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The glass is not the problem. The temperature of the glass is.

Why Is There Mold On My Windows? Condensation, Dew Point, and the Fix

Window mold is a surface condensation problem. How dew point, glazing, and frame thermal bridging cause it, and what actually stops it coming back.

16 min read

Mold grows on windows because the window is the coldest interior surface in the room. When room air touches glass or a sash that is below the air's dew point, water vapor condenses there, and the dust and soap film already sitting on that surface feed the growth. EPA's moisture control guidance puts the mechanism plainly: "When humid air contacts a surface that is cold enough, the water vapor in the air will condense onto that cold surface," and "the dew point is the temperature of the air at which condensation occurs." You have exactly two levers: lower the moisture in the indoor air, or raise the temperature of the window surface. EPA's targets are 30 to 50 percent relative humidity and a 10-square-foot DIY cleanup limit. One thing that is not a mold problem at all: fog sealed between two panes of glass, which means the insulating glass unit's edge seal has failed.

The earlier version of this article leaned on two ideas worth correcting. It suggested new windows would "significantly reduce condensation" as a fix, and the Minnesota Department of Commerce's home moisture guide is blunt that "condensation is not necessarily an indication that your windows are bad and need to be replaced." It also raised mycotoxins as a reason to worry about window mold; CDC's position is that all indoor mold should be treated the same way regardless of type, and that CDC does not recommend mold testing.

The Physics, In One Page

Warm air can hold more water vapor than cold air. The dew point is the temperature at which the air you have right now becomes saturated. EPA's guidance describes the relationship this way: "The higher the dew point, the greater the risk of condensation on cold surfaces. If the air is very dry and has few water molecules, the dew point is low and surfaces must be much cooler than the air for condensation to occur."

Your hygrometer reads the middle of the room. The air in contact with the glass is colder than that, and colder air at the same moisture content has a higher relative humidity. That gap is why windows sweat while the walls stay dry.

It also means you do not need visible water for mold. Building scientist Joseph Lstiburek notes that the critical relative humidity for adverse biological activity on building envelope surfaces is about 70 percent, and that with room air at 70°F and 40 percent relative humidity, any surface below roughly 54°F is already at that threshold. Liquid water shows up at 100 percent surface humidity. Growth starts well before that. A window sash can go moldy in a room that never visibly fogs.

Cornell Cooperative Extension's fact sheet on moisture condensation at windows adds the reason the window loses this contest so reliably: "Glass is a poor insulator. The R value of a single glazed window is approximately 1; a double glazed window is approximately 2. Compared to a typical adjacent wall with an R ranging from 11-20, the window is cold."

Why It Starts At The Edges And The Frame

Look at where the growth actually is. In most homes it is not in the middle of the pane. It is along the bottom rail, in the corners, on the sash, and in the caulk joint between glass and frame.

That is thermal bridging. The metal spacer bar that holds an insulated glass unit apart runs around the perimeter, and Lawrence Berkeley National Laboratory's review of window spacers and edge seals explains that traditional aluminum and galvanized steel spacers "have high thermal conductivity and thus create a thermal bridge on the interior glass pane surface in the edge-of-glass area." That cold band runs roughly 2.5 to 4 inches in from the frame. LBNL notes that "low temperatures in the edge-of-glass region increase the potential for condensation, which, in turn, can lead to mold growth and deterioration of window frames, window seals, and wall sections, especially in cold climates."

The size of that penalty is larger than most people expect. The Minnesota guide, using Lawrence Berkeley data, gives a worked example: insulated glass with a low-E coating and argon fill "may tolerate 61 percent humidity at 0 degrees, but the edge may show signs of condensation at only 26 percent humidity."

So a good window can have dry glass and a wet, moldy perimeter at the same time. Aluminum frames with no thermal break behave the same way, and so does any sash where the glass is set shallowly into the frame.

Glazing type sets the baseline. Approximate U-factors from the same guide: single glazing around 1.10, plain double glazing 0.52, double glazing with low-E and argon 0.37, triple glazing with two low-E coatings and argon 0.32. Lower U-factor means a warmer inside surface and more indoor humidity tolerated before anything condenses. If you are shopping, the NFRC label also carries an optional Condensation Resistance number. ENERGY STAR describes it as a measure of "how well the window resists water build-up," scored on a scale from 0 to 100, with higher being better.

Where The Moisture Is Actually Coming From

Before you blame the windows, find the water. EPA's guidance notes that in homes, "people and their activities, especially cooking and washing of floors and clothes, are usually the leading sources of humidity."

