30–50%, seasonal — 45–50% in summer, 30–40% in winter
A mixed climate is the one that catches people out, because both halves of this site apply to the same house at different times of year. In August the problem is a clammy basement and an air conditioner that cannot keep up with the latent load. In January the problem is static shocks and cracked floorboards.
The mistake is to look for one setting. There isn't one — there are two seasons and a changeover in between.
Two climates, one thermostat
Summers here behave like a hot-humid climate: outdoor dew points in the 60s and 70s °F for weeks, so ventilating adds moisture, basements sweat, and AC run time decides whether the house feels comfortable or clammy.
Winters behave like a cold climate: heating collapses indoor relative humidity, windows become the coldest surface, and the safe indoor humidity has to fall as the outdoor temperature does.
The changeover seasons are where damage happens quietly. In spring, warm humid air arrives before the AC starts running, and the house has no active moisture removal at all. In fall, the reverse. These are the weeks when basement musty smells appear.
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
What to do here
Ranked by what it costs you, and ordered for this climate rather than in general.
Run two mental seasons: dehumidify May to September, humidify December to March, and watch carefully in between.
Check the window calculator before ventilating — the correct answer here genuinely flips across the year.
Keep a hygrometer in the basement year-round. It is the room that tells you which season you are actually in.
Start the basement dehumidifier in April rather than June, and stop it in October rather than August.
A basement dehumidifier on a humidistat handles both the summer peak and the shoulder seasons unattended.
A furnace humidifier for winter, on its own humidistat, so the two systems do not fight.
If both seasons are a struggle, the envelope is leaking in both directions — air sealing pays twice here.
What will not work here
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One humidity target all year
45% is right in July and will wet your windows in January.
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Only thinking about it in summer
The shoulder seasons cause the musty basement, because nothing is running.
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Running the humidifier and dehumidifier in the same month
If that is happening, one of them has the wrong setpoint. Check both humidistats.
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Ventilating on instinct
The right answer reverses twice a year here. Check the numbers rather than the season.
Through the year
Spring
The blind spot. Warm humid days, AC not yet running, nothing removing moisture. Start the basement dehumidifier early.
Summer
Hot-humid rules apply: windows shut, AC on proper cycles, target 45–50%.
Fall
The second blind spot. AC run time falls before the heating starts. Watch the basement again.
Winter
Cold-climate rules apply: humidify to 30–40% and lower it as the outdoor temperature drops.
Common questions
What humidity should I keep my house at year-round in a four-season climate?
There is no single number. Target 45–50% in summer and 30–40% in winter, dropping further in a hard freeze. Two setpoints, two seasons.
When should I start the basement dehumidifier?
April, not June. The shoulder season — warm humid days before the AC starts running — is when basement moisture builds up unnoticed.
Why does my basement smell musty in spring but not summer?
In summer the AC and dehumidifier are running. In spring neither is, while outdoor dew points have already climbed. Nothing is removing moisture.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions