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Use air conditioning or move to an air-conditioned place when indoor heat is dangerous. Fans can improve comfort only while indoor temperatures are below 90°F; the Centers for Disease Control and Prevention (CDC) warns that above 90°F, a fan can increase body temperature. Close shades or curtains on sunny windows, ventilate only when outdoor air is cooler and safe, and choose a properly sized room air conditioner when mechanical cooling is needed.
The right combination depends on four conditions: indoor temperature, outdoor temperature and humidity, sun exposure through windows, and whether you can reach air conditioning. This guide gives a safety-first decision process, practical setups, sizing figures, and fixes for common failures.
Start with the heat-safety decision
Check the indoor temperature before choosing a cooling method. If the room is approaching or above 90°F, do not treat a fan as your primary protection. CDC guidance states: “Use fans, but only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature.” Use air conditioning, or go to a cooled location instead. CDC also advises people without home air conditioning not to rely solely on fans and to find an air-conditioned shelter or local cooling option.
For cooling locations, contact 2-1-1, your local health department, or emergency management office to ask about cooling centers and other public facilities. Availability, hours, transportation and eligibility vary by community, so verify the current details before traveling.
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- Indoor temperature below 90°F, outdoor air cooler and healthy: Use a fan for air movement and consider controlled window ventilation.
- Sun hitting windows: Close shades or curtains on those windows; reflective window coverings can send solar heat back outdoors.
- Outdoor air hotter, very humid, smoky or polluted: Keep windows closed and use air conditioning or relocate to a cooled building.
- Indoor temperature above 90°F: Do not rely on fans or open windows as the main protective measure. Get mechanical cooling or move to an air-conditioned location.
These are safety boundaries, not comfort guarantees. A room below 90°F can still be dangerous for someone who is ill, dehydrated, pregnant, very young, older, or taking heat-sensitive medications. Check on people who live alone or have chronic conditions, and seek medical help for signs of heat illness.
CDC prevention guidance is available at About Heat and Your Health: Prevention, with additional advice for households without air conditioning at Heat and People without Air Conditioning.
What each cooling method actually does
Fans: air movement, not heat removal
A fan moves air across skin and can make a person feel cooler through evaporation. It does not lower the air temperature or remove heat from a room. When the room is very hot, moving hot air over the body can add heat instead of removing it, which is why CDC sets the 90°F limit for fan use.
Use a ceiling, pedestal or box fan to improve comfort while the room remains below 90°F. Point the airflow toward occupied areas rather than assuming a higher speed is always safer. Turn fans off when nobody is in the room; they cool people, not empty spaces.
Shades and curtains: stop solar heat before it enters
Window coverings reduce incoming sunlight. Close drapes, blinds or shades on the sun-facing side before direct sun reaches the glass, and keep them closed during the hottest part of the day. Ready.gov recommends drapes or shades and window reflectors designed to reflect heat outside. Reflective products should be installed according to their instructions so they do not create a hazard or interfere with window operation.
Shades do not mechanically cool a room. They reduce the heat load that an air conditioner, fan or later evening ventilation must handle. No universal temperature reduction is established for a particular shade, curtain or reflector, so treat them as heat-blocking measures rather than a guaranteed number of degrees.
Ventilation: useful only under the right outdoor conditions
Opening windows can flush out indoor heat when outdoor air is cooler and reasonably dry and clean. It can make a home hotter when outdoor air is hotter or more humid. The U.S. Department of Energy (DOE) specifically notes that ventilation is not an effective cooling strategy in hot, humid climates.
Do not draw in wildfire smoke, heavy traffic pollution, dust, ozone alerts or other unhealthy air. Check local air-quality information and close windows when outdoor air is unsafe. Ventilation also has little value when outdoor humidity is so high that evaporation is limited.
Air conditioning: removes heat and usually moisture
An air conditioner transfers heat outdoors and, during normal operation, removes moisture from indoor air. It is the dependable choice when indoor temperatures are high, outdoor air is not suitable for ventilation, or occupants are vulnerable to heat. A correctly sized room unit is important: ENERGY STAR warns that an oversized air conditioner can cool a room before removing enough humidity, leaving a damp, clammy feeling.