Work through this list:

  • Showers and baths with no exhaust fan, or a fan that runs for two minutes and stops.
  • Cooking without a vented range hood. A recirculating hood filters grease and returns the water vapor to the room.
  • A clothes dryer venting indoors. The Minnesota guide calls this "a very bad idea" because of the moisture and other pollutants it dumps into the living space.
  • Humidifiers. Cornell's first recommendation for reducing water vapor production is blunt: "Do not operate humidifier or other moisture-producing devices in cold weather."
  • A wet basement or crawl space, which feeds the whole building's humidity through stack effect.
  • Unvented combustion heaters. Kerosene and vent-free gas heaters release water vapor and combustion byproducts directly into the room.
  • A new or newly tightened house. If you air sealed and insulated without adding mechanical ventilation, the moisture you used to leak away now stays.
  • Closed blinds and heavy drapes. Cornell notes that "condensation will often occur on windows when shades or draperies are closed because most window treatments separate the cold glass from the warm room air, allowing the glass to further cool below the dew point temperature."

Houseplants are on the list, but far down it. A few pots do not come close to what one unvented shower adds.

Condensation Between The Panes Is A Different Problem

If the fog is inside the glass and you cannot wipe it off from either side, stop treating it as a humidity problem. It is a failed insulating glass unit.

Every sealed double-pane unit contains a desiccant that absorbs the small amount of water vapor that works its way through the edge seal. LBNL describes the endpoint: "The capacity of desiccants is limited, and any water vapor infiltrating the interpane space after the desiccant is saturated can result in fog on the internal surface of the IG unit." The review adds that "IG unit failure most commonly results from water vapor condensation between the glass panes."

You cannot clean it, because the condensing surface is sealed inside the unit, and it is not a health issue, because nothing in that cavity exchanges air with your room. Drilling-and-venting services will clear the haze, but the unit no longer holds its gas fill afterward. The durable repair is replacing the sealed glass unit in the existing frame and sash, which is far cheaper than replacing the whole window.

Condensation on the inside face of a storm window is a third case, and it means something specific. The Minnesota guide explains that "moisture on the inside of the storm window indicates that the prime window is allowing air (carrying moisture) to leak out to the storm window where it condenses." The fix there is weatherstripping and caulk on the interior window, not on the storm. Exterior storms should keep their weep holes open, not be sealed airtight.

What To Do This Week

  1. Buy a hygrometer and put it in the room with the worst windows. EPA notes these cost roughly $10 to $50. You are managing a number you currently cannot see.
  2. Aim for 30 to 50 percent relative humidity, and drop toward the bottom of that range as it gets colder outside. The windows themselves are your feedback loop: persistent morning condensation means the number is too high for the surfaces you have. Our guide to indoor humidity targets by room and season covers how to hit it through the year.
  3. Run the bathroom fan during every shower and for 20 minutes after. That run time is the Home Ventilating Institute's recommendation, and a timer switch makes it automatic. For sizing and ducting, see why bathroom ceilings mold.
  4. Confirm every exhaust terminates outdoors, not into an attic, soffit, or crawl space.
  5. Turn the humidifier down or off for a week and watch what happens to the glass.
  6. Dry the sills each morning while you work the problem, and open blinds and move drapes off the glass during the day. EPA's instruction is to "act quickly to dry the wet surface and reduce the moisture/water source" when you see condensation collecting.

Cleaning What Is Already There

For a window sash, sill, and surrounding trim, this is a small job. EPA's threshold is that if the moldy area is "less than about 10 square feet (less than roughly a 3 ft. by 3 ft. patch), in most cases, you can handle the job yourself."

  1. Ventilate and protect yourself. Open a window. Gloves and eye protection at minimum; EPA suggests goggles without ventilation holes and an N-95 respirator.
  2. Scrub with detergent and water. EPA's instruction for hard surfaces is to "scrub mold off hard surfaces with detergent and water, and dry completely." Mechanical scrubbing is what removes the biofilm. A stiff toothbrush reaches the glazing bead and the track.
  3. Dry it fully. A damp sill restarts the clock immediately.
  4. Deal with the caulk separately. Silicone or acrylic caulk that has gone black is usually stained through. Cut it out, dry the joint, and re-caulk rather than scrubbing indefinitely.
  5. Replace materials you cannot dry. EPA notes that "absorbent or porous materials, such as ceiling tiles and carpet, may have to be thrown away if they become moldy." The same applies to a rotted wood stool or a soaked drywall return.

Vinegar is a reasonable household option on hard, non-porous surfaces, but it does not substitute for scrubbing and drying; we cover what it can and cannot do in how long vinegar takes to kill mold. If you are unsure whether the growth is mold or surface mildew, the distinction changes less about your next step than you would expect.

