Swamp Cooler Making the House Too Humid: What to Check and Fix
House feels sticky with the swamp cooler on? Learn why: too little relief air, low fan speed, humid weather or pad carryover, and how to bring humidity down.
Short answer
A swamp cooler always adds moisture, but a sticky house usually has too little relief air, a fan running too slowly, outdoor dew points that are too high, or droplets blowing off over-wet pads. Open more windows on the far side of the house, run high speed, check pads for overspray, and accept limits on monsoon days.
Key takeaways
- Indoor humidity is set mostly by the supply air dew point and how fast air moves through the house.
- Relief air is the first fix: roughly 1 to 2 sq ft of open window per 1,000 CFM.
- When the outdoor dew point passes about 60 F, expect indoor humidity to climb no matter what you do.
- Sustained indoor humidity above 60% encourages mold and dust mites; the EPA advises keeping it below that.
- Droplet carryover from overflowing pads adds moisture without cooling and can wet ducts.
On this page
A direct evaporative cooler works by adding water vapor to air, so some rise in indoor humidity is normal and expected. What is not normal is a house that feels sticky, clammy or muggy, where towels never dry and windows fog in the morning. That usually means the moist air is not leaving the house fast enough, the fan is moving too little air, the weather is too humid for evaporative cooling, or liquid water is blowing off the pads. Each has a different fix.
Quick diagnosis table
| Symptom | Most likely cause | Fix |
|---|---|---|
| Sticky everywhere, little air movement at windows | Too little relief air | Open more windows or add up-ducts |
| Muggy on low speed, better on high | Low airflow means slower air exchange | Run high speed during the day |
| Fine in June, sticky in July and August | Rising outdoor dew point (monsoon) | Fan only on humid evenings; accept limits |
| Mist or droplets at registers, damp near vents | Pad overspray or overflowing troughs | Reduce pump flow, reseat pads, level unit |
| One or two rooms clammy | No exit path from those rooms | Crack a window in each problem room |
| Humid after cooler shuts off at night | Pads drain moisture into still air; house sealed up | Run fan only to dry pads, then close up |
Where the humidity comes from: a worked example
The moisture a cooler adds is fixed by physics. What changes is how high indoor humidity gets as that air warms up inside and picks up moisture from people, cooking and showers.
Worked example: dry day vs monsoon day
Dry day. Outdoor air is 100 F with a wet bulb of about 66 F (dew point in the mid 40s F). Aspen pads at 75% efficiency deliver air at about 74.5 F. That supply air holds noticeably more moisture, with a dew point around 60 F. As it warms to 80 F inside the house, relative humidity settles around 50%. Comfortable.
Monsoon day. Outdoor air is 95 F with a dew point around 68 F (about 40% relative humidity, wet bulb near 76 F). The same pads deliver about 95 - 0.75 x 19 = 81 F air, with a dew point in the low 70s F. Warmed to 84 F indoors, relative humidity lands near 70%. That is sticky, and it is the weather, not a broken cooler.
You can run your own numbers with the cooling temperature calculator, and see typical summer conditions for your city in the climate index. For how indoor humidity affects comfort and health in general, see swamp coolers and indoor humidity.
Diagnostic sequence, easiest first
- Measure before you guess. Confirm: put an inexpensive digital hygrometer in the main living area and note temperature and relative humidity, plus the outdoor dew point from the National Weather Service. Fix: this gives you a baseline to judge every change below.
- Open more relief air. Confirm: hold a tissue at a partly open window on the far side of the house. It should be pulled firmly outward. Weak or no movement means air is trapped. Fix: open windows to roughly 1 to 2 sq ft per 1,000 CFM of cooler, favoring rooms you use and the downwind side. Up-ducts or barometric relief dampers into a vented attic do the same job with windows closed. See venting and relief air.
- Run the fan on high. Confirm: humidity worse on low speed. Fix: high speed replaces indoor air faster, so moisture from people and cooking gets flushed out and the supply air spends less time warming up. Use low speed for quiet evenings when it is drier.
- Check for water carryover. Confirm: damp ceilings around registers, a fine mist you can feel or see in the supply air, or water in the duct. Fix: make sure troughs are not overflowing, distributor tubes are aimed into the troughs, pads are fully seated with no gaps, and the cooler is level. See swamp cooler leaking for water in ducts.
- Switch modes with the weather. Confirm: when outdoor dew point is above about 60 F and outdoor air is cooler than indoor air (often evenings and nights), the pump is adding moisture for little cooling. Fix: run fan only to ventilate with outdoor air. Turn the pump back on when it gets hot and drier. Our monsoon season guide has a full strategy.
- Control indoor moisture sources. Confirm: bathrooms and kitchen stay damp. Fix: run bath and range exhaust fans, and keep a relief window open near them so the cooler pushes humid air out of those rooms.