A condition-by-condition cooling plan
Case 1: The room is below 90°F and outdoor air is cooler
- Close shades on windows receiving direct sun.
- Open windows on the cooler, shaded side if outdoor air is clean.
- Use a fan to move air across occupants.
- If the layout permits cross-ventilation, place a window fan to exhaust air outward from an upstairs or leeward window, while opening windows elsewhere for intake. DOE guidance describes this as a possible setup; its effectiveness depends on the building layout, wind and temperature difference.
- Close windows and coverings when outdoor air becomes hotter than indoors or air quality deteriorates.
An exhaust fan should send air out rather than simply blowing more hot outdoor air into the room. Never leave a window fan unattended where rain, unsecured windows or electrical hazards are possible.
Case 2: Sun is heating the windows
- Identify windows receiving direct morning, afternoon or low-angle sun.
- Close interior shades or curtains before the sun reaches the glass.
- Use a reflector designed to reflect heat outward if it is suitable for your window and permitted by local rules or building requirements.
- Keep doors between sunny rooms and cooler occupied rooms closed when practical.
- Run air conditioning after reducing the solar load, rather than expecting the covering alone to cool the room.
Exterior shading can be effective because it blocks sunlight before it passes through the glass, but the supplied guidance does not provide a single performance figure for any shade type. Choose products based on safe installation, window compatibility and the amount of sun they block.
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Keep windows closed and stop relying on window fans. Close shades on sun-exposed windows and use air conditioning. If you do not have AC, move occupants to a cooled public or community location; CDC recommends this rather than relying solely on fans. Use a cool shower or bath, drink fluids as appropriate, and avoid adding indoor heat while arranging a cooler location.
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Case 4: Indoor temperature is above 90°F
Treat this as a protective-action threshold. Turn off fans if they are the only intervention and are making occupants feel hotter, and arrange air conditioning or relocation immediately. Do not wait for a fan, shade or open window to produce a predictable temperature drop; no comparable reduction figure has been established for those methods.
How to use fans safely and effectively
Choose the fan position
- Ceiling fan: Use the normal cooling direction that pushes air down onto occupants. Reverse-direction settings intended for winter circulation are not the choice for direct cooling.
- Pedestal or box fan: Aim across the occupied zone, not at a closed wall. Lower speeds may provide adequate comfort with less noise.
- Window fan: Use as an exhaust fan when outdoor air is cooler and clean; provide another opening for replacement air. Stop when outdoor conditions are worse than indoor conditions.
Prevent common fan mistakes
- Do not use a fan as a substitute for AC above 90°F indoors.
- Do not blow outdoor smoke or polluted air into the home.
- Keep cords, plugs and fan motors away from water used for cooling.
- Keep blades and grilles clear of curtains and bedding, and place portable fans on a stable surface.
- Do not assume a fan lowers the room temperature; check a thermometer rather than judging only by airflow.
Fans can be paired with AC to distribute cooled air through a larger room, allowing occupants to use a higher thermostat setting while maintaining comfort. The fan still does not replace the AC’s heat and moisture removal.
Blocking heat with shades, curtains and reflectors
Prioritize the windows that matter
Start with glass receiving direct sunlight, especially broad west-facing exposures during afternoon heat. Close coverings on those windows first. If a room has several windows, cover the sunny ones while leaving shaded windows available for ventilation only when outdoor conditions are favorable.
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Select and install coverings carefully
- Use drapes, blinds or shades that fully cover the window opening.
- Choose reflective products specifically designed to reflect heat outside when that is the goal.
- Follow the product’s mounting instructions and keep emergency exits and window operation available where required.
- Inspect coverings for gaps around the edges; a partially exposed pane still admits direct sunlight.
Window coverings reduce heat entering through glass but do not cool a room that is already overheated. Combine them with AC or safe evening ventilation.
Choosing a room air conditioner
Measure the room first
Measure length and width and multiply them to get floor area. For an open plan, estimate the total area that the unit must cool and consider whether doors can remain open. A room unit cools the space it can exchange air with; walls, closed doors, high ceilings and strong sun can change the practical requirement.
