Humidity, climate and ventilation

Two things decide whether an evaporative cooler keeps a home comfortable: how dry the outdoor air is, and whether the air can flow through the house and out again.

Why coolers struggle in humid weather

Cooling capacity depends on the wet-bulb depression, the gap between air temperature and wet-bulb temperature. Humid air is closer to saturation, so the gap shrinks. DOE gives an example: at 100 °F and 10% RH (wet bulb 63 °F), a typical aspen cooler delivers about 73 °F, a 27 °F drop. At 95 °F and 50% RH (wet bulb 79 °F), it delivers about 84 °F, a drop of only 11 °F.^[1]

There is a second effect. A direct cooler adds moisture to indoor air. If that air cannot escape, indoor humidity climbs and both comfort and cooling fall further. This is why ventilation matters so much (below).

Interactive: how the cooling limit shrinks as humidity rises

Which climates suit evaporative cooling?

Use the local summer design wet-bulb temperature, the wet bulb that coincides with the hottest few percent of hours, published in engineering climate tables (ASHRAE design conditions, or ACCA Manual J in the US). DOE guidance:^[1]

• Design wet bulb below about 21 °C (70 °F): evaporative cooling works best.
• Up to about 23 °C (74 °F): still provides cooling, but humidity reduces performance. Indirect cooling may be best.
• Higher: choose refrigerated air conditioning or heat pumps, which also dehumidify.

Hot, dry inland regions on several continents often fall in the first band, at least outside their rainy seasons. Many coastal and tropical climates do not. Seasonal humidity matters: a location can be excellent in early summer and marginal during a monsoon or rainy season. Check both with the suitability calculator.

Ventilation: give the air somewhere to go

Unlike AC, an evaporative cooler does not recirculate indoor air. It pushes outdoor air through the house, replacing it every 2–3 minutes when running, so there must be an exit path.^[1]

• How much opening: DOE (citing PG&E) specifies 2 ft² of unrestricted relief per 1,000 CFM (about 0.11 m² per 1,000 m³/h). Screens and louvres reduce free area, so enlarge accordingly. An older DOE fact sheet suggests 1–2 ft² per 1,000 CFM on the leeward side.^[1][2]
• Too little opening: humidity builds indoors. Too much: hot outdoor air enters through the openings instead of cooled air reaching the rooms.^[2]
• Zoning: air flows toward open outlets, so opening windows only in occupied rooms directs the cooling there.^[1]
• Ceiling relief vents (up-ducts): an alternative to open windows. They exhaust into a ventilated attic and use dampers that close when the cooler is off.^[1]

The sizing calculator estimates the relief area for your airflow.