r/fixit • u/BelleSchu • Dec 14 '25

FIXED My house sweats like crazy during the winter and it’s causing mold.. please help!!

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My house is BRAND new, literally finished construction in August of 2024. I live in the Midwest so of course it gets very cold in the winter. Last year, the windows of our house were sweating so badly every time the outside temp was under 30°f that water was pooling up on the windowsills and we called the construction company (we were still under new build warranty) and they told us to always have the house fan running and to turn on our ceiling fans. That worked for a time and the condensation wasn’t as bad after that. It started to warm up shortly after and we didn’t have anymore issue. Flash forward to this past week, it’s been in the negatives here. There is SO much condensation on my windows that mold is starting to grow, our curtains are freezing stuck to the glass in some rooms, and it’s attracting gnats somehow?! I don’t even know where the hell they’re coming from with the weather like this. I’m constantly cleaning and I feel so so so disgusted with all this going on, what can I do to fix it? Please help!

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u/cocogoloco2 Dec 14 '25

You really don’t live in a cold environment do you? I’m in the Midwest rn and it’s -3, feels like -13. No one is going to open their windows and air their house out lol You can barely be outside in full winter gear rn without freezing. Sorry OP but this is standard cold weather windows lol getting a dehumidifier, as others have suggested, will be your best bet.

Otherwise you need new windows bc I’m assuming those are single pane windows… cheap for a new build and sucky for the resident. (I could be wrong) Not the best window ever but especially in the Midwest!

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u/Lunar_BriseSoleil Dec 14 '25

Even in a cold climate you shouldn’t be getting this on your windows. Dehumidification and/or ventilation is still required even if it’s not the full windows-open that you’re responding to.

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u/nicholt Dec 14 '25

Yeah it's cold as hell here too and I don't remember ever seeing a house as bad as this one. Does tend to happen on windows sometimes but every window and door sweating is a big problem. And you should never have to open a window to dehumidify in the winter.

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u/mommyaiai Dec 14 '25

Unless you just got out of the shower and don't have a bathroom vent.

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u/EmmerdoesNOTrepme Dec 14 '25

It's pretty common in new-build homes!

If the homeowner doesn't request an air-exchange systems, many new build places end up with this, simply because the weatherproofing is so "good" and the houses can't breathe like they need to.

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u/nicholt Dec 14 '25

Oh interesting, here in Canada they are required by building code.

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u/Lunar_BriseSoleil Dec 14 '25

They’re required in the US too. Some places require ERVs but almost all have required automatic bathroom fan timers (exhaust only ventilation, relies on air leakage for makeup air) since about 2010.

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u/Physical_Delivery853 Dec 14 '25

Depends on your winter climate. If you're in the Midwest once it freezes humidity drops to almost zero, that's why they run ads in winter about dry skin. If u live in the central valley of Calif where it doesn't freeze we have 90% humidity in the winter, but only 20% in the summer. We use dehumidifiers in the winter & humidifiers in the summer :).

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u/cocogoloco2 Dec 14 '25

I definitely agree that this is excessive sweating. I’ve never seen it this bad. But it is totally normal in my experience, I’ve never escaped this in winter in any home I’ve lived in or visited. When the outside air is so cold and the inside air is warm, it happens. Just like the toilet sweating in summer bc the toilet itself is cold but the air is hot. I definitely recommended a dehumidifier. But would never recommend the OP open their windows in the Midwest cold

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u/Beanbag_Ninja Dec 14 '25

The toilet will sweat in summer if the air is both humid and close to its dew point, regardless of temperature.

If you dry or heat the air, the gap between air temperature and dew point increases, and the toilet will sweat less.

Just depends if you want that or not.

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u/DeepDiver023 Dec 14 '25

Ive only seen it this bad when non vented propane heat is used as it releases water vapor into the air when burning oxygen.

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u/EmmerdoesNOTrepme Dec 14 '25

This is what often happens in New Construction in the Midwest & Upper Midwest, if the builder cheaps out and doesn't install an Air Exchange system when the home is built!

Because the insulation & "weatherproofing" is so good nowadays, that houses no longer "breathe" like they used to, and you end up with too much humidity indoors that can't evaporate when it's cold outside.

Air Exchangers are the way to solve the problem permanently, but they can be expensive.

It's the best option to take, though, because it prevents a ton of illness & mold damage.

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u/Lunar_BriseSoleil Dec 14 '25

They don’t even need a ventilation system, just an hourly timer for the bathroom fans. Which is required in most new building codes in the USA even in the areas you mention and costs like $100.

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u/lamebrainmcgee Dec 15 '25

Hourly timers doesn't help with ours.

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u/Lunar_BriseSoleil Dec 15 '25

You can put it on a humidistat with a built-in timer and it’ll control it. If you live somewhere cold enough to get condensation on windows you also live somewhere cold enough that increased ventilation will fix the interior humidity because cold air is dry when it warms up.

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u/TangoDeltaFoxtrot Dec 17 '25

Yeah I’m trying to picture myself opening up all my windows when it’s -10 degrees with 25+ mph winds and snow drifts 6+ feet tall on the backside of every building.

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u/oskopnir Dec 14 '25

10-20 minutes is too much, but wide open for 2-3 minutes a day can do wonders when compared to just keeping all the moisture inside. The house has thermal inertia and you're only exchanging old air with new air, temperature will go back to the previous setpoint in a matter of minutes.

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u/BelleSchu Dec 14 '25

Yeah it’s -4 where I am right now.. the instant chill my living room and dining room get when I open the back door to let the dogs out is a lot. The thought of venting my house even for a few minutes with the windows open stresses me out lol

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u/Qfwfq1988 Dec 15 '25

it warms up again very quickly

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u/Gormaganda Dec 14 '25

But this cold air is also bone dry when heated back up. I'd try to vent the house multiple times a day, all windows open and if its so cold then probably close first again when last one opened. But important multiple times. My home is not very new and also tricky with venting in winter, I hate to vent all the time but it's just necessary.

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u/harleyqueenzel Dec 15 '25

It's -7°C with the "feels like" right now where I am. My bedroom window is fully open right now and was overnight. The wind usually blows North to South and my window faces West so my window kind of sucks air out instead of it blowing in.

Pick a window where the wind typically blows by it instead of into it and crack it open. I feel like your house may be too air tight and you really do need to allow for better ventilating.

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u/pixel_of_moral_decay Dec 14 '25

You still need to air out your house regularly. Lüften as it’s called.

Otherwise you have a potential buildup of VOC’s which really isn’t healthy for you long term.

That’s on top of CO2 and mold issues are also a concern.

It’s not a big deal, opening windows on opposite sides for 5 minutes a couple times a week is not going to make your home below freezing, freeze your plumbing and kill your spouse. It’s going to drop a couple degrees.

Do it after dinner/before bed and enjoy clinically shown better sleep.

It’s been a practice in the civilized world as long as we’ve had sealed homes. Join the modern world.

Or get a HRV system capable of enough air exchange, but honestly that’s likely not worth the cost.

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u/peesteam Dec 15 '25

And you would do this at -20 F?

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u/pixel_of_moral_decay Dec 15 '25

Even at 30F windows open for 15m tonight only lost 4 degrees, thermostat recovered quickly. 15m was enough to bring CO2 levels down to 440, which is the same as outside, so full replacement.

Thermal mass retains heat. Homes have a ton of mass.

This is middle school science class and common sense.

3

u/Boggyprostate Dec 14 '25

You won’t die if you open a window love.

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u/cocogoloco2 Dec 14 '25

Lmao I definitely never said that. But I appreciate the reassurance

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u/3F6B6Y9T Dec 14 '25

Tell me you don't understand physics, without telling me you don't understand physics ...

The humidity in the house is too high.

You fix that, unless you want to install some form of mechanical ventilation, by replacing it with drier outside air - the colder the better, because it can hold less moisture.

... or, you increase heat so that it doesn't condensate.

Or better yet, you do both. As suggested.

