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Heated garage and frosted windows?

So I just got a heated attached garage added to my house, and it is lovely on those -30º days (we’ve had a couple already this winter, though it hasn’t hit -40º yet) to climb into a already warm car for the drive to work. The only problem is when I get to the other end, where I then have to park the car outside all day at work. When I come out after work, the insides of the windows are all completely frosted over, to an extent that the back window rarely defrosts before I get home (about a 15 minute drive). Annoying at best, potentially dangerous at worst, unless I waste a lot of gas just letting the car idle until the windows defrost. Scraping, while quite effective on outside window frost, doesn’t work well on the insides.

What can I do to prevent this? I tried turning on my defroster full for the drive to work, but that didn’t seem to help much - I think my car puts it on internal circulation when on full defrost, thereby keeping all the most warm air inside.

If there is enough moisture inside the car to cause completely frosted-over windows, there are only two conclusions, as far as I am concerned:

You are using your HVAC system on “recirculate”, rather than on the setting that allows it to pull in outside air


You have a leaking heater core

For reasons that I have never been able to understand, a LOT of people use their HVAC system on recirculate, even though that is one of the best ways to kill yourself with Carbon Monoxide.
If you are not recirculating the HVAC air, then you really need to have a mechanic check for a leaking heater core, which–believe it or not–is also a health hazard, as aerosolized coolant can cause very serious respiratory problems.



Or perhaps, in this case, the car’s been parked in a nice, warm garage overnight, allowing it to fill up with nice, warm, relatively humid air, the moisture in which condenses out onto the windows once the car is off and cools down? This problem only started last week when my nice new garage became available to park in. For the past three years I’ve been parking outside in the winter, and the only frosting has been to the outside of the windows, as one would expect.

I suppose one option might be to drive with my windows down long enough to dump the warm humid air and replace it with frigid, dry air? Of course, that at least partially negates the benefit of having the garage in the first place… :slight_smile:

As far as the recirculate mode, perhaps someone can correct me on this, but I seem to recall reading in the manual that on my car when you put it in full heat defrost mode, it automatically goes to recirculate mode (for faster heating) and AC on (for dehumidification, I suppose). I could be wrong about that though - it’s just a vague recollection of something I think I saw in passing. If so, however, perhaps another option might be to put the heating system on something other than defrost/full heat?

During the winter months, the relative humidity is almost always so low that I don’t understand how there would be humid air in your garage. And, if you are heating the garage, the heater/furnace will actually reduce the humidity even further. In areas that experience freezing temperatures, high humidity is rarely an issue–especially inside a heated space.

As to your HVAC system going into auto-recirculate mode, that is usually only the case if you use “Max A/C”.
You should be able to override that possibility when heating the car, just by making sure that the system is set for fresh air, rather than recirculated air.
Many people never both to check those controls.

There is another possibility that I overlooked earlier. Even though you didn’t mention it, if your region has been snowy recently, and if you and your passengers tracked snow into the car, that could have introduced enough moisture to soak the carpet and lead to the problem that you are experiencing.

Have you checked the humidity in the garage? If it is high, you could try running a dehumidifier…I think it would be wise, though, to consider taking @VDCdriver’s advice and have your mechanic check for a leaking heater core, even if just for peace of mind.

Are you dragging in a lot of snow on your shoes/boots? This can lead to inside window frost in the winter as well.

As mentioned, it’s not the heat, it’s the humidity. The defrost mode may not help much as the AC system will not operate at those low temps so it’s not helping to dry out the air.

I used to have the opposite problem- leaving the heated garage and the windows would immediately flash over. Although it took a lot less time for the engine to start producing heat and clearing them with hot air.

One thing you might try is to leave the windows open overnight in your heated garage. The trapped humidity in the vehicle may dissipate into the heated garage air…

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There’s probably some amount of snow being tracked in, yes, but I wouldn’t think a lot. I do have all-weather floor mats, and I haven’t noticed any moisture on them in the mornings when I get into the car. That said, there is always snow, and some will always get tracked inside despite best efforts to keep it out. As such, I’ll try leaving the windows open while in the garage, as suggested by TwinTurbo - perhaps the garage air is dryer than the air inside the car, and by leaving the windows open, the car itself can dry out better from the snow tracked in.

Relative humidity inside, however, is around 30%. Pretty sure that 30% at 70º translates into a fair amount of moisture being available to condense on the windows when the temp gets down to -20 or so, much less -30. Yes, the outside air is quite dry, however my house is fairly well sealed, so the inside air stays relatively moist.

