Why a cooler that suits Colorado can still cause mold
An evaporative cooler pulls outdoor air through wet pads and blows the cooled, moistened air into the house. In Denver's normally dry summers that is efficient and comfortable, and plenty of mid-century ranches around Hilltop, Crestmoor and Lowry's older edges still have one on the roof or in a window, sometimes alongside a newer central air system.
The trouble is not the moisture the cooler is meant to add. It is moisture going where it should not: water standing in a cracked pan, a float valve stuck open, an overflow line dripping onto roof decking, a duct boot sweating into the ceiling, or a cooler run with the windows shut so indoor humidity climbs during a rare muggy stretch. Each of those can wet drywall, insulation or roof sheathing for weeks before anyone notices.
Warning signs worth checking
- Ceiling stain
- A brown ring or bubbling paint around the cooler's ceiling register
- Smell at startup
- A sour or musty odor in the first minutes of running each season
- Roof evidence
- Rust streaks, algae or soft shingles below the cooler cabinet
- Indoor humidity
- Sticky air or condensation on windows while the cooler runs
- Winter drafts
- Cold air or frost at the register when the duct was never closed off
A season with a swamp cooler, done right
Timing varies with the weather and the cooler model; use this as a general rhythm and follow the manufacturer's instructions.
- Late spring
- Have the roof-mounted unit opened safely, replace or clean pads, flush the pan, check the float, pump and water line, and remove the winter duct cover.
- Early summer
- Run the cooler with a window or relief vent open so humid air can leave the house rather than build up.
- Midsummer
- Look at the ceiling around the register and the roof below the unit after any storm. Watch for standing water or a bleed-off line that drips onto the roof.
- Early fall
- Shut off and drain the water supply line before the first hard freeze, empty the pan and let the pads dry fully.
- Winter
- Cover the cooler and seal the duct opening from inside so warm, moist household air does not condense in a cold duct.
What to do with what you find
- IfOnly the pads or pan are moldyReplace the pads and clean the pan; this is normal maintenance, not remediation.
- IfThere is a stain on the ceiling but the drywall is firm and dryFind and fix the leak path first, then monitor; stains alone do not prove growth inside the ceiling.
- IfThe ceiling drywall is soft, sagging or shows growthTreat it as a wet-ceiling mold problem: contain the room, remove affected drywall and insulation, and check the roof deck above.
- IfThe duct itself shows growth insideAsk whether the duct can be cleaned or should be replaced, and whether the cooler is still worth keeping.
- IfThe cooler is no longer usedConsider having it removed and the roof and ceiling openings properly closed, rather than leaving an idle unit to leak.
General information for Hilltop, not a quote or professional advice. In an emergency, call 911.