What this category is about
There is no such thing as the humidity of a house. There is the humidity of each room, and in a typical home the gap between the best and worst room is twenty percentage points or more. The kitchen and bathroom produce moisture in bursts. The bedroom produces it slowly for eight hours a night. The basement produces almost none and is still the wettest room in the building, because it has the coldest surfaces.
This category exists because the same advice gives opposite results depending on where you apply it. Opening a window helps a bathroom in any weather and can actively harm a basement in July, when warm outdoor air meets a cold slab and condenses on it. Running a dehumidifier in a garage in February may do nothing at all, because the compressor ices up below 65 °F.
Each article takes one room, describes what it does over a day and over a year, and gives the measurement that tells you whether it is behaving.
The four things it keeps coming back to
Where the moisture is made
A sleeping adult adds up to a pint a night. A shower adds half a pint in ten minutes. An uncovered pan on a rolling boil adds around a pint an hour. A load of laundry dried indoors adds four to five pints. Knowing which rooms make water tells you which rooms need to move it out.
Where the cold surfaces are
Basements and crawl spaces have cold floors and walls all summer. North-facing bedrooms have cold exterior walls all winter. Closets and the wall behind a dresser have still air and no heating. These are the places that fail first regardless of what the rest of the house reads.
Rooms that need the opposite treatment
A bathroom needs fast extraction and does not care about outdoor conditions. A summer basement needs the windows shut and a dehumidifier running. Treating them the same way is the most common self-inflicted damp problem there is.
One hygrometer is not enough
If you are chasing a problem, the reading that matters is in the room that is failing, taken early in the morning when surfaces are coldest. A second cheap unit costs less than takeout and settles most arguments about where the problem actually is.
Common questions
Why is one room in my house always more humid than the rest?
Usually one of three reasons: it makes more water than the others (bathroom, kitchen, laundry), it is colder than the others so the same water reads higher (north-facing, unheated, below ground), or the air in it does not move (door kept shut, furniture against the exterior wall, no vent). Work out which of the three applies before buying anything.
Should I keep interior doors open or closed to manage humidity?
Open, with two exceptions. Open doors let the drier air of the whole house dilute a damp room and even out temperatures, which raises the cold surfaces. The exceptions are during and just after a shower or heavy cooking, when you want the moisture contained and extracted rather than distributed through the house.
Does an unheated spare room need heating even if nobody uses it?
It needs enough background heat to keep its surfaces above the dew point of the air reaching it, which in practice means a low, steady setting rather than nothing at all. An unheated room with the door shut in a heated house is where mold appears first, because warm moist air migrates into it and meets cold walls.