Prepping for a heatwave (Part 2): Passive cooling tactics
The grandmother who kept her brick bungalow survivable through the worst week of the heat dome had one trick the neighbors laughed at until the blackout: aluminum foil on the windows, shiny side out, mounted on cardboard. It looked like the house was wearing tinfoil to church. Her upstairs stayed fifteen degrees cooler than the neighbor’s, whose “nice curtains” absorbed the sun all day and quietly turned the living room into an oven. The ugliest house on the block was the coolest, and physics does not care about curb appeal.
Heatwaves place massive stress on the electrical grid. Transformers blow, lines sag, and power stations overheat. In a severe heat event, you must assume the air conditioning will fail. A modern home without AC is effectively a greenhouse, sunlight enters through the glass, heats the air and furniture inside, and the insulation traps that heat. Within hours, the indoor temperature can soar dangerously higher than the outdoor temperature.
By employing passive cooling techniques, methods that require no electricity, you can keep your home’s interior 10 to 20 degrees cooler than the outside air. This temperature differential is the margin between discomfort and heat stroke. This is Part 2 of the heatwave series.
Disclaimer: The information provided in this article is for general informational and educational purposes only. It is not intended as, and should not be considered, architectural or professional advice.
The Readiness Audit
Could you keep your house 15 degrees cooler with no power?
- Green: You have exterior shading or foil-ready cardboard, blackout curtains, and know your night-purge routine.
- Yellow: You have blinds but have never managed windows on a schedule.
- Red: You assume the AC will always work.
If you are Yellow or Red, execute Phase 1 immediately.
Phase 1: Block the Solar Gain (Shield the Glass)
Goal: Intercept sunlight before it enters.
Once sunlight hits your window glass, the heat is already inside. You must intercept it.
- Exterior shading (the gold standard): blocking the sun outside the window is 7x more effective than blocking it inside with blinds. Install exterior shutters, awnings, or solar shade screens, even hanging a tarp or bamboo shade outside the window frame makes a massive difference.
- Interior shading (the backup): use thick, lined blackout curtains. The side facing the window should be white or reflective to bounce heat back out, dark curtains absorb heat and become radiators.
- The foil hack (emergency protocol): tape standard kitchen foil directly to the window glass, shiny side facing out. It reflects about 95% of radiant heat. It looks ugly, but it keeps the room habitable. Mount it on cardboard sheets for easier removal and reuse.
The Takeaway: The sun must be fought at the glass, outside first and inside second, and the foil-on-cardboard trick is the poor household’s 7x advantage.
Phase 2: The Cool Room Strategy
Goal: Retreat to a defensible perimeter.
Do not try to keep a 2,000 sq ft house cool without power, it is impossible.
- Select the bunker: choose the lowest floor (heat rises, the basement is king), a north-facing room to avoid direct solar gain, and a small volume that is easier to manage.
- Seal the perimeter: close the door to the cool room and stuff towels under the door gap. The family sleeps, eats, and exists here, the rest of the house is surrendered to the heat.
The Takeaway: The cool room is chosen by floor level, compass direction, and smallness, then defended like a position.
Phase 3: Air Management (The Night Purge)
Goal: Become a human thermostat.
You must manage the windows aggressively.
- The daytime seal: from sunrise to sunset, windows and doors stay closed. If it is 100°F outside and 85°F inside, opening a window does not let a breeze in, it lets the heat in.
- The night purge: monitor the temperature, and the moment the outside air drops below the inside temperature (usually late evening), open everything. Use the chimney effect, open low windows on the ground floor and high windows on the second floor so hot air escapes the top and cool air is drawn in the bottom. Place box fans blowing out on the top floor and in on the bottom floor to force circulation. Close everything again the moment the sun comes up (5:00 to 6:00 AM).
The Takeaway: The house breathes at night and holds its breath all day, and the schedule is kept by a thermometer, not a feeling.
The Essential Kit Checklist
- The reflector: 2 rolls of heavy-duty aluminum foil and painter’s tape.
- The barrier: blackout curtains installed on west and south windows.
- The monitor: an indoor/outdoor thermometer to know exactly when to open windows.
- The fans: battery-operated fans for the cool room.
- The insulation: cardboard boxes broken down to mount foil on.
Scenario Planner (Contingencies)
“It’s hotter inside than outside, but the sun is shining.” The trap: the greenhouse effect trapping heat. The fix: cross-ventilation, open windows, but only those in the shade, and create a wind tunnel. Hang wet sheets over the open windows so incoming air passes through the wet fabric and cools slightly via evaporation (a poor man’s swamp cooler).
“I live in an apartment on the top floor.” The trap: heat rising from units below you. The fix: the floor bed, sleep on the floor, where the coolest air settles. If you have a balcony, shade the sliding glass door from the outside using a blanket or tarp to stop the solar gain.
Next Steps
- Do the curtain audit this weekend. West and south windows get reflective blackout curtains, and two sheets of cardboard get foil-mounted and staged.
- Rehearse the night purge once. Low windows in, high windows out, close at sunrise. One rehearsal makes it muscle memory for the whole family.
- Read Part 3: The human radiator next, because a cool house still needs a cool body inside it, and the body is its own cooling system to engineer.
Originally adapted from public DOE and FEMA passive-cooling guidance (public domain).