- Consider equipment changes. Confirm: humidity is regularly above 60 to 65% for weeks every summer despite good relief air. Fix: options include an indirect or two-stage cooler (adds less or no moisture to supply air), a different system for the humid months, or a heat pump. See direct vs indirect evaporative cooling and swamp cooler or heat pump by region.
Why relief air controls humidity
A cooler does not recirculate. It runs a one-way stream of air through the house. The faster that stream moves, the less time each cubic foot of air spends picking up heat and moisture before it leaves. Starve it of exits and three things happen: delivered CFM drops, so the house runs warmer; the same indoor moisture sources load a smaller volume of fresh air; and air movement across your skin falls, which by itself makes a room feel muggy.
A 1,600 sq ft house with 8 ft ceilings has 12,800 cu ft of air. A 4,500 CFM cooler at full flow gives 4,500 x 60 / 12,800 = about 21 air changes per hour, roughly one complete air change every 3 minutes. If closed windows cut flow to 2,500 CFM, that falls to about 12 per hour. The house feels warmer and stuffier even though the cooler is unchanged. Sizing for adequate air changes is covered in how to size a swamp cooler.
How humid is too humid?
| Indoor RH | What it means | Action |
|---|---|---|
| Below 40% | Very dry outdoor conditions; cooler working with lots of headroom | None needed |
| 40 to 55% | Normal for a well-vented cooler | None needed |
| 55 to 60% | Upper end of comfortable; check relief air | Open more windows, run high |
| 60 to 70% | Sticky; mold and dust mites favored if sustained | Fan only when possible, maximize venting |
| Above 70% | Evaporative cooling ineffective; condensation risk | Switch to fan only or another cooling method |
The EPA recommends keeping indoor relative humidity below 60%, ideally between 30 and 50%, to limit mold growth. Short spikes are not a problem, but weeks at 65%+ in closets, behind furniture and on cold surfaces can be. See mold and swamp coolers.
Note: Swamp coolers are a poor match for homes that also run central AC, because AC needs a closed house and the cooler needs open windows. If you have both, never run them together; switch fully from one to the other.
Daily operating strategy for humid stretches
On days with a high dew point you get the most comfort by changing how you run the cooler, not by fighting the weather. A practical routine many owners in monsoon regions follow:
| Time of day | Typical outdoor conditions | Cooler setting |
|---|---|---|
| Early morning | Coolest air, highest relative humidity | Fan only with windows open, to flush the house with cool outdoor air |
| Late morning to late afternoon | Hot, relative humidity at its lowest of the day | Pump on, high speed, generous relief air |
| During or right after a storm | Temperature drops, humidity spikes | Pump off, fan only or off |
| Evening | Cooling, humidity rising | Fan only once outdoor air is cooler than indoor air |
| Before shutdown | Any | Fan only 15 to 30 minutes to dry pads |
The logic is simple: the pump only helps when the air is dry enough for evaporation to cool it meaningfully. When outdoor air is already cooler than the house, plain ventilation does the job without adding a single pound of water to indoor air. A thermostat or control with separate fan and pump modes makes this easy; see thermostats and controls.
Pro tip: Watch the dew point, not relative humidity. Relative humidity swings all day with temperature, while the dew point tells you how much moisture is actually in the air. If the morning dew point is already in the 60s F, expect a sticky afternoon and plan to lean on fan-only mode.
When to call a pro
- You see mold growth on walls, ceilings or near registers.
- Water is collecting in ductwork or ceiling insulation.
- You want to add up-ducts or barometric relief dampers through the ceiling, which involves attic and roof work.
- You are considering converting to an indirect, two-stage or refrigerated system.
Frequently asked questions
What humidity is normal in a house with a swamp cooler?
In dry weather, indoor relative humidity commonly sits around 40 to 60% with a properly vented cooler. On humid monsoon days it can rise above 65 to 70%, which is when comfort and mold risk become concerns.
Can I run a dehumidifier with a swamp cooler?
It works poorly, because the cooler is constantly replacing indoor air with fresh moist air. A dehumidifier makes more sense in a closed room on days you switch the cooler to fan only or off.
Does turning off the pump reduce humidity?
Yes. Running fan only brings in outdoor air without adding moisture, which is useful on humid evenings when outdoor air is cooler than indoors.
Why do my windows fog up with the swamp cooler on?
Fog forms when glass is colder than the dew point of the indoor air, which can happen in early morning with high indoor humidity. It is a sign to increase relief air or switch to fan only until humidity drops.
Will an indirect evaporative cooler fix the humidity problem?
Indirect coolers cool air without adding moisture to it, so they avoid the problem, but they cool less per unit of energy and cost more. Two-stage indirect-direct units are a middle ground.
Sources and further reading
- Mold, U.S. Environmental Protection Agency
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- National Weather Service, NOAA National Weather Service
- ASHRAE Handbook: Fundamentals (Psychrometrics chapter), ASHRAE