it makes the house wetter, whatever the two percentages say.
These pages follow the standard US climate zones, because they line up with the way houses in each region are actually built — and construction is half the story. The same weather produces different problems in a slab-on-grade ranch, a house over a vented crawl space and a hundred-year-old building with solid walls.
What each climate does to a house
Cold climates: the winter ceiling
The problem is dry air indoors and condensation on glass whenever you try to fix it. How high you can safely run the humidity depends on how cold it is outside and what your windows are made of — from about 40% at 20 °F with double-pane glass down to 25% at −10 °F.
Hot humid climates: summer is the wet season
Outdoor dew points in the seventies for months. Ventilation stops being a tool, air conditioning becomes the main dehumidifier, and the failure mode is a house cooled so efficiently that it never runs long enough to dry the air.
Marine and mixed climates: both problems, six months apart
Damp cool winters and humid summers in the same building, needing opposite treatments. The mistake is leaving the summer strategy running into fall, or the winter one into spring.
Hot dry and high altitude: the opposite failure
Very low indoor humidity for most of the year, splitting timber and cracking instruments, plus evaporative cooling that adds moisture deliberately. Thin air at altitude also shifts the numbers — the same relative humidity carries less water.
Find your climate
| Climate | Target | Where |
|---|---|---|
| Hot and humid | 45–55% | Florida, Louisiana, coastal Texas, Georgia, the Carolinas, coastal Alabama and Mississippi |
| Hot and dry | 30–40% | Arizona, Nevada, inland Southern California, New Mexico, west Texas |
| Cold winters | 30–40% | Minnesota, Wisconsin, Michigan, Illinois, New York, New England, Colorado, Montana |
| Mild and damp | 40–50% | Western Washington and Oregon, coastal Northern California, coastal British Columbia |
| Four seasons | 30–50%, seasonal | Virginia, Maryland, Delaware, Kentucky, Tennessee, Missouri, southern Ohio and Indiana |
| High altitude | 30–40% | Colorado, Utah, Wyoming, northern New Mexico, eastern Nevada, mountain Montana and Idaho |
Hot and humid
Florida, the Gulf Coast, the Southeast. Outdoor air carries more moisture than your indoor air for months at a time.
Hot and dry
Phoenix, Las Vegas, Albuquerque. Almost every month is a dry-air problem — with a short, sharp exception.
Cold winters
Upper Midwest, Northeast, Mountain West. The target is not one number — it falls with the outdoor temperature.
Mild and damp
Seattle, Portland, the coast. Nothing is extreme and everything is damp — the hardest climate to notice a problem in.
Four seasons
Mid-Atlantic, Ohio Valley, Tennessee Valley. Dehumidifying in July, humidifying in January, in the same house.
High altitude
Denver, Salt Lake City, the Mountain West. Cold winters and thin dry air compound each other.
Why one number cannot serve every house
The 30–50% band is a national average of a moving target. What actually decides your number is the temperature of your coldest surface and the moisture content of the air outside — and both of those are climate, not preference.
In a cold climate the winter target has to fall as the outdoor temperature does, or the windows run with water. In a hot-humid climate 40% is technically achievable and financially foolish. In the desert, 50% is a fight against physics. Pick the page that matches your weather rather than the average of everyone else's.
Common questions
How do I know whether opening the window will help today?
Compare dew points. Look up the current outdoor dew point — every weather app shows it, though you may need to tap through to the details — and compare it with the dew point the calculators here give for your indoor reading. If the outdoor dew point is lower, opening the window dries the house. If it is higher, opening the window wets it, whatever the two humidity percentages say.
Does air conditioning count as dehumidifying?
Yes, and in a humid climate it is usually the main one. A cooling coil below the dew point condenses water out of the air, which is why an air conditioner drips. The catch is that an oversized system cools the room to temperature so quickly that it never runs long enough to remove much water, leaving a house that is cold and clammy at once.
Do I need different equipment in different seasons?
Usually different settings rather than different equipment. The same dehumidifier that earns its keep in a humid August is the wrong machine to run in a cold January, when ventilation does the job for free. The seasonal change that matters most is the target humidity itself, which should drop as the outdoor temperature falls.