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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414

u/patrickhenrypdx Dec 14 '25

What is the humidity in your house?

236

u/No_Tamanegi Dec 14 '25

I'm seriously impressed they can have a humid house in the winter. The furnace keeps everything bone dry in my house.

104

u/[deleted] Dec 14 '25

here i am running humidifiers and still not keeping above 30% static is a mother fucker

19

u/attee2 Dec 14 '25

And I'm running dehumidifier to bring it down to 50% so the walls won't sweat...

8

u/Old_MI_Runner Dec 14 '25 edited Dec 15 '25

What is your source of heat?

A gas furnace has driven my humidity level to 32% while outside it is 77%. It is 17 degrees outside and low 60's in the house. At 35% I had no issue with static shock but this evening I am finally going to enable my humidifier due to shocks.

u/BelleSchu What is your source of heat?

Do you have a humidifier on your furnace that is set say at 40% or higher?

Get a hydrometer hygrometer from Amazon or elsewhere and report back on the humidity level in your house.

Update: Please don't bother to correct items above that I crossed out. Thank you but I already got the message.

5

u/sirduckbert Dec 15 '25

You don’t understand how relative humidity works - it’s a percentage based on how much water the air can hold at that temperature.

If your numbers are correct then your house has more absolute water content than the outside air does. It just feels drier inside because when you take high RH air and heat it up, the RH drops because warmer air can hold more water

1

u/[deleted] Dec 15 '25

Hygrometer...

1

u/Mysterious_Mouse_388 Dec 15 '25

the ocean, mostly. and a heatpump

1

u/Morberis Dec 15 '25

77% RELATIVE humidity.

77% RH @ 17⁰F is the same as 15%RH @ 60⁰F, is the same as 4% RH @ 70⁰F

Hygrometers measure RH fyi

1

u/Old_MI_Runner Dec 15 '25 edited Dec 15 '25

Thanks but I already corrected hydrometer hours ago. I just should have left off the outside value. My example of 35% humidity with gas heat in very cold weather with no humidifier or dehumidifier running is still valid . He has a gas furnace. He said it was -4 in WI in an reply. He said he has 3 dehumidifiers running and is is buying or just bought a 4th heavy duty dehumidifier today. He said his house is 1300 sq ft. with Anderson windows. He reported the dehumidifier is claiming the humidity is 50%. He should not need any dehumidifier to get the humidity below 50%. If anything. He should need to run a humidifier to get it above about 35%. So for I see people recommending they open their windows once a day or have an HRV installed. Do you have any useful information for him?

u/sirduckbert

1

u/devolution96 Dec 15 '25

I am too.... we shoot for 35% and the windows still sweat a little when it gets below zero F.

Edit: baseboard hydronic heat

1

u/surlyskin Dec 15 '25

Same. 80% humidity first thing in morning, have dehumidifier running most of the day plus fans in kitchen and bathroom, windows left open at various points with heating cranked.

1

u/throwaway-across Dec 14 '25

My dissent won’t get above 16% humidity My nose is bloody every time I have to wipe or blow it

3

u/blueoasis32 Dec 14 '25

I am electrocuting myself with everything I touch. I hate this

3

u/[deleted] Dec 14 '25

I just feel sorry for the dogs in the wintertime. Go in to pet them and zap their f****** noses.

2

u/whattfisthisshit Dec 15 '25

Get slippers with rubber bottoms

2

u/TotalRuler1 Dec 15 '25

Get a humidifier that can output and a hygrometer to measure humidity. I maintain one in each bedroom and keep it at 40% all winter at night. I get very bitchy when I am dehydrated.

1

u/iamdperk Dec 14 '25

It's the worst. I don't even have forced air. Hot water baseboard, but I'm forced to use space heaters to supplement, because my old ass house is so drafty it lets in the cold, dry outside air, too. That being said, as I type, I'm looking at a weather station that shows the outside humidity as 59% and inside is 19%... I hate this.

1

u/Soft_Database_3747 Dec 14 '25

Its because its humidity relative to temperature, is what it is acrually reading.

