Short answerIn a Michigan winter, keep indoor relative humidity between 30 and 45 percent, and lower it toward 30 percent or below when the outdoor temperature drops under 10 to 20°F to prevent window condensation. In summer, keep it under 55 percent, which the air conditioner handles if it is sized right. Above 60 percent in any season invites mold; below 30 brings static, dry skin and cracked wood.
Relative humidity is relative to temperature
The number on a hygrometer is relative humidity: how much water vapor the air holds as a percentage of what it could hold at that temperature. Warm air can hold far more than cold air, so the same amount of moisture reads as 80 percent at 10 degrees and about 12 percent at 70. That is the whole story of a Michigan winter indoors. Outdoor air that feels damp at 15 degrees becomes desert-dry the moment the furnace warms it. And it is why the target is not one number; it has to move with the outdoor temperature, because the coldest surfaces in your house, the window glass, are tied to what is happening outside.
Winter targets by outdoor temperature
The general range is 30 to 45 percent. Within that, the upper limit is set by the windows. Glass on a cold night is the coldest surface in the room, and when the indoor air touching it is humid enough to reach its dew point at that glass temperature, water condenses. Keep the indoor humidity below that point and the windows stay dry.
- Outdoor temperature above 40°F: up to 45 percent indoors is fine for most houses.
- 20°F to 40°F: 35 to 40 percent.
- 10°F to 20°F: 30 to 35 percent.
- 0°F to 10°F: around 30 percent, lower in a house with single-pane or older aluminum-frame windows.
- Below 0°F, polar vortex conditions: 25 percent or less until it warms up.
Houses with modern double- or triple-pane windows, like the 2000s builds in Saline and Canton, can run a few points higher at each step because the inside glass surface stays warmer. A 1950s ranch in Garden City with original single-pane windows and aluminum storms needs to run at the low end. The practical method is to watch the bottom corners of the coldest window in the house on the first cold morning: a thin line of condensation on the lowest inch is at the limit; water running down the glass means back it off.
Summer targets
In summer the issue reverses. Outdoor dew points along the Huron River valley sit in the 65-to-72 range through July and August, and every bit of outdoor air that gets into the house brings that moisture with it. The target indoors is under 55 percent, and under 50 feels noticeably better at the same thermostat setting. Above 60 percent, mold grows on anything organic, dust mites multiply, and the house feels clammy at 74 degrees.
Central air removes moisture as a side effect of cooling; the evaporator coil runs below the dew point and water condenses on it and drains. An air conditioner that is sized right runs long cycles and does a good job of this. One that is oversized, which is common in the 1990s subdivisions across Washtenaw and Wayne County, cools the house in ten minutes and shuts off before the coil has pulled much water out. The result is a cold, damp house, and the fix is sizing, a variable-speed system, or a whole-house dehumidifier. A house at 50 percent feels comfortable at 76 degrees; the same house at 65 percent feels sticky at 72, so the humidity number has a direct line to the DTE bill as well as to comfort.
Basements are the exception to "the AC handles it." They are already cool so the thermostat never calls on their behalf, and they can sit at 65 percent all summer. A dehumidifier set to 50 percent and drained to the sump is the usual answer; the basement moisture guide covers the rest.
How to measure it
A hygrometer, a small digital humidity meter, costs very little and is the only way to know. The humidity reading on a thermostat is a reasonable guide if the thermostat has a sensor; the humidistat on a bypass humidifier is not, since it is often mounted on the return duct and reads whatever is passing by. Put a meter in the main living area and one in the basement. Cheap meters can read 5 points off, so if two disagree, the truth is probably between them. What you care about is the trend and the window check, not decimal precision.
When the house is too dry
Below 30 percent, static shocks start, skin and sinuses dry out, hardwood gaps and trim joints open. The fix for a whole house is a furnace-mounted humidifier, which adds moisture every time the blower runs and holds a setpoint through the winter. Bypass and fan-powered units handle most houses in our area; steam units handle large or tight ones. The whole-home humidifier guide compares them, and a humidifier installation estimate is free. The humidistat should be mounted where it reads the living space, and the bypass damper has to be opened each fall; a humidifier that was switched off for summer and never switched back on is a common reason a house with a humidifier is still dry in December. Portable humidifiers work for one room and need filling and cleaning constantly; a furnace unit does the whole house and drains itself.
Before adding moisture, it is worth reducing how much dry air gets in. Sealing the rim joist in the basement, weatherstripping doors and adding storm windows all lower the air exchange rate, which keeps humidity up and also lowers the gas bill.
When the house is too humid in winter
The opposite problem shows up in tight newer houses and in any house where the humidifier is set too high. Windows sweat, frost forms on the inside of the glass on cold mornings, mold appears in the corners of window frames and in closets on exterior walls, and the attic hatch drips. In a tight 2010s build in Pittsfield Township with a family of five, two showers a day and a kitchen that cooks, the house makes enough moisture on its own and the humidifier should be off or set very low. Turn the humidistat down first. If the windows still sweat with the humidifier off, the house needs more ventilation: bath fans actually run during and after showers, a range hood vented outside and used, and possibly a mechanical ventilation system.
What we check
On a furnace maintenance visit we measure the humidity in the house and the basement, check the humidistat setting against the outdoor temperature, confirm the humidifier solenoid and drain are working and the pad is not scaled, and note any condensation on the windows or the ductwork. If the house is dry with the humidifier running, the usual causes are a scaled pad, a stuck solenoid, a bypass damper closed for summer and never reopened, or a unit too small for the house. If the house is damp, we look at the AC sizing and the return air before suggesting equipment.