ENERGY STAR starting capacities
The following room-area bands come from ENERGY STAR’s current Room Air Conditioners guidance, whose specification criteria are effective October 30, 2023. They are starting points, not a guarantee for every building.
| Room area | Starting capacity |
|---|---|
| 100–150 square feet | 5,000 BTU/hour |
| 150–250 square feet | 6,000 BTU/hour |
| 250–300 square feet | 7,000 BTU/hour |
| 300–350 square feet | 8,000 BTU/hour |
| 350–400 square feet | 9,000 BTU/hour |
| 400–450 square feet | 10,000 BTU/hour |
| 450–550 square feet | 12,000 BTU/hour |
| 550–700 square feet | 14,000 BTU/hour |
| 700–1,000 square feet | 18,000 BTU/hour |
| 1,000–1,200 square feet | 21,000 BTU/hour |
| 1,200–1,400 square feet | 23,000 BTU/hour |
| 1,400–1,500 square feet | 24,000 BTU/hour |
| 1,500–2,000 square feet | 30,000 BTU/hour |
| 2,000–2,500 square feet | 34,000 BTU/hour |
Use the area band closest to the actual space, then apply the adjustments below. Do not select a larger unit simply because it promises faster cooling; oversizing can leave humidity behind.
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- Heavy shade: Reduce the starting capacity by 10%.
- Very sunny room: Increase the starting capacity by 10%.
- Occupancy: Add 600 BTU/hour for each regular occupant beyond two.
- Kitchen: Add 4,000 BTU/hour when the unit serves a kitchen.
These adjustments are from ENERGY STAR’s room-air-conditioner guidance. For example, a 300-square-foot room starts at 7,000 BTU/hour. A very sunny room would add 10% (700 BTU/hour), producing a 7,700-BTU/hour calculation before any occupancy or kitchen adjustment. Select the nearest available certified capacity rather than treating the arithmetic as a promise of exact performance.
Check more than BTUs
- Window fit: Confirm the minimum and maximum opening dimensions, sill depth and support requirements.
- Airflow direction: Make sure the discharge path reaches the occupied area and the hot side can exhaust outdoors.
- Noise: Compare published sound ratings if the unit will operate in a bedroom or work area.
- Efficiency: Look for current ENERGY STAR certification and compare efficiency information on the product label.
- Controls: Features such as a thermostat, timer or variable-speed operation can affect comfort and energy use, but do not compensate for incorrect sizing.
- Electrical capacity: Use the required circuit and plug configuration; do not defeat grounding or overload an extension cord.
ENERGY STAR provides a rebate finder on its room-air-conditioner page, but local offers, eligibility and dates change. Verify the current certification and any rebate directly at ENERGY STAR Room Air Conditioners.
Install and operate a window AC correctly
- Read the installation requirements. Confirm the window type, opening size, support hardware, drainage instructions and electrical requirements.
- Stabilize the unit. Use the supplied brackets, rails and fasteners. A heavy unit should not rest precariously on a movable sash.
- Seal the side gaps. Install the included insulation panels or approved sealing materials so hot outdoor air cannot flow around the unit.
- Keep the outdoor side clear. Do not block the condenser grille with furniture, curtains or debris.
- Keep heat sources away from the thermostat. Lamps, televisions and other appliances can make the control sense a warmer local temperature and run longer.
- Set a stable temperature. Avoid extreme settings that cause long, inefficient cycles; allow the unit time to remove both heat and moisture.
- Clean the filter as directed. A clogged filter restricts airflow and reduces cooling performance. Turn the unit off before servicing.
- Inspect the seal after installation. Check for daylight, drafts, water intrusion or movement, and correct the problem before extended operation.
Improvised mounting, uncovered gaps and blocked airflow can create safety risks and undermine the unit’s capacity. If the window cannot safely support the model, use another cooling option or professional installation rather than forcing a fit.
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Use a cooled location when heat is dangerous
If indoor temperatures are above 90°F or occupants are becoming ill, the safest substitute for home AC is a cooled public or private location. Ask local officials, libraries, community centers or emergency services about current cooling options; CDC specifically points people toward 2-1-1, local health departments and emergency management.
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Reduce heat production indoors
- Use the stove and oven less during the hottest period; choose foods that require little or no cooking.
- Turn off unnecessary lights and heat-producing electronics.
- Close shades on sunny windows.
- Use a cool shower or bath to help lower body temperature.
