fogged up inside windows

We probably have similar HVAC systems. Prius windshields have always been subject to condensation, or at least all of mine have been. It only happens under very specific temperature and humidity conditions. High humidity and an evening with clear skies that will promote radiational cooling of the windshield. It can also happen when the vehicle is warming up and set interior temperature is higher than outside temperature and the heater core evaporates any moisture accumulated in the HVAC box which then condenses on your colder windscreen. The defrost cycle runs the A/C to dry the air directed at the windshield, so it accumulates the moisture which then after turning off the defroster evaporates and recondenses on your windshield. Gets to be a vicious circle sometimes. In these specific moisture and temperature conditions I usually just keep the airflow directed at the windshield in the defrost mode position but have the HVAC off. If you manually turn on the fan to set this position then turn it off it will bring outside air in and direct it across the inside of the windshield. This will usually work at highway speed, but looses efficiency at slower speeds. If you turn the fan on manually it will trigger the A/C cycle and you'll be accumulating moisture in the HVAC box again.

 

For starters, make sure any moisture stored in the car is removed. That usually means wet clothing or coats or boots or gym clothes, damp mats or carpet, water spillage or accumulation in the trunk, etc. Any wet mats should be removed and dried outside the car. Any rags or towels used to wipe morning condensation from the inside windows must be removed and replaced with dry, otherwise the moisture will migrate overnight back to the windows. Wet carpets demand more work to get them dried out.

When driving, if not using AC, then do keep the climate control operating and blowing in some fresh outside air. It you always leave it on Recirculate, the car may not adequately flush out the moisture of ordinary human respiration. Any mammal that breathes, emits this moisture

If the above isn't enough, then consider turning off Recirculate when using AC, to improve the flushout of this moisture.

I never turn my climate control completely off, but leave the fan on, even if at lowest speed, to keep some fresh air coming in to force moisture out.

If parked outside, it might be useful to put a frost cover over the windshield at night to keep it from getting so cold. This should reduce overnight condensation.

If that still isn't enough, it might be time to get some desiccant, such as DampRid. But this should be a last resort.

 

All time spent in the car while it isn't ventilating, would certainly add to the problem.

Cleaning the windows, per se, won't help, the moisture causing the problem is still present. But coating or wiping them with certain anti-fog products, just might. Not using any myself, I don't have any recommendations.

 

Standard practice is to drive with A/C on if it's fogging up. We lived in a high-humidity area where there was often fog outside - my first couple of cars with no A/C would fog up at a moment's notice. Towels, heater on full etc only partly worked.

Bought a 1974 VOLVO with A/C - and reading the Owner's Manual it mentioned defrosting/defogging with A/C. So we tried it first night out - the fog just peeled from the windscreen.

A/C dries the air - and you'd have noticed a puddle underneath the car where the condensate from the evaporator dribbles out. Up there, we had to keep a container in the garage to collect the huge amount of water.

 

Yes - I think RAIN-X (or something similar??) makes something which is supposed to do that. Personally - I found it worked a little in a marginally moist environment - but as it doesn't actually remove any moisture, is quite limited - and in a properly foggy area, it just gives up.

 

No real way around this. Using A/C removes a bunch of humidity quickly. You would have to get the windshield and glass really warm so that moisture can't condense on the inside of it.

Wiping the windshields to defog (until the defroster got "hot") was normal in the days before A/C and rear window defoggers were standard equipment.

....OR, you can stop breathing. When you exhale, your body releases a bunch of water vapor. That increases the humidity inside the car, = fog.

Posted via the PriusChat mobile app.

 

What's wrong with just running the A/C? It's designed to dehumidify the cabin air keeping the windows clear.

 

breath through your nose, limit talking.

 

if you don't want to have to open the windows after turning the defroster off, leave the defroster on longer to compensate. After a few times you should be able to figure out how long you need defrost on to be able to make your trip (if it's the same trip) without having to open the windows.
Another member posted a year or two ago that power mode doesn't cycle the HAVC system the same as ECO mode. But if you're turning off the defroster manually you are cycling the HAVC system yourself.

 

I keep one of these gizmos in the centre console:

 

We manage without AC in rainy weather, at least 50% of time. Just ever-so-slightly cracking the front windows helps a lot. And setting climate mode to heat/defog. Come to think of it, that kicks on the AC, without showing an indicator light on the AC button, albeit at lower wattage (monitored AC compressor watt consumption with ScanGauge II).

I try to turn the AC off at least a mile before destination, purposely run the fan speed and temp up, to try to clear the humidity off the evaporator in the dash. NEVER want to be dealing with accessing that puppy.

 

There's one specific place we park, near ocean shore, and on cool/cold nights, getting back in the car, the inside of the windshield would be very fogged up. Sitting for 5~10 minutes watching the vent system try to deal with it gets very frustrating; with that paddle we're on our way much faster.

 

Reducing the source of the moisture inside is the best way to prevent this from happening. I keep a couple large bags of desiccant in a car and sometimes leave them on the top of the front dash to help reduce the moisture.


I also keep a temperature/humidity sensor inside the cabin to monitor both temperature and relative humidity. You can anticipate when condensation occurs on the window based on the cabin temperature and humidity and the dew point using a calculator like this one. Dew Point Calculator

If the humidity gets too high and you expect the outside temp is low enough to cause the glass surface temperature to get down to the dew point, then you either have to increase the glass surface temperature using HVAC sending warm air to the window or decrease the cabin humidity by opening a window or both. I rarely have to resort to turning on the A/C on my PP. In my experience, as long as I keep the RH in the cabin to be below 40%, I rarely get condensation on the glass.



You can also use the anti-fog agent on the glass surface to retard the fog (small water droplet) formation. Rain-X makes specifically formulated anti-fog spray for this purpose. Rain-x Interior Glass Anti-Fog - 12 fl. oz - 630046W - Walmart.com It generally works but does wear off with time, so needs to reapply frequently. Also, some anti-fog spray makes visibility and reflection of the front glass irregular. You can get a very similar anti-fog effect using shaving foam to coat the surface of the window. If you don't want to go out and get a bottle of anti-fog spray, and already have shaving foam (do not use the gel type) give it a try on the side window first.

 

The RainX they sold once as a demister for inside - doesn't work.