12v humidity fan controller

Hello all. Just received a $12 12v humidity meter with programmable 12v output controller which I will use for a fan.

My issue is condensation inside the vehicle when it’s cold out.

I’m wondering if anyone has some insight on humidity and it’s relation to water build up inside the vehicle. Does it happen based on RH outside vs inside?

Will one small fan be enough? Is there a certain % of humidity that starts to create condensation inside? It’s just a wagon so it’s not hard to fog it up. I’m concerned about the massive amounts of water build up on all windows and especially the windshield as it drips down into the dash to who knows where.

This is the one I got. Very efficient and works exactly as it should. Great little device.


The solution for cold weather condensation is ventilation, circulation, and dry heat. The warmer, drier air can hold more moisture, which can then be exhausted outside. To be comfortable, you need enough heat to compensate for the the necessary ventilation. It doesn’t work well in the cold without the heat, as can be witnessed with your vehicle defroster before the engine warms up enough to start throwing warmer air.

The solution for warm weather condensation is ventilation, circulation, and a surface cold enough to attract and collect the moisture in the air. Most commonly a swamp cooler that incorporates a radiator for the heat exchanger, or a traditional A/C unit.

Insulation does not prevent condensation, it only hides it, and possibly traps it. Insulation alone does not provide either heating or cooling, it can only slow the transfer of heat into or out of your vehicle. Shade in the summer & sun in the winter is much more effective than any amount of insulation. Only active heating & cooling can keep you comfortable year round, with or without insulation.

Humidity is a measure of moisture in the air. Unfortunately, I don’t think humidity alone is enough of an indicator to predict when condensation can form, the calculation also involves the temperature and a formula to determine the dewpoint. Then any surface at or below the dewpoint can collect condensation.

My solution is just to run enough dry heat to keep the whole interior, windows included, above the dewpoint, without knowing the dewpoint. If condensation starts to form, like the windows start to steam up, I turn up the heat, which with proper ventilation and air circulation, will quickly defrost the windows back to dry. If your breath steams up the window, and it doesn’t dissipate fairly quickly, it’s too cold, so the heat needs to be turned up. Generally if I keep my interior in the 70’s, it prevents condensation, and keeps everything dry.


“Everything should be made as simple as possible." ~ Einstein

Wow. Excellent explanation. Can you explain details on this process for the opposite weather : summer high heat and humidity? I’m looking for help on traveling with heat/humidity sensitive electronics and hard drives.
I have window covers ( a minivan with lots of windows). But sometimes outside will be 90F, had AC blasting prior to, park at Walmart 30 minutes later the inside thermometer reads 115F! Bad for computers.
Any tips or help would be greatly appreciated.