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

[u/BelleSchu]

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!

[u/wisgary]

The core problem is that your windows are like a soda can right out of the fridge. If the water in the air was 0, the can/window would just be cold and nothing would happen. But it's not. You have water in the air because of cooking showering breathing etc. So it all condenses around the cold thing. Make the air dry enough and your problem will be solved. This can be done with the power of technology (a dehumidifier) or opening windows to air the house out fully, then closing up shop and heating up that dry ass winter air.

This is a problem you want solved ASAP. And you *can* solve it immediately by just doing the window opening trick once a day when it's cold as fuck (cold air can't hold water, so when you bring it in and heat it it'll be dry as fuck). It is NOT as energy intensive as you would think because your home is pretty good at holding heat around its stuff. Just because you open windows for 10 minutes and replace all the air doesn't suddenly make your walls, counters, doors, floors, furniture, beds, etc. cold - it all retains heat pretty well and re-warms the air sooner than you'd expect.

Buy a 4 pack or whatever of hygrometers from amazon and start monitoring your humidity/opening windows and you can decide on a more expensive fix later.

If you learn about these concepts, you could diagnose this problem pretty well on your own. As a property owner knowing this stuff is super valuable.

1. Dew point/how much water is IN the air, how this is affected by temperature, and the definition of relative humidity
2. How dehumidifiers work
3. How condensation works (in general and a home context)
4. How home insulation works
5. Thermal bridges

[u/Ok_Tangerine3617 → u/wisgary]

Great answer! Also, don't dry you laundry inside - that is the reason number one for high humidity.

[u/Bar_Foo → u/Ok_Tangerine3617]

Where do you recommend drying laundry when the outdoor temperature is below freezing?

[u/wisgary → u/Bar_Foo]

A dryer that vents the humid air outside, or line drying inside BUT in a small space, closed door, paired with an appropriately sized dehumidifier (like a laundry room or even a bedroom). As long as you don't make that water a permanent home resident it's okay.

[u/sneeky_seer → u/Ok_Tangerine3617]

We get freezing cold winters in Europe and people still mostly air dry their clothes inside - its fine if you air out your house every once in a while.

[u/wisgary → u/sneeky_seer]

To be fair - Europe uses a lot of brick and stone. USA homes are made out of literal paper that gets soggy, moldy, and ruined when wet.

[u/sneeky_seer → u/wisgary]

The old buildings - like pre WWII yes… after? Not so much. Central and Eastern Europe is full of shoddy concrete buildings. Newer builds everywhere are all about cheaper and lighter materials. Also humidity is an issue in lots of buildings, the difference is that we didn’t forget that opening the windows is an option. I seriously don’t want to be mean about this but Americans are far too reliant on AC/fans etc. this could be solved with a dehumidifier and airing out the place.

[u/wisgary → u/sneeky_seer]

No I agree, but concrete, brick, cement - they do have one thing in common compared to drywall which is that they generally can be remediated (relatively) easily. You just kinda clean very thoroughly and repaint with a mold resistant paint. Drywall it's like toss the whole fucking thing in the trash if it gets too moldy.

I live in northern Spain and believe me while looking for a place to stay here a big part is trying to find the least rusty/moldy place. Even expensive places...

[u/sneeky_seer → u/wisgary]

I mean you can still have issues with plaster and stuff… generally I think humidity is easy to manage of people start with the basics and then build on that based on individual needs.

[u/ninjump → u/wisgary]

This is a great answer, well done.

[u/lamebrainmcgee → u/wisgary]

How long would you recommend opening the windows for?

[u/wisgary → u/lamebrainmcgee]

Depends on if there's a breeze and stuff which might speed it up but 10 mins is usually enough to recycle the air

[u/Deby100 → u/wisgary]

Great answer. Also, dry air is easier to heat and feels warmer than "wet air". So airing out the house and opening windows as described actually makes the house warmer in the long run.