Do Open Windows = Mold?

David J. Allen

I spent the better part of 2 ½ years learning about mold and indoor air quality because my child's life depended on it. Now I help others avoid ever having to experience the same thing. The mold industry suffers from conflicting opinions and I do my best to distill loads of data into something practical and useful. I love hearing from and helping readers. • "A house desecrated by mildew, mold, or fungus would be a defiled place to live in, so drastic measures had to be taken." — Leviticus 14:45

There’s no universally applicable answer when it comes to the effect that open windows have on mold growth, but in most cases opening a window at the right times will do more good than harm to a home’s indoor air quality and humidity level.

It depends on the weather outside: If outdoor humidity is high or if it’s raining, the moisture that comes in through an open window contributes to mold growth.

If outdoor humidity is low, the air is breezy, and it isn’t raining, opening windows can improve a home’s air quality and reduce indoor humidity—both of which help prevent mold.

Mold spores can and do enter into homes through open windows, but always keeping your windows closed only to “prevent mold from coming in,” is not at all productive, since mold spores already exist in ALL homes and buildings, and many many more are brought in daily through doors, on shoes, on people, on packages, etc., etc.

Closing up and sealing off a home from the outside world actually promotes mold growth. Buildings need to breathe and windows help them do so.

“The mere act of opening and closing doors and windows, or turning air-conditioning and heating on and off at appropriate times, and being aware of moldy smells offers excellent protection against Stachybotrys and its allies.”

Money, Nicholas P.. “Carpet Monsters and Killer Spores: A Natural History of Toxic Mold.” (2004).

When to open windows

When it’s “a nice day out.”

  • Sunny
  • Breezy
  • Not muggy or humid
  • Not raining, hailing, snowing.

Before you turn to opening windows as a strategy for preventing mold, check the weather.

If it’s a dry, breezy day, go ahead and pull the trigger.

That fresh, cool, dry air will help remove some of the moisture from your home, making it harder for fungus to flourish.

Windows should also be opened while higher-than-normal levels of steam and moisture are being generated inside a room or apartment, such as while cooking or showering.

Exterior-facing bathroom windows should be opened while showering and for some time afterward, to allow the moist air to ventilate.

“…if the occupants of an apartment generate a significant amount of moisture from cooking and cleaning activities without opening windows or using exhaust fans, excess moisture accumulation and mold growth may occur.”

ASHRAE, On Limiting Indoor Mold and Dampness in Buildings, Reaffirmed June 23, 2021

Additionally, stagnant air is a good friend to mold, and the clean breeze flowing through your home will keep the air moving and cycle out much of the stale oxygen, replacing it with fresher air. 

But there are also times when opening the windows will do more harm than good.

When the humidity level outside is anywhere near 50-60 percent, you’d be wise to keep your windows closed if you’re concerned about mold growth.

Otherwise, that muggy air circulating into your home will only add to the amount of moisture, which may increase the likelihood of a mold colony developing.

The same goes for when it’s raining outside. If you’re able to keep your household dry, there’s virtually no chance that fungus will be able to flourish, so there’s no reason to invite water into your home unnecessarily.

“…avoid opening windows for long periods when it is humid outside (e.g. during nights and evenings) if you are experiencing mold growth problems in your home.”

UCF, FSEC Energy Research Center, “Mold Growth”

Pick one: An open window, or a running AC

Your air conditioner (if installed correctly) was sized specifically for your home or apartment.

It’s neither too large or too small, and so it can control humidity levels in the home.

Sometimes, however, it needs your help, and that’s what you’re doing by opening a window while cooking, or while showering.

If you open a large window in your house to welcome in some fresh air, shut the air conditioner off for the hour or three that you have your window open.

Otherwise, your a/c will be working overtime and unwanted condensation will form, which is exactly what you’re trying to prevent by opening your window.

A bathroom window can be opened briefly even while the home’s air conditioning system is running, but larger windows in the house shouldn’t be opened while the AC is running—do one or the other, to avoid condensation.

“Do not open windows while heating/cooling units are operating. This can cause condensation and may contribute to mold growth.”

Villanova University, “Mold and Mildew Prevention”

Opening windows ≠ mold remediation

Will opening windows remediate or fix an existing mold problem? No.

Some homeowners mistakenly believe that by opening their windows and letting a cool, dry breeze flow into the house, they can remediate an existing mold problem.

Unfortunately, it doesn’t work that way.

Professional mold remediation should be done, and fans or forced air flow should never be applied to a moldy area.

Once active mold growth is known to be present in a room, the area has to be cordoned off and air flow carefully controlled (through negative pressure) so that the mold spores that will inevitably and always escape during clean up or remediation are channeled outside of the building, rather than to nearby spaces or other rooms.

Granted, if the conditions are right, ventilating the home may indeed slow down the growth and expansion of an existing mold colony.

Additionally, opening the windows and letting the air circulate may also reduce that unpleasant musty odor and alleviate any respiratory issues you may be experiencing.

But simply letting fresh air into your home won’t eliminate or even significantly reduce a serious mold problem.

To rid yourself of the issue, you’ll need to either have it professionally removed or, if you have the necessary knowledge, tools, and safety gear, remediate it yourself.