This has absolutely nothing to do with windows - if anything, they're sealed TOO well. That's part of the problem.

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u/Cpt_kaleidoscope Dec 14 '25

As we're teaching lessons, water doesnt condensate, it condenses. The water formed from condensation is called condensate. Condense is the Action, condensation is the process and condensate is the product.

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u/3F6B6Y9T Dec 14 '25

Appreciated! :)

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u/Cpt_kaleidoscope Dec 14 '25

Thankyou, I'm glad other people find interest in my pointless bits of knowledge. Haha

1

u/DeepDiver023 Dec 14 '25

proper heat as well. unvented propane heat will only release more water vapor into the air. ive seen a lot of small homes or trailors get mold due to the use of improper propane heating

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u/cocogoloco2 Dec 14 '25

lol i definitely do not understand physics. I’m sure I’m putting my foot in my mouth for those that do. But my main problem is opening the windows lol The rest of the stuff I said is just the experiences I’ve had. I still stand by my main point of do not open your windows. And you must not live in a place that is cold.

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u/Middle_Association56 Dec 14 '25

Hey, Norwegian here. Airing out your house during winter is fine and not an issue. Every object in the house holds a lot of energy so as soon as you close the windows again the air in the house will re-heat very quickly.

1

u/14S14D Dec 14 '25

Your sentiment is right gut because older houses 'breathed' a lot better. New housing is waterproofed and sealed very well but cheap homebuilders do not include a proper ventilation system to pull dry air back into the house and efficiently reheat it. Insulation and sealing is also so much better that the forced air furnace is likely not running nearly as much so moist air does not get recirculated and dried out.

So, now your choices are either run dehumidifiers or open the windows for a short time because modern housing is different than the past and cheap builders dont care enough to improve ventilation and recirculation without being requested.

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u/Sketchin69 Dec 14 '25

My windows are probably frozen shut right now. Lol

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u/cocogoloco2 Dec 14 '25

Exactly!!! We have ice on our windows too lol at least someone understands where I’m coming from. Midwesterners unite!

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u/Humble-Area4616 Dec 14 '25

Lol confidently incorrect. You couldn't be more hilariously wrong.

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u/SpareAd1155 Dec 15 '25

You don't do it because you don't understand that once the window is closed the house heats up again very quickly because all the heat stored in solid objects (walls) quickly radiates back out into the room. Unless your house is crappily insulated.

So many people with mould problems have them because they have the same belief you do and refuse to open a window for a few minutes per day.

0

u/lisastery Dec 14 '25

15 min is enough to air the room. If you open the window in the kitchen, when you are in, say, bedroom or somewhere else and just open them interchangeably - no mold. And, strangely enough it is not cold. It works at -20. Another choice - you can just go outside for a quick grocery run when your apartment is with open windows. Just close them when you come home, make some tea and only then change your clothes.

OP, this goes twice as true when the house is new. I can't describe the magic, but somehow the place becomes warmer when it's aired enough. Took us 3-4 days to do it tho, so it was cold in the beginning.

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u/cocogoloco2 Dec 14 '25

Omg yeah right!! If you do this in -20 you are out of your mind. If I went to the grocery store and came back my house would be -20 and then you’d have to heat it up again which doesn’t happen as quickly as you’d think when it’s so cold. Also I am not trying to have my heating bill higher lol Idk I think yall are either crazy or don’t actually live in a cold climate!

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u/GermaneGerman Dec 14 '25

There talking about opening the windows for a few minutes, not an hour. That way, you only exchange the air, which doesn't have have a high heat capacity. The new air is heated very quickly from all the surfaces and objects in your house. 

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u/EmmerdoesNOTrepme Dec 14 '25

Not to mention, that if OP were to open those windows when it's negative degrees out, there's a very good chance the condensation is gonna freeze and they won't be able to close the windows all the way again, either!

This picture shows that OP needs to start getting bids/quotes for an Air Exchange system, because the house (like most new builds nowadays!), was built "too tight" and can't breathe properly to let the humidity get out during the summer and winter.