How to Keep Your House Cool in a Heatwave

A heatwave is not a hot day — it is multiple consecutive days of extreme heat where your home's ability to cool down overnight is overwhelmed. Buildings absorb more heat each day than they can shed at night, creating a ratchet effect where indoor temperatures climb steadily higher. By day three of a heatwave, your home may be 10-15°F warmer inside than it was on day one, even with the same outdoor temperatures. This guide covers emergency-level cooling strategies specifically for sustained extreme heat — not just a warm afternoon.

The Heatwave Ratchet: Why Your Home Gets Hotter Each Day

During normal summer weather, nighttime temperatures drop enough for your home's thermal mass (walls, floors, furniture) to release stored heat. By morning, the interior has reset to a comfortable starting point. During a heatwave:

• Nighttime temperatures stay elevated (often above 75-80°F)

• Thermal mass cannot fully discharge overnight

• Each morning starts warmer than the last

• By day 3-5, indoor temperatures may be 5-15°F higher than outdoor nighttime lows because the building itself has become a heat reservoir

This is why heatwave cooling requires more aggressive tactics than normal summer strategies. You are not just fighting today's heat — you are fighting the accumulated heat of the previous days stored in your walls and floors.

Day 1: Immediate Defense

1. Close Everything During the Day

From 8 AM to 8 PM (or whenever outdoor temp exceeds indoor), close every window, door, blind, and curtain. Your home is cooler than the outside air — every opening lets hot air in. This is counterintuitive for people used to "letting fresh air in," but during a heatwave, you are defending your indoor temperature like a fortress.

• Close blinds and curtains on ALL windows, not just sun-facing ones (reflected heat enters from all directions)

• Close interior doors to unused rooms — cool the rooms you use, abandon the rest

• Hang light-colored sheets or emergency blankets over windows that receive direct sun if you do not have curtains

2. Flush with Cool Air at Night

When outdoor temperature finally drops below indoor (typically 9 PM to 7 AM during a heatwave), open everything. Place fans in windows — facing outward to exhaust hot indoor air, and open windows on the opposite side of the house to draw in cooler night air. The goal is to flush as much stored heat as possible before morning.

A whole-house fan, if you have one, is invaluable here — it can exchange the entire home's air volume in 3-4 minutes, dramatically accelerating overnight cooling.

3. Eliminate Internal Heat Sources

During a heatwave, every watt of heat generated inside your home matters:

• Do not use the oven — An oven generates 3,000-5,000 BTU of residual heat. Grill outside, use a microwave, or eat cold meals.

• Run dishwasher and laundry at night — or not at all during the worst days

• Switch off all unnecessary electronics — Desktop computers, gaming consoles, TVs all generate significant heat

• Use LED lighting only — Incandescent bulbs convert 90% of energy to heat

• Limit hot water use — Showers add heat and humidity; take brief, lukewarm showers

Days 2-3: Escalation

4. Designate a "Cool Room"

Stop trying to cool the entire house. Pick one room — ideally on the ground floor, north-facing, with minimal windows — and concentrate all cooling efforts there. Close doors to the rest of the house. This is your household's base of operations during peak heat hours (noon to 8 PM).

The cool room should have:

• Your most effective cooling device (AC, fan, or personal cooler)

• Water and snacks

• Activities to keep everyone occupied

• A way to monitor indoor temperature (a simple thermometer)

5. Use Water for Emergency Cooling

When mechanical cooling is insufficient, water-based cooling provides significant relief:

• Wet sheet over a fan — Hang a damp (not dripping) sheet in front of a fan. As air passes through, evaporation cools it by 5-10°F. Re-wet the sheet every 30-60 minutes.

• Wet towels on neck and wrists — Pulse points where blood runs close to the surface allow efficient body cooling

• Cool foot bath — Fill a basin with cool water and sit with your feet in it. Your feet have extensive blood vessel networks that transfer cooled blood throughout your body.

• Mist spray — A spray bottle of cold water misted on skin provides immediate evaporative cooling

6. Manage Your AC Wisely

If you have AC, resist the urge to set it as low as possible. During a heatwave, electrical grids are under extreme strain. Rolling blackouts are common. If your AC is running at full capacity and the power goes out, your home heats up rapidly with no backup plan.

