Condensation on Car Windshields

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Condensation on Car Windshields

There are four types of window condensation, and each calls for a different fix

Posted on Feb 2 2018 by Martin Holladay

A surprising number of people don’t understand the causes of condensation. If you ask a stranger on the sidewalk, “Does condensation happen when cold air encounters a warm surface, or when warm air encounters a cold surface?,” many people will shrug their shoulders.

Here’s an example of this type of confusion: When drivers see condensation on their windshield during the summer, they are often unsure of the best remedy. Should they turn on the heater or the air conditioner?

Let’s look at four different scenarios.

1. Water droplets or frost on the outside of your windshield in winter

This is the most common type of cold-weather condensation on a windshield.

Where did the moisture come from? The moisture came from the outdoor air.

What’s the cause? Due to night-sky radiation, the temperature of the windshield dropped below the dew point of the outdoor air.

What’s the solution? If all you have is dew, you can jump in the car and use the windshield wipers. If you have frost on your windshield, you either have to scrape the outside of the windshield, or let your engine idle for a while so that the engine coolant gets warm enough to provide heat through your defroster vents.

2. Water droplets or frost on the interior of your windshield in winter

This phenomenon only happens when all of the vehicle’s windows are closed. It occurs more rarely than exterior condensation.

Where did the moisture come from? The moisture came from the air inside the vehicle.

What’s the cause? This phenomenon happens when the interior of the vehicle is very damp (as might occur if the carpeting has been soaked by melted snow from the driver’s boots). At night, the outdoor temperature drops below the temperature of the air inside the car, and the windshield is chilled by the outdoor air (even on a night without significant night sky radiation). The temperature of the windshield drops below the dew point of the interior air.

What’s the solution? If your windshield has interior frost, it can be cleared by scraping. (You’ll discover that typical scrapers have curved blades that are designed for exterior use, not interior use, making the chore somewhat difficult.) The condensation can also be cleared by operating the defroster — that is, by directing hot air to the bottom of the windshield. To clear the condensation quickly, you want the controls set to the “outdoor air” mode instead of the “recirculation” mode.

3. Water droplets on the interior of all of the car’s windows when the car is parked on a rainy day

If you’re sitting in a parked car with a friend on a cool, rainy day, with all of the car windows closed, condensation can build up on the interior side of all of the car’s windows.

Where did the moisture come from? The moisture came from the air inside the vehicle. The relative humidity of the air inside the vehicle is likely to be high, because the occupants give off a lot of moisture due to perspiration and respiration. A parked car is poorly ventilated, especially if the ventilation fan is turned off.

What’s the cause? Cool rain keeps the temperature of the vehicle’s windows below the dew point of the interior air.

What’s the solution? Open one or more windows to lower the relative humidity of the indoor air.

4. Water droplets on the exterior of your windshield in summer

This phenomenon only happens in an air-conditioned car. If the ventilation system is in the “defrost” position (aiming the air flow at the base of the windshield), it’s common to see areas of condensation at the base of the windshield. The moisture is on the exterior side of the windshield.

Where did the moisture come from? The moisture came from the outdoor air.

What’s the cause? The car’s air conditioner has cooled a portion of the windshield to a temperature that is below the dew point of the outdoor air.

What’s the solution? There are several possible solutions: (a) Operating the windshield wipers will clear the windshield temporarily. However, the condensation is likely to return rather quickly. (b) Turn on the heater and blow hot air on the windshield. (The idea is to raise the temperature of the windshield so that the windshield is no longer a condensing surface.) This works, but the solution makes occupants uncomfortable. (c) Turn off the air conditioner and open this windows. This raises the temperature of the windshield. Occupants will complain, but not as much as they would have if the heater were running. (d) Adjust the controls so that the air from the cooling system blows at the driver’s face, not at the windshield. This adjustment may allow the windshield to warm up enough to avoid condensation.

Hot air or cold air?

Most readers know that condensation happens when relatively warm air encounters a cold surface — specifically, a surface that is below the dew point of the adjacent air. That knowledge will help you figure out what to do when you’re fiddling with the levers and knobs on your car’s heating and cooling system in hopes of clearing up your foggy windshield.

Bonus question

Q. At the beginning or end of an airplane flight, you'll sometimes see what looks like condensed water vapor coming out of the air conditioning vents under the overhead baggage compartments. Is this insecticide or water vapor?

A. While some airlines do spray insecticide on commercial passenger jets — you can read about the practice here and here — insecticide isn't distributed through the air conditioning vents. What you are seeing is simply condensed water vapor, as explained here.

Martin Holladay’s previous blog: “Brick Chimneys With Multiple Flues.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Jessica Merz - Flickr

Feb 6, 2018 10:59 AM ET

Air Conditioner + Heat
by John Ranson

There's a missing solution here for condensation on the inside of your window when it's humid out (and in your car). This usually manifests as such: you get into your car and turn everything on, and suddenty all of the windows fog up entirely. The solution is to turn on your A/C, turn up the heat, and turn the system to defrost. The A/C strips humidity from the air while the engine heats the air, so you push hot dry air at the windshield. This removes condensation and warms the windshield, preventing further condensation.

Feb 6, 2018 1:19 PM ET

Response to John Ranson
by Martin Holladay

I'm not sure whether you are talking about interior condensation or exterior condensation. In your scenario, is the weather hot or is the weather cold?

I'm 63 years old, but I've never experienced what you experienced: "You get into your car and turn everything on and suddenly all of the windows fog up entirely." Perhaps you are describing a humid car filled with 4 or 5 passengers on a cold day.

