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Use the method that matches both indoor and outdoor conditions. Close sun-facing shades before sunlight reaches the glass, use a fan for comfort only while indoor air is below 90°F, ventilate with outdoor air only when it is cooler and safe, and use air conditioning—or move to an air-conditioned location—when the room is very hot. The Centers for Disease Control and Prevention (CDC) warns: “Use fans, but only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature.” (CDC heat-prevention guidance.)
Start with heat safety, not equipment
A heat wave can make an ordinary room hazardous, especially for older adults, infants, people with chronic illness, and anyone who lives alone. CDC guidance is to use air conditioning or find a location that has it. If you do not have home air conditioning, do not rely on fans alone; find an air-conditioned shelter or another local cooling option instead. Call 2-1-1, your local health department, or emergency management office to ask about available cooling centers and transportation. Sources: CDC, About Heat and Your Health and CDC, Heat and People without Air Conditioning.
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- Below 90°F indoors: A fan can improve comfort by moving air across your skin. Combine it with shade and, when appropriate, cooler outdoor air.
- At or above 90°F indoors: Do not make a fan your primary protection. Use air conditioning or relocate to a cooled building.
- Hot, humid outdoor air: Open windows and window fans can add heat and moisture. Keep the home closed and use mechanical cooling or a cooled public space.
- Polluted or smoky outdoor air: Keep windows closed and avoid drawing that air inside.
Check regularly on people with chronic conditions and those who live alone. CDC also recommends taking cool showers or baths and using the stove and oven less during extreme heat (CDC guidance).
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A quick decision guide for your house
| Indoor and outdoor conditions | Best first action | What to avoid |
|---|---|---|
| Indoor temperature under 90°F; outdoor air is cooler, clean and reasonably dry | Close sun-facing shades, then use a fan. Ventilate with a window fan if the layout allows. | Running a fan while windows admit hotter air. |
| Sun is striking windows, but indoor temperature is still moderate | Draw curtains or shades on exposed windows; use a reflective window product designed to send heat outward. | Leaving bright, uncovered glass to heat the room. |
| Outdoor air is hotter or much more humid than indoors | Keep windows closed, shade the glass and use air conditioning if available. | Open-window ventilation as the main cooling method. |
| Indoor temperature is 90°F or higher | Use air conditioning or go to an air-conditioned location. | Relying on a fan alone. |
| No home AC and the home is becoming dangerously hot | Contact 2-1-1, a local health department or emergency management office for a cooling location. | Waiting for a fan to make an unsafe room safe. |
This approach separates heat removal from comfort. Shades reduce solar heat entering through glass. Ventilation can exchange indoor heat for cooler outdoor air. A fan mainly increases evaporation from your skin. Air conditioning removes heat and, depending on the system, moisture. No source cited here establishes a universal number of degrees that any one method will save; the result depends on weather, construction, sun exposure and room layout.
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Use fans where they help—and know their limit
What a fan actually does
A room fan does not refrigerate the room. It moves air over your skin, which can make you feel cooler while the air temperature remains nearly unchanged. That makes a fan useful for a person in a room below the CDC’s 90°F boundary, particularly when humidity is not so high that evaporation is ineffective.
Position a fan so air crosses the occupied area rather than blowing into a blocked corner. Turn it off when nobody is in the room; moving air over empty furniture does not cool the building and still uses electricity. If you are using air conditioning, a fan can help distribute cooled air between nearby spaces, allowing you to set up the room for comfort without lowering the thermostat as far.
When a window fan can ventilate
Window ventilation works only when outdoor conditions are favorable. Open windows or use a window fan when outdoor air is cooler than the indoor air, humidity is acceptable, and there is no smoke, pollution or other hazard. Close the openings again when outdoor air becomes hotter or more humid.
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- Check the outdoor temperature, humidity and air-quality advisories before opening windows.
- Choose an upstairs or leeward window for exhaust when your layout permits; point the window fan outward.
- Open windows on another side of the home for intake, keeping the path as clear as possible.
- Feel the incoming air after several minutes. If it is warm, damp, smoky or dusty, close the windows and stop ventilating.
- As the outdoor temperature rises, close windows and shades before the heat enters the room.
Do not use a fan as a high-heat rescue
CDC’s exact warning applies to indoor conditions: “Use fans, but only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature.” (CDC.) If a room has crossed that boundary, move people to air conditioning or a cooled location instead of adding more fans.
Block solar heat with shades and window coverings
Sun through glass can keep raising the indoor heat even when the outdoor temperature temporarily drops. Ready.gov recommends covering windows with drapes or shades and using window reflectors designed to reflect heat outside (Ready.gov, Extreme Heat).
Use coverings at the right time
- Identify windows receiving direct sun, including those that become exposed later in the afternoon.
- Close blinds, curtains or shades before direct sunlight reaches the glass, rather than after the room has heated.
- Use a reflector only if it is designed for window use and directs radiant heat outward; install it according to its instructions.
- Keep coverings closed during the hottest, sunniest period. Reopen them when the sun has moved away if you need daylight or a view.
Interior curtains and blinds are easy to deploy, while exterior shading can stop sunlight before it reaches the pane. The guidance does not establish one universal temperature reduction for a particular fabric, color or installation, so choose based on window orientation, fit and how much direct sun you need to block.
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Shade lowers the heat load that an air conditioner must handle; it does not mechanically cool a room. Keep sun-facing coverings closed while a window or central AC runs. If you use a window AC, do not block the unit’s intake or exhaust with a curtain. Leave the airflow path clear and use the insulation supplied with the unit to close gaps around the frame.
Air conditioning is the dependable option in dangerous heat
Air conditioning is the primary protective measure when indoor temperatures are very high or outdoor conditions prevent useful ventilation. A home system, a properly installed room unit or a cooled community location can provide the mechanical heat removal that a fan cannot.
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Choose the right type for the space
- Central or ducted AC: Useful for cooling multiple rooms when the system is operating correctly. Close doors to rooms you are not trying to cool if doing so improves distribution, but do not block supply or return vents.
- Window AC: A practical choice for one room or a small apartment when the window can safely support the unit and the unit’s capacity matches the room.
- Portable AC: Follow the manufacturer’s exhaust and clearance requirements exactly. A hose that leaks or is routed incorrectly can return heat to the room, so verify the installation rather than assuming any window opening is adequate.
ENERGY STAR emphasizes that bigger is not always better for a room air conditioner. An oversized unit may cool the room before it removes enough humidity, leaving a damp, clammy feel. Capacity should reflect floor area, sun and shade, occupancy and whether the room includes a kitchen (ENERGY STAR Room Air Conditioners).
Size a window air conditioner from the room area
Use the table as an ENERGY STAR starting point, then apply the listed adjustments. The figures are cooling capacity in BTU per hour; they are not a promise that every room will reach a particular temperature. Measure the room’s area and check the unit’s electrical, window and installation requirements before buying.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11| 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 |
| 1,400–1,600 square feet | 25,000 BTU/hour |
| 1,600–1,800 square feet | 28,000 BTU/hour |
| 1,800–2,000 square feet | 30,000 BTU/hour |
| 2,000–2,500 square feet | 34,000 BTU/hour |
These room-area bands and the following adjustments come from ENERGY STAR’s current room-air-conditioner guidance, whose specification is effective October 30, 2023 (ENERGY STAR):
- For a heavily shaded room, reduce the starting capacity by 10%.
- For a very sunny room, increase the starting capacity by 10%.
- Add 600 BTU/hour for each regular occupant beyond two.
- Add 4,000 BTU/hour when the room includes kitchen use.
For example, a 200-square-foot very sunny bedroom with three regular occupants starts at 6,000 BTU/hour. Add 10% for the sun (600 BTU/hour) and 600 BTU/hour for the third occupant, producing a calculated starting point of 7,200 BTU/hour before you check the manufacturer’s available sizes and electrical requirements. The table is a sizing guide, not a reason to choose the largest unit that fits.
Install a room AC so it can actually remove heat
- Measure before purchase. Record window width, opening height, sill strength and nearby outlet location. Confirm that the model is designed for that window style and that the circuit can support it.
- Support and level the unit. Follow the manufacturer’s brackets, angle and drainage instructions. A poorly supported unit can fall, leak or vibrate.
- Seal the side panels and frame. Use the included insulation materials to close gaps around the unit. Unsealed openings let outdoor heat and humidity bypass the cooling system.
- Keep airflow clear. Do not place curtains, furniture, lamps or televisions where they obstruct the intake or discharge. ENERGY STAR specifically advises keeping lamps and TVs away from the thermostat because their heat can make the control sense a warmer room.
- Set a realistic target. Cooling a single occupied room is more efficient than trying to force a small room unit to cool an entire open-plan home. Close doors where that improves containment, while keeping return and supply paths unobstructed.
- Maintain the filter and clearances. Clean or replace the filter as the manufacturer specifies and keep the outdoor coil area free of obstructions. A dirty filter reduces airflow and can cause icing or weak cooling.
ENERGY STAR also points readers to efficiency features such as variable-speed operation and a rebate finder. Rebate eligibility, product availability and certification listings change, so verify the current details for your location on the ENERGY STAR room-air-conditioner page.
Reduce heat made inside the home
Cooling equipment has less work to do when the house is not generating extra heat. During the hottest hours:
- Use a microwave, toaster oven or no-cook meal instead of the stove or full-size oven when practical. CDC specifically recommends using the stove and oven less during extreme heat (CDC).
- Turn off unnecessary lights and electronics that release heat into the room.
- Run heat-producing appliances, such as a clothes dryer or dishwasher, during a cooler period if you must use them.
- Take a cool shower or bath to help lower body heat; this is a personal cooling measure, not a substitute for bringing a dangerously hot room under control.