• Set the thermostat to 78°F and use fans for wind chill, rather than 72°F with no fans

• Pre-cool the house aggressively during early morning hours when demand (and electricity rates) are lowest

• Have a backup plan: battery-powered fans, personal coolers, or a plan to relocate to a cooling center if power fails

7. Deploy Personal Cooling

During a heatwave, the most energy-efficient approach is cooling people, not spaces. A personal evaporative cooler on a desk or nightstand creates a comfortable 3-4 foot zone using just 7-12 watts — critical if you are trying to reduce grid strain or survive a power outage with a small battery.

Devices like the Evapolar are especially valuable during heatwave conditions because:

• They continue working if the grid goes down (7-12W runs on a small portable battery for 12-24 hours)

• They add moisture to air that may be dried out by heavy AC use

• They allow you to keep the AC thermostat higher (saving energy and reducing blackout risk) while maintaining personal comfort

• Multiple units for different family members cost less to run combined than a single AC turned down 2 extra degrees

Days 4+: Extended Heatwave Protocol

8. Know When Your Home Is No Longer Safe

Consider leaving your home and going to a cooling center if:

• Indoor temperature exceeds 90°F and you cannot cool it below 85°F

• Anyone in the household is elderly, very young, or has a chronic health condition

• Power has been out for more than 2 hours with no sign of restoration

• Anyone shows signs of heat exhaustion (dizziness, nausea, heavy sweating, cool clammy skin)

Call 211 to locate your nearest cooling center, or go to any air-conditioned public space: library, mall, community center, movie theater.

9. Emergency Exterior Cooling

Hosing down your roof with water (if you have a garden hose and your roof is safe to reach with spray) can reduce roof surface temperature by 40-50°F through evaporation. This reduces heat radiating through the ceiling into the living space. This is an emergency measure, not a daily practice — it uses significant water and is only worthwhile during extended extreme heat.

Exterior shade structures (tarps, shade sails) rigged over the most sun-exposed walls and windows also help reduce heat gain, even if they are improvised.

10. Check on Vulnerable Neighbors

During extended heatwaves, check on elderly neighbors, people living alone, and anyone without AC at least twice daily. Many heat deaths occur because people do not recognize the signs of heat illness in themselves or do not have the means to seek help.

Preparing Before the Next Heatwave

Frequently Asked Questions

Should I open or close windows during a heatwave?

Closed during the day when outdoor temperature exceeds indoor. Open at night when outdoor temperature drops below indoor. During a severe heatwave, the crossover point may not come until 9-10 PM and the window for night flushing may only be 7-8 hours. Use that window aggressively with fans to exhaust as much stored heat as possible.

How hot is too hot inside a house?

For healthy adults: sustained indoor temperatures above 90°F are uncomfortable and potentially dangerous. For elderly, young children, and people with chronic conditions: above 85°F requires active intervention. Above 95°F indoors: evacuate to a cooling center regardless of age and health status. Body temperature regulation fails when ambient temperature approaches body temperature.

Does AC use more electricity during a heatwave?

Yes — significantly more. The greater the temperature difference between inside and outside, the harder the AC works and the longer the compressor runs. At 100°F outdoor temperature, your AC may run nearly continuously to maintain 75°F inside — compared to 50% runtime at 90°F. This can double your daily AC electricity cost and strain the electrical grid.

What should I do if the power goes out during a heatwave?

Open windows for ventilation, use battery-powered fans, apply wet towels for evaporative cooling, drink plenty of water, and move to the lowest level of the home (basements can be 10-15°F cooler). If indoor temperature exceeds 90°F and you cannot cool it down, go to a cooling center or any air-conditioned public space. For vulnerable individuals (elderly, infants), this should be treated as an emergency — do not wait.

Do blackout curtains really help during a heatwave?

Yes — blackout curtains with a white or reflective backing reduce solar heat gain through windows by 33-45%. In a heatwave, this is one of the most impactful low-cost interventions. For windows receiving direct sun, the difference can be 5-10°F of indoor temperature. Close them early (by 8-9 AM) and keep them closed until sun has moved past.