- Keep vulnerable people, children and pets in the coolest available room and check them frequently.
These actions reduce heat exposure but do not make an overheated home safe indefinitely. Continue pursuing air conditioning or relocation.
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Ventilate only during a cooler outdoor window
When evening or early-morning outdoor air is cooler, open windows on opposite sides if the air is clean and the building layout supports cross-flow. Use an exhaust fan outward on an upper or leeward opening and an intake opening elsewhere. Close everything when outdoor temperature, humidity or pollution rises. In a hot, humid climate, DOE says ventilation is not an effective general cooling strategy, so do not wait for it to solve severe indoor heat.
Reduce the heat your home gains
Manage sun exposure
Close coverings before direct sun enters. Focus first on large, unshaded panes and rooms occupied during the afternoon. Keep unused rooms closed if doing so prevents cooled air from escaping into areas that do not need it.
Limit appliance heat
Delay laundry, baking and other heat-producing tasks until cooler hours. Use the stove and oven less, as CDC recommends for people without air conditioning. Turn off devices that are not needed, especially those that exhaust warm air into the room.
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Keep cooling targeted
Room AC works best when doors and windows are closed and the cooled area is limited to the space occupants use. A fan can distribute conditioned air within that area, but opening a window or leaving a door to a hot, sunny space open can overwhelm the unit.
Troubleshoot common cooling failures
The fan is running but the room feels hotter
Check the thermometer. If indoor temperature is near or above 90°F, stop treating the fan as the solution and move to AC or a cooled location. If the room is below 90°F, verify that the fan is moving air across people rather than drawing hot or polluted outdoor air inside.
Opening windows made the room hotter
Close the windows when outdoor air is hotter or more humid than indoors. Remove window fans from intake duty and use shades on sunny glass. Reopen only during a cooler, clean-air period.
The AC cools quickly but the room feels clammy
The unit may be oversized, cooling the air before removing enough moisture. Recheck the room area, sun and occupancy adjustments, and compare the installed capacity with the ENERGY STAR starting table. Keep doors and windows closed, clean the filter and ensure the outdoor side can exhaust heat.
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- Inspect and clean the filter according to the manual.
- Check that side panels and window gaps are sealed.
- Confirm that shades are closed on sunny windows.
- Make sure the condenser side is not blocked.
- Check whether the room is larger, sunnier or more occupied than the sizing assumptions.
- If the unit is properly installed but still cannot cool, arrange qualified service or move occupants to a cooled space.
The window unit does not fit securely
Do not operate a unit that is unstable, unsupported or leaving large gaps. Recheck the manufacturer’s dimensions and brackets. Use a different window, a correctly sized model or professional help; sealing an unsafe installation with improvised materials is not an adequate fix.
Protect people, not just the room
Heat risk depends on the person as well as the thermometer. Check frequently on older adults, infants, people with chronic conditions, people taking medications that affect heat response, and anyone who lives alone. Move them to air conditioning before conditions become severe. Keep water available when appropriate, use cool showers or baths, and reduce strenuous activity during the hottest hours.
Watch for confusion, fainting, severe weakness, unusual behavior, hot dry skin or other signs of heat illness. Call emergency services for a medical emergency. Do not delay professional help while trying another fan position or shade.
Put the methods together: a practical sequence
- Measure indoor temperature. If it is above 90°F, arrange AC or relocation immediately.
- Check outdoor conditions. Compare outdoor temperature, humidity and air quality with indoor conditions.
- Block sunlight. Close shades or curtains on sun-exposed windows.
- Choose ventilation or closure. Ventilate only when outdoor air is cooler and safe; otherwise keep windows closed.
- Add air movement. Use a fan for comfort below 90°F, positioned toward occupants.
- Provide mechanical cooling when needed. Operate a properly sized, securely installed AC and seal the window gaps.
- Reduce indoor heat. Use the stove and oven less, turn off unnecessary heat-producing equipment and limit the cooled area.
- Recheck people and temperature. Conditions change quickly during a heat wave; repeat the assessment and relocate vulnerable occupants if cooling is inadequate.
For detailed federal guidance, see DOE Home Cooling, DOE’s Building America HVAC Guide, and Ready.gov Extreme Heat.
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