What Causes Dampness Inside the Car on Rainy Days?

[CalebAnn]

It is generally due to water leakage inside the car, which often occurs at the door or trunk openings. The main causes are deteriorated, aged, or broken rubber waterproof seals, which can be replaced. Below are some dehumidification methods for a damp car interior: 1. Turn on the AC for dehumidification: When the air inside the car is humid, turn on the AC cold air. The car AC can regulate cabin temperature, similar to cooling the car before driving in summer. This not only quickly circulates air but also rapidly removes moisture. 2. Give the car a 'sunbath': Sunlight contains ultraviolet, infrared, and visible light. Like humans, cars also benefit from sunbathing, especially during damp and rainy seasons. The heated car body quickly expels accumulated moisture. 3. Frequently replace interior items: Carpets, seat covers, and other cotton/linen materials in the car are prone to dampness, mold, and bacteria growth. Therefore, it's best to air-dry or regularly replace these items.

[Ursula]

I can totally relate to this, especially when driving older cars. The door seals become aged and deformed after just three to five years of use, turning stiff to the touch, and rainwater seeps in through the door gaps. If the rubber around the window edges cracks, you can even feel the moisture hitting your face while driving. Sunroof-equipped cars are even more troublesome—once the rubber tracks wear out, heavy rain causes the headliner to drip incessantly. One time, I didn’t even notice water pooling in the back seat, and when I lifted the floor mat half a month later, it was covered in mold spots. Now, every six months, I use a high-pressure car wash gun to flush out the four drainage holes of the sunroof, and I replace any seals that turn white or hard as soon as I spot them. Don’t forget to check the trunk seal either—last year, I neglected it, and rainwater leaked in, rusting the spare tire.

[Gideon]

I reckon this mostly has to do with AC usage. Turning on recirculation mode immediately to dehumidify when getting in the car on rainy days is indeed crucial, but frequently using the wrong mode makes things worse. Once I kept recirculation on for a full hour, yet the windows remained foggy. The mechanic said my cabin air filter hadn't been changed for six months and was completely clogged with dust, rendering it incapable of absorbing moisture. Here's my current trick: After ignition, first switch to fresh air mode for 30 seconds to vent stale air, then revert to recirculation with AC cooling activated. Once window condensation clears, switch to ventilation mode. For extremely muggy rainy days, placing small desiccant bags on the dashboard works wonders. Also remember to clean AC vents monthly - cabin filters must be replaced within five months at the latest, otherwise you'll always get that damp rag odor lingering in the car.

[CalvinMarie]

Ultimately, it's the rainy weather itself to blame. When humidity hits 90%, just the moisture from passengers breathing can instantly fog up the windshield. Last week while driving my kids to school, three soaked people got in with wet jackets tossed on the backseat—the front windows turned completely white in under three minutes. Parking under trees makes it even worse; once I found water droplets condensed inside the door handles. My solution is simple: shake the umbrella dry thoroughly before entering, and store wet coats in waterproof bags in the trunk. Keeping a dedicated defogging spray handy helps clear mist faster. Of course, keeping a box of tissues in the car for quick window wipes is practical too—far better than using sleeves to smear the glass.

[Wesley]

A professional mechanic told me that clogged drainage systems in cars can be the most troublesome. The most common issue is the thin drain pipes at the four corners of the sunroof—when they get blocked by willow catkins or leaves, rainwater can backflow directly into the headliner. My SUV had this exact problem; I only noticed it when water seeped near the reading light after rain. The mechanic used an air gun to clear the drain pipe from the engine bay and removed half a bottle cap's worth of sludge. It's also common for the drainage holes along the bottom edge of the doors to get clogged with sand and mud, leading to soaked carpets if the water can't drain. Since then, I've made it a habit to check these drainage points every time I wash the car, especially the small hole under the driver’s door—regularly poking it with a thin wire is the safest bet. If you notice any water stains on the roof, check immediately, or else the soundproofing foam will get soaked and require a full replacement.