What this category is about
Half the traffic on humidity sites is people trying to get a number down. This category is for the other half. In a cold climate a heated house in January is one of the driest environments most people spend time in — routinely drier than the outdoor air in a desert, because cold outdoor air carries almost no water and heating it does nothing to add any.
The mechanism is worth understanding once. Air at 20 °F holds very little water even at 100% relative humidity. Bring that air indoors and warm it to 70 °F and the water content has not changed, but the air can now hold five or six times as much — so the relative humidity collapses. Nothing has dried your house out. The heating simply revealed how little water was in the air to begin with.
The catch, and the reason this category keeps pointing back at the condensation pages, is that you cannot simply raise the humidity until it feels comfortable. Every percentage point you add lands on your coldest window.
The four things it keeps coming back to
How low is too low
Below about 30% the effects are consistent: dry eyes and airways, cracked lips, static shocks, shrinking timber and gapping floorboards. Below 20% for weeks, instruments and furniture start to move measurably. Comfort and health both sit in the 30 to 50% band.
The winter ceiling
How high you can go depends entirely on your windows and how cold it is outside. With double-pane glass at 0 °F outdoors, about 30% is the ceiling before the glass runs wet. With single-pane glass it is closer to 10%. Ignoring the ceiling turns a dry-air problem into a mold problem.
Humidifier types and what they cost you
Evaporative units cannot over-humidify a room and leave no residue. Ultrasonic units are silent and cheap but atomize whatever is in the water, which is why hard water leaves white dust on every surface. Steam units are the most controllable and the most expensive to run.
Static shocks as a measurement
Reliable static discharge from carpet is a fairly good indication that the room is below about 30%. It is not a substitute for a hygrometer, but it is a free signal that something has changed, and it usually arrives before anyone notices the dry throat.
Common questions
Is dry indoor air actually harmful or just uncomfortable?
Both, mildly. The consistent findings are irritation of eyes and airways, worse symptoms for people with asthma or eczema, and some evidence that very dry air impairs the mucous membranes that catch airborne particles. It is not dangerous in the way sustained damp is, but it is not purely a comfort issue either.
Will a bowl of water on the radiator raise the humidity?
Barely. Evaporation from an open bowl is limited by surface area, and a bowl offers very little. Measured against the several pints a day a room actually needs, it is closer to a gesture than a fix. A wet towel over a radiator does noticeably more, and drying a load of laundry indoors does more again.
Should I run a humidifier overnight in a bedroom?
It is the room where it makes the most difference, since dry-air symptoms are worst on waking. Set a humidistat to stop at 45%, keep the unit away from the wall and off soft furnishings, and check the window in the morning — if the glass is wet, you are above your winter ceiling and need to come down.