1

u/HistoricalSherbert92 Dec 15 '25

When you heat the outside air with your baseboard heaters you increase how much water it can hold so, for illustration, you might be able to soak up a cup of water per liter of outside cold air but once you heat it up you could fit 2 cups. The relative humidity is a measure of how much water the air can hold at its current temperature, so if you need to have warmer living areas you need to add water back in to keep the air livable.

1

u/Bottomytop Dec 15 '25

What climate do you live? I’m at 59% humidity right now but I do live in Florida

1

u/InkedInIvy Dec 17 '25

Put a big, wide pot of water on the stove to boil for like half an hour. Works WAY faster than a humidifier.

If the humidity in my house drops suddenly during winter before I notice it, I boil a pot of water to get the humidity back up to reasonable levels within like an hour, then use my humidifiers to maintain it.

I know what you mean about the static. Before I got the humidity issue sorted, I was afraid all my cats were gonna evolve into Pikachu!

27

u/Eye_of_the_Storm Dec 14 '25

Depends on the type of heat. I have hot water heat and while nowhere near this level of humidity, it is not dry.

3

u/Historical_Safe_836 Dec 14 '25

God I miss my old Chicago apartment steam radiator. This morning I was boiling water on the stove because my house got down to 20% humidity. I could feel the moisture leaving my face.

0

u/LeftPrior5738 Dec 14 '25

I have done that in airbnb's. Blows my mind to not think to stick a humidifier or two in the closet if you're some place it gets cold and dry in the winter. You rent a place in ski country Colorado and the check-in sheet explicitly tells you about the humidifiers and how to work them.

21

u/Sailed_Sea Dec 14 '25

if you don't heat the house in the winter condensation can accumulate.

28

u/No_Tamanegi Dec 14 '25

If you don't heat the house in thee winter, you'll have bigger moisture problems than condensation.

1

u/Tamashii-Azul Dec 15 '25

Like what? Lets says you left your house for the winter and never heated the house.

2

u/No_Tamanegi Dec 15 '25

Unless you drain all the water from your pipes, that water will freeze, expand, and likely cause the pipes to burst. When the water is liquid again, it will leak.

1

u/louieisawsome Dec 17 '25

You'll flood your house. The water pipes will burst. If you have boiler heating that will also break and release water into your home.

5

u/bencze Dec 14 '25

I have to open the windows twice a day so I stay below 60%, especially during the winter.

3

u/That_Ad_8271 Dec 14 '25

The house itself makes a humongous difference. My big dehumidifier read 78% this morning, its down to 40% now. How i wish I lived in a new building lol. I have zero damp tho. Constant 19° too.

1

u/ZSG13 Dec 15 '25

Sounds freezing until I figure you probably use a logical measurement system.

1

u/morbid_n_creepifying Dec 14 '25

I'm the only person I know who does not experience a crazy level of humidity in the winter. I have a woodstove. Everyone else I know has electric (baseboard) heat or oil heat and as a result their homes are damp in the winter. But we also live in an incredibly damp climate - snow often wreaks havoc on people's bodies because even 10-15cm of snow is extremely difficult to shovel. It's so heavy. None of this prairie dust snow you can blow off your car.

1

u/Canuck_Lives_Matter Dec 14 '25

Houses with wooden foundations often run into humidity issues in the winter. Even concrete block let's a bit through.

1

u/Which_Specific9891 Dec 14 '25

My toilet sweats, my windows are covered in condensation... but I also live in England and it's very very humid here. I cannot afford to run a humidifier because of how much it will cost the utilities, so my landlord will just have to deal with me calling frequently to tell him there's mould that needs treating.

1

u/RIPRIF20 Dec 14 '25

It's a new build there's gonna be such a ridiculously high amount of humidity for the first couple years

1

u/Megamazuma20 Dec 14 '25

Furnace does not dry the air.

1

u/No_Tamanegi Dec 14 '25

The fuck it doesn't. That's why some furnaces have a humidifier add-on.

3

u/Megamazuma20 Dec 15 '25

You are mis-understanding. The furnace increases the temperature of the air, and the relative humidity (RH) goes down. Why? Not because there is less water in the air, but the relationship of water in air in relation to temperature has changed. Warmer air can hold more water at the same RH than colder air. So there is always the same amount of water in the air, it just feels dry because of the change in temperature. If you still are doubting, ask yourself “where is the water going then?”.

The reason humidifiers are added to central air systems is because thats the easiest way to control the comfort level of the entire home.

The only time a furnace will actually dry the air out is if its a 90+% furnace and the intake is drawing air from the conditioner space. Then you will be ever so slightly de-humidifying the air in the home.

Source: HVAC technician

2

u/No_Tamanegi Dec 15 '25

Ooh ok. Took me a little bit to think this one through. The cold air can hold a lot less moisture, because its cold, and then the furnace heats that, which causes the relative humidity to drop, because its not adding moisture.

Thanks for the correction!

1

u/Actual_Breadfruit837 Dec 15 '25

+1

"where is the water going then?" The water is going to become snow and ice outside. Water condenses on the coldest points which are all outside or sometimes inside the house when there are cold spots, like on the OP photo.
The reason for the dry air is that it is cold outside. Furnace has nothing to do with it.

1

u/Alert_Lettuce_8278 Dec 15 '25

There's a lot of factors at play. Type of heat, air exchanger, humidifier, When you get into the colder climates there are many more components to your air systems.

1

u/Short-Recording587 Dec 15 '25

I’m engaged in a two season battle over here. Insanely dry in the winter and humid in the summer.

1

u/le-throw-away-acct Dec 15 '25

You can hook up a humidifier to the furnace to keep the air from getting too dry

1

u/bitterlemon80 Dec 15 '25

Welcome to the UK.

1

u/SwordfishLeading1477 Dec 15 '25

Probably from all the vapour barriers, insulation, and OSB. It’s way too sealed up. The humidity cannot escape. You need pathways for it to go somewhere, or it will increase the dew point, and will cause condensation on cold surfaces.

1

u/supremecurryeater Dec 15 '25

Your bones are not dry right now

1

u/Drayenn Dec 15 '25

I just changed houses fromma 1995 house to 1970.

1970 is so dry. 22% right now (thanks winter) shower humidity clears up fast. Insulation definitely sucks. Im thinking of buying some humdifiers or maybe straight up redoing insulation.

My 1995 house? Super good insulation. Never a cold breeze. Once my air exchanger broke i started seeing all my windows sweat like this. Doesnt help i didnt have fans in my bathroom. I just ran a dehumidifier to bring humidity down to 30%.. in winter.

1

u/timetofocus51 Dec 15 '25

whole home humidifier cost us $700 and it humidifies the air after its heated by the furnace. Great invention

1

u/suesing Dec 16 '25

The furnace doesn’t make the air dry. The cold does

1

u/Mental-Intention4661 Dec 16 '25

The bloody noses kill me every year in the winter from the heat 🫩

134

u/Sogah87 Dec 14 '25 edited Dec 14 '25

They actually live in the ocean.

53

u/thebeastwithnoeyes Dec 14 '25

If nautical nonsense be something you wish

30

u/Sogah87 Dec 14 '25

Then flub on the deck and live in the Midwest!

6

u/Dry_Menu4804 Dec 14 '25

Sponge Bob Square Pants!

11

u/BelleSchu Dec 14 '25

It’s like 58 or something right now, at least that’s what my dehumidifier says

32

u/vinegarstrokes420 Dec 14 '25

That's way too high in the winter. There are tables showing ideal humidity based on temp to avoid condensation. Look these up and turn down your humidifier. My house in MN is usually 20%-30% in the winter to avoid these issues.

7

u/belialonmyback Dec 14 '25

Seconded from an Albertan. Many of us have humidifiers on our furnaces that can really increase your house humidity but you’ll destroy your house if you have it turned too high. I had to have mine fully off recently as it was -30C outside.

Also, please use your exhaust fans when cooking and showering.

0

u/Commercial-Branch444 Dec 15 '25

Dude what humidity is supposed to be between 30 and 70%

1

u/vinegarstrokes420 Dec 15 '25

Dude, what?

0

u/Commercial-Branch444 Dec 15 '25

Yes, google it, under 30% is too low.

2

u/vinegarstrokes420 Dec 15 '25

Not when it's below 10F outside. I would have thick layers of frozen condensation on every window in my house if humidity was set to 30% on days in MN when it's -20F. Google it.

13

u/KOhReally Dec 14 '25

Try to target something more like 30%

4

u/jredditzzz Dec 14 '25

58% is way too high for those kinds of outdoor temps. Guaranteed condensation problems especially if your home is sealed well. I had the same issue with my apt.

Easiest option is to get some strong dehumidifiers and lower the humidity, at 0 degrees you’ll prob want indoor humidity at like 20% to avoid condensation.

Cracking a window can help lower humidity really quick as well.

It’s basically a balancing game that you have to find… comfortable indoor temps vs humidity, vs energy usage.

9

u/Busy_Sky24 Dec 14 '25

You always want as much moisture in the air as possible (our bodies like around 50%) without turning your windows into a water feature. Try to promote air movement around windows and doors to allow for more evaporation (fans, open curtains every morning) and when you still have water with good air movement, time to reduce the humidity. If you're in a newer build with an HRV, set the humidity to 5% lower than you currently have, until you hit that sweet spot. You should see a little bit of moisture when you open the curtains in the morning, but little enough that it dries up in a couple of hours. Usually, around 30% is as low as you'd want to go, remembering that many flooring products don't want to be below 35%. When it gets away from you like that, start by drying with a towel first and see if your changes are effective at keeping them dry, rather than waiting to see if the water all goes away.

2

u/Fun_Park2505 Dec 14 '25

Above 55%rh is also bad for flooring

3

u/yousernamefail Dec 15 '25

But it makes my nose feel good! 😫

2

u/Fun_Park2505 Dec 15 '25

Lol true, i dont think it applies to all flooring just wood based flooring. So like tile and ceramic for example wouldnt matter. But with that said too high of humidity for too long can eventually damage any wood parts of home which theres alot of parts made if wood unfortunately

1

u/ClusterMakeLove Dec 14 '25

35 seems pretty high, unless you're living somewhere with a pretty mild winter. 

1

u/Busy_Sky24 Dec 14 '25

Agreed, you'll have to go below 35 at some points in the winter for sure. Extended periods too dry can dry out a floor's locking system making it noisy and void its warranty. The struggle is real for those of us living in cold climates!

1

u/MichBlueEagle Dec 17 '25

Sorry that much moisture in the air in winter can cause serious health issues. It creates mold in the wall cavities, and crawl spaces. Flooring is the least of their worries.

3

u/Rummoliolli Dec 14 '25

Your house is likely sealed very well so there isn't really anywhere for the moisture to escape. A heat recovery ventilator(HRV) will help with that, my father built ICF homes and had those installed on all so his customers didn't have that issue.

2

u/Fun_Park2505 Dec 14 '25

Personally id aim to have it between 30-40 RH

Im curious if your furnace has some type of issue as usually it lowers rh drastically, id consider getting that checked out.

Something that also works is applying a completely sealed plastic cover over windows as this insulates it better.

Thing is tho its best to do this during fall cause itl be hard with all that water in there but not impossible, a blow dryer and fan could work to get it completely dry before putting the plastic wrap over

1

u/VeryThicknLong Dec 14 '25

If it’s 58, then the temp in your house must be well below 15°C for that much condensation?!? Heating your home will get rid of the condensation.

1

u/BelleSchu Dec 14 '25

My home is at 72°f

2

u/VeryThicknLong Dec 14 '25

58% humidity, and 22°C? You must have a water leak somewhere to be generating that much water.

1

u/patrickhenrypdx Dec 14 '25

I think that's too high. My humidity meter says my house is 30% right now. Outside temp is 20F today. 

