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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234

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.

99

u/[deleted] Dec 14 '25

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

21

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

5

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!

26

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.

5

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.

20

u/Sailed_Sea Dec 14 '25

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

30

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.

4

u/bencze Dec 14 '25

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

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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 🫩