I can, of course, get things checked - but given that the problem is directly tied to being parked in the garage overnight, and doesn’t occur when parked outside, even though the heater core is running the same amount, it seems to me that would be a waste of time and money. Please do let me know if I am wrong about that somehow - i’m not trying too be stubborn here, it just doesn’t seem to me to logically fit the observed facts. But I might be observing the facts wrong :slight_smile:

I’ll look into a dehumidifier. Would definitely be worth it if it fixes this problem. Although I’ll probably wait on that until the door between my garage and my house is installed - I don’t want to suck the moisture out of my entire house!

My house is fairly well-sealed also, and the interior humidity today is 27%, versus ambient humidity of ~50% today.
If you measure the humidity in your house, you might be surprised to see how low it can get as a result of the heating process.

and I haven’t noticed any moisture on them in the mornings when I get into the car

That’s because it’s in gaseous state by then :wink:

Does your garage have a separate heater or is it branched off the house furnace? (ignoring the lack of a door for now). You may be surprised to find the garage humidity will vary widely in winter. When you bring the car(s) in, they will track in a lot of snow/ice. I bang my fenders and rocker panels off before backing in and still have a lot of water on the floor once it warms up. A heated garage will be fairly well sealed so until you open the door, it may have pretty high relative amount of humidity. If your house furnace is also serving the garage and it is humidified…

Yeah - as I said, the humidity in my house is around 30%. Actually, I generally see it between 30% and 40% during the winter - I don’t think I’ve seen it below about 25-30% at any point in the last 8 years that I’ve lived in this house, although the garage is new this year, and might affect things.

This morning, for example, my house was reading 36% inside with a temperature of 68º. Outside is 24% with a temperature of -20º. My office, on the other hand, is only reading 3% humidity, but then I’m not parking my car inside my office :slight_smile:

In looking up my car’s owner manual, I see that I did, in fact, misread the statement about AC and circulation. What it actually says is:

…the air conditioner compressor automatically operates to more quickly defrost the windshield. At the same time, the air inlet selection is automatically set to the outside air mode.

So it is pulling outside air, which is good.

Makes sense. So leaving my car windows open overnight could well help with that, as it would allow that moisture to escape into the garage.

Hydronic heating system, not forced air. Garage is heated slab, radiant floor heating. And yes, snow melting off the car definitely would contribute to the humidity in the garage, as it hits the warm slab and evaporates into the warm air. So I’d say that it is quite likely that the humidity is actually fairly high until I open the garage door in the morning.

This air has a dew point of 40degF. As you’ve surmised, there’s your problem.

Well, when I was a kid we’d put the heater on recirculate because the heater wasn’t hot enough for comfort on fresh.

Haven’t experienced that problem in anything remotely modern that I’ve driven, but old habits die hard.

OP, flip on your air conditioner. It doesn’t just make things cold - it dries out the air. Running it with the temperature on hot should get all that moisture from the garage air out of the car by the time you get to work.

According to the Owners manual for my car, it’s automatically turned on when setting the controls to full defrost. So maybe I just need to turn the fan up higher for the drive. Or at least that might help. It’s just so loud :stuck_out_tongue:

There may be a lot of moisture in the carpet and seats. I have sometimes brought the floor mats inside to dry, and sometimes also put a small electric heater in the car overnight, with the windows slightly open, to drive off some of the trapped moisture.

If by day you can crack the windows open a bit and park in the sun, that will aid the process.

That would be nice :slight_smile: Unfortunately, we don’t have any sun to speak of around here in the winter. Total daylight today: 4 hours 14 minutes, all of which occur while I am at work :smiley: Tomorrow will be four minutes shorter. By the time we start gaining sun again, we’ll be down to 3 hours 42 minutes. Fun!

So, to summarize what I’ve been hearing:

  • Crack the car windows in the garage overnight to prevent trapping moisture from melting snow inside the car
  • Look into getting a dehumidifier for the garage to lower the humidity in there
  • Run the AC (on hot) full blast on the drive to work to pull in as much fresh air as possible and dry out the air as much as possible

It might also be a good idea to get the heater core checked. Does that about sum it up? I can try these things tomorrow morning (other than the dehumidifier) and see how they go.

My brother taught me about leaving the car windows open a small amount to release the inside humidity and it works! Now, whenever this problem develops, I find that if I do this one night (when no snow is expected), the problem is gone for a long time.

I don’t think running the fan on high will make any difference, medium should be enough. Just make sure you are not on recirculate. Your area is so cold that the AC won’t actually run, even though the defrost is on. The outside air will have almost no moisture in it to begin with so as soon as it hits the heater, it warms up and its relative humidity (RH) easily drops into the low single digits.

Try this, set the HVAC to floor and defrost, heat on high, fan medium and crack a window while driving. Just a small opening in the window, not enough to feel much cold. This will help the humidity to escape while driving to work.

Ok, sounds good. I’ll give it a shot and see what happens. Maybe opening my moonroof to vent would be a good option, as the airflow should suck air out without creating a cold draft (theoretically).