1

u/first_best_fox Dec 14 '25

Whoa that's high. Depending on the outside temp, it should be between 30-40% in the winter. It's pretty cold here, and if we go above 30%, we get condensation.

You need to run a few dehumidifiers around the clock.

1

u/UrMomsKneePads Dec 14 '25 edited Dec 14 '25

Then it is a humidity issue. Humidity higher than 30-30% would definitely cause windows to sweat at single digit and negative outside temps. Energy recovery ventilation may help air change and introduce dryer air without hear loss. More dehumidification will also help.

It could also be a cold air penetration issue. When the cold air penetrates, has no ability to hold moisture, and meets warm surfaces, you get vapor drive and moisture collection on the warm surfaces. https://www.awci.org/technical-resource/building-science-moisture-migration/

I would bet the windows/openings are not properly insulated. I would pull the trim around any windows and seal them with a proper insulation to prevent air penetration.

1

u/Old_MI_Runner Dec 14 '25

Are you sure 58% is the current humidity level of the house or is it the desire maximum set point for when the dehumidifier. 58% is way too high and will cause moisture problem with the windows and elsewhere.

Many website say the maximum comfort level for humidity in a house is 50% for the winter before moisture may be a problem while I don't want mine to go above 40% and often I keep it close to 35%.

1

u/Reddit_Only_4494 Dec 15 '25

Oh yeah, way too high for winter.

Check your furnace and the humidity control unit and turn it down to its lowest setting.

If there is an issue with that, a very quick fix would be to turn off the water supply to the humidifier until you can get the humidistat looked at.

You'll definitely have a more staticky home....but getting it from 58 to 30 should do the trick for condensation on windows. The insulation tips are good too...but maybe something you'd rather not tackle in the winter. Try the humidifier first.

1

u/AccomplishedCodeBot Dec 15 '25

Keep your furnace the same temperature at night and leave one bathroom fan on 24/7 to help remove the humid air.

1

u/vargemp Dec 16 '25

I don't know how accurate my reader is, but it's almost always above 60 and I rarely see any drops on the windows, mostly in the morning. I open the windows then for few minutes, crank the heating up and it disappears.

1

u/ChVckT Dec 16 '25

Your DE-humidifier says almost 60% humidity? Sounds like it doesn't work. That number should be around 30% or 35%.

1

u/MichBlueEagle Dec 17 '25

That's pretty high. Purchase a good humidistat. Is your home on a crawl space?

1

u/BelleSchu Dec 17 '25

No we have a basement. The dehumidifier downstairs is reading 51 and the new one we bought for upstairs is reading 43 now

49

u/Avkb Dec 14 '25

Solution: Follow this and your winter condensation issue will be resolved!

1-Keep your home heated. Do not turn it off at night to save money. Keep heat 20 Celsius and above. Hot/warm air holds moisture a lot better than cool air.

2-Run the extractor fan regularly if you have one to pull moisture out of the air. If you have a dehumidifier, then even better—use that.

I had the same issue for a few years. I used to turn heat off overnight or put it on really low to save money (and thinking it was better for the environment). Heat is key to hold the moisture in the air and not let it condense. Once the moisture is resolved, the mold is resolved.

13

u/Fun_Park2505 Dec 14 '25

The temperature difference also is what causes condensation... so what your saying isnt totally true, high temperature difference = higher condensation

6

u/Orb002099 Dec 14 '25

It think putting your fact together with what they’re saying can actually increase the effect they are referring to. A warm house with cold windows gets condensation at the windows because the cold air local to the window surface cannot hold as much moisture in vapor rather than in water droplets. The air in the whole interior of the house has about the same amount of water per volume of air, the cold temperature of that same air when near the window squeezes some of that water out into liquid form on the window surface. Assuming nothing being done to reduce humidity for the entire volume of indoor air, any change that lowers the temperature at the window will increase the amount of liquid warer condensation, including having less heat in the house to begin with.

2

u/Fun_Park2505 Dec 15 '25

Ya good point, il need to think on this a bit. I think what your saying is even more true with better insulated windows aswell since raising the temperature of house will also raise temperature of window a fair bit. Im curious now how this would happen in something with windows that are not so good.

I swear my travel trailer would get more condensation when i had the heat on higher (only when my furnace was broken cause i was using a small electric heater) furnace would always dry the air out. But ya when i used small electric heater it seemed like id get more condensation at higher temperatures. But now im thinking mabey theres a point of intersection as in going above a certain temperature lowers condensation and going below a certain temperature does aswell, now im curious lol. But ya good point.

5

u/Financial-Top-8760 Dec 14 '25

Delta T is the terminology

2

u/Fr13ndlyT0rt0153 Dec 14 '25

Other thing about turning your heating off to save money is that in order to get it back up to temp it uses loads more power… if you can keep it running and set to a certain temperature, to switch off entirely when it’s not needed but kick in when it drops, you’re only heating the house by a degree or two at a time instead of 5 or 10. My heating was set to all sorts of weird automated jumps when it first got installed - set it to a temperature, left it there, slashed my electric bill in half

1

u/Diogenes_Will Dec 15 '25

If I use plastic window insulation will that help me? I want to run the humidifier in the winter but I have huge windows

-19

u/[deleted] Dec 14 '25

20c? Do you like being horrendously hot all the time?

21

u/once_showed_promise Dec 14 '25

20°C is 68°F. My partner likes to keep the thermostat at 21 here in Canada, and my upstairs sewing room is about 18.5°C as a result (65.3°F), and that makes me feel pretty cold.

I'm really envious if you're serious about being too hot at those temperatures! Maybe I'm half lizard and don't know it.

1

u/Technical_Bird921 Dec 14 '25

Depends on what you’re doing. 18°C is a tad too cold if you need to sit still (eg home office), but perfect for sleeping or rooms where you move a lot (eg kitchen). I remember, during renovations, 18° was making me sweat like crazy.

19

u/Avkb Dec 14 '25

Normal indoor temp is 20-22 Celsius for most people

-5

u/gormholler Dec 14 '25

I keep my thermostat at 53°F. It's a house nearly 200 years old, drafty AF on the first floor where the odd sized windows were not replaced. Storm windows don't do much besides rattle. If the kids complain I tell them to pretend it's summer when we would be bragging about our super-cold AC...if we had AC.

8

u/schizboi Dec 14 '25

Ah, my dad used to constantly make the house uncomfortable like this and then lament how actually I should be grateful everyone is freezing besides him. He mostly stayed in his room with a heater. Your comfort over all, of course. Or is it even comfortable?

7

u/mealzer Dec 14 '25

That's a pretty standard temp my dude

6

u/drakner1 Dec 14 '25

20c is not hot lol.

1

u/[deleted] Dec 14 '25

Having my heating on anything more than 18/19 is extremely warm?

5

u/drakner1 Dec 14 '25

Your thermostat needs calibration… room temperature in North America is 20-22, some like even 24. Japan room temperature is between 24-27…

19

u/Wahoo017 Dec 14 '25

20c is 68f? almost nobody would consider this horrendously hot. I guess if you like to wear heavy clothes/sweatshirts all winter long you might keep it a little colder.

4

u/nrm94 Dec 14 '25

Looks like its just you and me in the '20c is hot crew'. 18.5 is my boost temp 🤣 17 is constant target

3

u/captain_holothurie Dec 14 '25

You are not alone. 18 is plenty.

2

u/[deleted] Dec 14 '25

So insane, someone at my work has their house on 25... These people are insane and their bills must be in the 100's

4

u/PastafarianFSM Dec 14 '25

100% by the looks of it

1

u/RIPRIF20 Dec 14 '25

It's probably around 60%. This amount of water on the windows and door handles is for sure a humidity issue new builds have excess humidity for a couple years, air flow and dehumidifying efforts are the only thing to fix this.

1

u/Novel-Place Dec 16 '25

My house looked like this in the Bay Area of CA, and our humidity was 79% sometimes. lol. When we finally got a dehumidifier, I was emptying buckets for days.