Cooling off in the summer

Curtains and an awning block some sun

Unlike dealing with extreme cold, one nice thing about hot weather is that, except in the most extreme cases, you at least have the option of getting out of the vehicle if it has accumulated too much heat.

Finding relief in the shade

Your first defense against extreme heat is to prevent as much heat as possible frome entering the vehicle in the first place. This means blocking sunlight from striking the vehicle and especially from penetrating the windows. If parking under a large tree is an option, that will obviously help. If you have to park in the sun without natural shade for long periods, building a canopy over the whole vehicle with a tarp will block sunlight. But a canopy is really only viable when at a park or recreational area – not on city streets or in parking lots.

For less conspicuous protection from the sun, you can cover the windows from the inside with curtains, or affix a white or reflective material (Mylar/BoPET or foil) to reduce the solar heating effect. Beware that highly reflective barriers in the windows will probably draw some attention to your vehicle. Police and thieves alike will suspect you of trying to hide something.

A more permanent action is to add or increase the tint on your windows, which provides a fair amount of protection from solar heat entering the vehicle. The laws about tinted windows vary from state to state. If you will be traveling between states, it’s better to have less tint on the windows than to keep getting pulled over in the more restrictive states.

Air circulation

To improve air flow, an effective modification is to install a vent in the roof. Heat rises. A vent with a small fan can pull the hottest air out of the vehicle using relatively little energy.

For a good source of cheap – often free – 12v DC fans, you can salvage the fans used for cooling inside computer cases. For the more electronically inclined, fan speed can be controlled by changing the voltage, or in many cases, using a method called pulsed width modulation (abbreviated PWM). These small fans move quite a bit of air and typically use 0.5 to 1 amp at 12 volts, meaning they will run on a small deep-cycle battery for many hours or even days.

Air conditioning

Mobile air conditioners usually don’t provide very much value in extreme heat (compared to even the tiniest heaters, which can save lives the cold). For one thing, air conditioners generate waste heat, which will just make the problem worse unless you come up with a good way to vent it. Air conditioners also use a tremendous amount of electricity or fuel.

That said, if the vehicle already has an air conditioner, it is worthwhile to maintain it. Just remember that using it burns quite a bit of extra fuel – enough to reduce gas mileage by 10-20%. Driving with the windows down also reduces gas mileage by some amount that increases with speed, but it’s inconclusive whether the effect is enough to even measure or not. (Simply driving faster also increases fuel consumption geometrically once you’re in top gear, so slowing down probably affects fuel use more than AC or having the windows down.)