In any case, you're right: If you want to lower the indoor humidity of a house or a car, simultaneous operation of the air conditioner and the heater is the fastest way to lower the indoor humidity. That said, your suggested approach isn't possible in many cars. (Many cars have controls that force you to choose either heating or cooling but don't allow both functions to operate simultaneously.)

The trick you describe is often used by disaster recovery contractors attempting to dry out a flooded house. If the HVAC systems allows it, simultaneous operation of the heating system and the air conditioning system is a fast way to dry out a house.

Feb 6, 2018 6:26 PM ET

You left one out
by Reid Baldwin

You left out the scenario in which the windows fog up while teenagers are parked in a remote location on Saturday night. (Related to #3, but without the rain.) I suppose the "solution" might depend on whether you are one of the teenagers or one of their parents.

Feb 6, 2018 7:56 PM ET

Response to Reid Baldwin
by Martin Holladay

In your scenario, the condensation is a benefit, not a problem, at least from the teenagers' perspective.

Feb 7, 2018 2:05 PM ET

what's happens in a garage
by Daniel Beideck

I remain a bit confused why frost doesn't occur in an unheated garage. If I park my car in a detached, unheated and VERY leaky garage, I never have frost on the windshield in the morning. With the open ventilation in the garage including a couple of windows open, I would think the air inside the garage must be very nearly the same as that outside the garage. Yet the conditions that produce frost on the windshield if the car were parked outside don't do the same inside the garage.

Feb 7, 2018 2:13 PM ET

Response to Daniel Beideck
by Martin Holladay

The answer is simple: parking in a garage stops night sky radiation.

You generally need a surface that is colder than the outdoor air temperature for condensation to form, and without night sky radiation, the windshield never gets cold enough to allow condensation.

For more information, see Night Sky Radiation.

Feb 8, 2018 10:19 AM ET

reply to Martin
by Daniel Beideck

I read the article on night sky radiation. I suspect the answer has something to do with that. However, I'm not sure it's the whole answer. The windshield will emit blackbody radiation, as do all objects, in a garage or outside. The IR photons from the windshield don't "know" or care if there is a roof above them. They will be emitted either way, and will therefore release energy and cool the windshield. If there is a roof above them, those photons will often be absorbed by the roof. the roof in turn will emit it's own blackbody radiation. Some of that will be absorbed by the windshield. Maybe that is enough to keep it warm enough to keep the frost at bay???

Another factor is the volume of air involved. Even though garages are often very leaky, airflow is still restricted. I don't have experience with carports. Do you know if a carport with a high ceiling will prevent frost on the windshield as effectively as a garage? A carport will block night sky radiation similar to a garage, but will have airflow pretty similar to the car parked outside.

Feb 8, 2018 11:31 AM ET

Response to Daniel Beideck
by Martin Holladay

Outer space is much, much colder than your garage ceiling.

Feb 8, 2018 12:06 PM ET

space is 3K
by Daniel Beideck

Yes, the temperature of space is around 3 Kelvin on average, i.e. very cold. It's also miles away from a car windshield. I suspect the radiation from the garage (and carport?) ceiling are what keeps the windshield from getting frosty. If that's the case, the terminology of 'night sky radiation' is a little misleading since it's more a lack of nearby blackbody radiation that's creating the conditions that make the frost possible.

Mar 2, 2018 2:11 PM ET

Soffit Frost Scenario?
by Gio Robson

Here's a particular scenario I'm trying to resolve related to this Blog topic. How likely is it for frost to be deposited/formed from outside air in a ventilated attic or soffit in Winnipeg during bitterly cold winter months?

In this case, the soffit in question is the extension of a flat roof (continuous sloped rigid insulation and mod-bit) and enclosed on all sides - air barrier between house and soffit appears to be good (but impossible to verify well with my blower door, infrared and theatrical fog tools due to finishes in place).

The soffit is finished with 1x5 cedar boards spaced around 3/8 inches apart over insect screen. Large amounts of frost is forming between board cracks, on roof sheathing in soffit, and on top of screen in soffit.

Everything screams air leakage from house to soffit as the culprit but I want to eliminate the possibility of hoar frost forming during evening night sky radiation or downward temperature swings....


Mar 2, 2018 2:28 PM ET

Response to Gio Robson
by Martin Holladay

If we are talking about "bitterly cold winter months," we know that the weather can make your roof sheathing bitterly cold. So we have a condensing surface.

If the boards in question can get cold due to night sky radiation, the boards might get colder than the outdoor air (on certain nights). But if the boards have rigid foam above them, it's unlikely that the boards' heat will radiate to outer space. So the boards are probably at the same temperature as the outdoor air.

Under those circumstances, the small amount of moisture in the cold outdoor air won't be able to be deposited on the boards as frost. However, if there is a leak of indoor air, and that warmer indoor can can contact the cold boards, you've found your mechanism.

Mar 2, 2018 4:17 PM ET

Reply to Martin
by Gio Robson

Thanks, Martin - that was also my feeling.

There is some modest insulation on the top of the soffit to direct water from the parapet part of the roof assembly.

That soffit space is also likely to be at least a few degrees warmer than outside temperatures as it is a buffered space, much like an attached garage or porch.

Therefore if the soffit/house air barrier was perfect, the absolute humidity in the soffit would be similar (or perhaps slightly higher) to the outside colder air, but the relative humidity would be lower, and the dew point might also be lower than the outside air. Is this correct?


Mar 2, 2018 5:58 PM ET

Response to Gio Robson
by Martin Holladay

Lots of speculation going on. I guess if you really want to know the temperature profile inside your soffit area, you could install temperature monitoring equipment. I don't really know the answer -- but my gut tells me that significant frost or condensation would be a sign of an air leak and a path that allows indoor air to reach the cold roof sheathing.

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