- Keep interior doors arranged to support the cooling plan. A closed door can contain conditioned air, while an open door may be necessary for a fan-assisted cross-flow; choose based on whether the system is cooling or ventilating.
A practical heat-wave schedule
Before the hot period
- Check that window coverings close fully and identify the windows that receive afternoon sun.
- Test fans, clean AC filters and inspect window seals before an emergency.
- Measure rooms you may cool with a window unit and verify safe electrical outlets and window support.
- Ask vulnerable relatives, neighbors or friends how they will reach an air-conditioned location if their home becomes unsafe.
During the morning
If the outdoor air is cooler than indoors and air quality is acceptable, ventilate briefly to release accumulated heat. Use an outward-facing window fan where the home layout supports it. Close windows and sun-facing shades before outdoor heat overtakes indoor conditions.
During the hottest hours
Keep direct sun blocked and windows closed when outdoor air is hotter or humid. Run AC in the occupied area, keep its airflow unobstructed and use a fan only for comfort while the room remains below 90°F. If indoor temperature approaches or exceeds 90°F, move people to air conditioning rather than trying to solve the problem with more fan speed.
Overnight
When outdoor air becomes cooler and safe, you can reopen windows for ventilation if doing so will not admit smoke, pollution, insects or security risks. If the night remains hot and humid, continue mechanical cooling or relocate to a cooled space.
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- Hassle-Free Operation: The extra-long three-prong power cord eliminates the need for extension cords, while the adjustable side panels easily extend to best fit your window, ensuring a secure installation with optimal efficiency and airflow
Common mistakes and how to correct them
Running a fan in a room above 90°F
Why it fails: Air movement can increase body temperature in the conditions CDC identifies as dangerous. Correction: Use AC or an air-conditioned shelter and check on people at higher risk.
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Opening windows because the air feels fresh outside
Why it fails: Outdoor air may be hotter, more humid or unhealthy even when it feels breezy. Correction: Compare outdoor and indoor conditions and check air-quality information before ventilating.
Leaving sunny windows uncovered
Why it fails: Direct solar radiation continues adding heat to the room. Correction: Close drapes or shades before sun reaches the glass and consider a reflector designed to send heat outward.
Buying the largest AC that fits
Why it fails: Oversized equipment can cool the air quickly without removing enough humidity, producing a clammy room. Correction: Start with the ENERGY STAR area table and apply shade, sun, occupancy and kitchen adjustments.
Leaving gaps around a window unit
Why it fails: Hot outdoor air bypasses the unit and conditioned air escapes. Correction: Install the supplied side-panel insulation and seal the frame according to the instructions.
Cooling an empty house while someone remains in a hot room
Why it fails: A cool thermostat reading elsewhere does not protect the person in the overheated room. Correction: Move the person into the cooled zone, keep doors arranged for actual airflow and check the room where they spend time.
How the options compare
| Option | Removes heat? | Best conditions | Coverage and limits | Main requirements |
|---|---|---|---|---|
| Portable or room fan | No; mainly improves perceived comfort | Indoor temperature below 90°F | Usually one room or occupant area; does not make a dangerously hot room safe | Electrical outlet and unobstructed airflow |
| Window ventilation or exhaust fan | Can remove indoor heat by exchanging air | Outdoor air cooler, clean and suitable | Depends heavily on cross-flow, floor plan and wind; ineffective in hot-humid conditions | Safe window fit, intake path and timely closing when conditions change |
| Shades, curtains or reflectors | Reduces solar heat entering through glass | Sun-facing windows | Does not cool air already inside | Correct fit and early deployment before direct sun |
| Window air conditioner | Yes, and typically removes some moisture | Hot indoor conditions or when ventilation is unsuitable | Primarily the room it serves; capacity must match area and heat load | Supported, sealed window; adequate circuit; clear intake and discharge |
| Air-conditioned public or community location | Yes, for people who relocate | Home has no AC or indoor heat is unsafe | Availability and transportation vary locally | Contact 2-1-1, local health department or emergency management |
What to do if you do not have air conditioning
Start by reducing the heat entering the home: close sun-facing shades, stop unnecessary indoor heat sources and keep windows closed when outdoor air is hotter or unhealthy. Use fans only while indoor temperatures stay below 90°F. Take cool showers or baths and drink fluids according to your health professional’s advice. If the home continues heating toward or above 90°F, do not wait for a fan to solve it. CDC recommends finding an air-conditioned shelter or other cooled location; local agencies can identify options through 2-1-1, a health department or emergency management office (CDC).
Bottom line
Use shades first to stop solar heat, fans for comfort only below 90°F, and window ventilation only when outdoor air is cooler and safe. In hot-humid weather or when indoor temperature reaches 90°F, rely on air conditioning or move to a cooled location. A properly sized, sealed room AC is more effective than an oversized unit, while regular checks on vulnerable people are as important as any equipment choice.
Quick Recap
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