What Not To Do

  • Do not paint or caulk over it. EPA is explicit: "Do not paint or caulk moldy surfaces. Clean up the mold and dry the surfaces before painting."
  • Do not mix bleach with anything. Bleach plus ammonia produces chloramine gases; bleach plus an acid, including vinegar and many bathroom cleaners, releases chlorine gas. EPA also notes that biocides like chlorine bleach are "not recommended as a routine practice" for mold cleanup.
  • Do not order a mold test to decide what to do about a window sill. CDC does not recommend testing, and the answer would not change the work.
  • Do not seal an exterior storm window airtight. It needs to drain and breathe, or you move the condensation somewhere worse.
  • Do not replace the windows first. Replacement can help, but if the house is generating more moisture than it removes, new windows shift the condensation to the next-coldest surface, which is often inside a wall or closet where you will not see it.

Warming The Surface

If you have cut indoor humidity as far as comfort allows and the glass is still wet, the other lever is surface temperature. In rough order of cost:

  • Air seal and weatherstrip the existing window. Cornell recommends caulking and weatherstripping the sash and the wall around it, noting that "in tightly sealed houses, moisture may pass through window frames (the path of least resistance) and cause condensation." Old double-hung windows with open sash-weight pockets are a specific offender.
  • Add a second layer of glazing. An interior storm panel or an exterior storm window raises the primary glass temperature substantially at a fraction of replacement cost. Interior panels must be sealed continuously to the frame, or you push moist room air into the gap.
  • Improve air movement at the glass. A register or a small fan aimed at a problem window keeps room air moving across the surface. The Minnesota guide suggests exactly this for condensation you cannot otherwise stop.
  • Add mechanical ventilation. In a tight house, Cornell's recommendation is "reduction of water vapor or mechanical ventilation of moisture-laden air, such as with an air-to-air heat exchanger." A balanced HRV or ERV solves the whole-house version of the problem rather than one window.
  • Replace the unit or the window. Worth doing when the glass is single-pane, the frame is unbroken aluminum, or the seal has already failed. Look for a low U-factor and, if published, a high Condensation Resistance number.

Note the sequence. Ventilation and air sealing come before glass, because they address the cause rather than the symptom.

DIY Or Call A Pro

Handle it yourself when the growth is confined to sashes, sills, and trim, the total area is under about 10 square feet, and the window dries out when you run a fan and turn the humidifier down.

Get help when:

  • The area is larger than roughly 10 square feet, EPA's threshold for considering professional remediation.
  • Drywall, plaster, or the wall below the window is soft, stained, or bulging. That is water in the assembly, not surface condensation. See how to tell if there is mold in your walls.
  • Condensation is heavy at windows throughout the house. Cornell treats this as a warning sign of concealed condensation, "that is, condensation in the attic or inside the walls of the house."
  • Water is entering around the window during rain. That is a flashing or sill-pan failure and needs a carpenter or window contractor, not a dehumidifier.
  • Someone in the home has asthma, a mold allergy, chronic lung disease, or a weakened immune system. CDC identifies these groups as being at greater risk, and they should not do the cleanup.

If the pattern is house-wide rather than one window, an energy auditor with a blower door and an infrared camera will locate the cold spots and air leaks far faster than guessing.

Frequently Asked Questions

What humidity level stops window condensation?

There is no single number, because it depends on how cold your windows get. EPA's general target is 30 to 50 percent relative humidity, and the colder it is outside, the closer to 30 you need to be. A double-pane low-E window with argon tolerates far more indoor humidity than single glazing at the same outdoor temperature, and the edge of any unit tolerates much less than its center. Use the windows as the indicator: if they are running with water in the morning, the number is too high for the glass you have.

Is the mold on my window sill dangerous?

For most people it is a nuisance and a maintenance problem rather than a hazard. CDC says exposure to damp and moldy environments "may cause a variety of health effects, or none at all," with people who have asthma, mold allergy, chronic lung disease, or weakened immune systems at greater risk. Color does not tell you anything useful about risk, and CDC advises treating all indoor mold the same way.

Why do my windows sweat more in winter than in summer?

Because the glass is colder. The moisture in your indoor air may not change much between seasons, but in winter the inside surface of the window drops far below the room temperature, so it crosses the dew point that summer glass never reaches. Mold in general follows a seasonal pattern indoors and out, which we cover in when mold is worst during the year.

I have new double-pane windows and still get condensation. Did I get bad windows?

Probably not. New windows are usually tighter than the ones they replaced, which reduces the accidental ventilation that was carrying moisture out of the house. Indoor humidity rises, and the condensation reappears on whatever surface is now coldest, often the frames or the edges of the new glass. The Minnesota guide makes the same point: condensation is not by itself evidence that the windows are defective.

Can I fix fogging between the panes myself?

You can have the sealed glass unit replaced, which is the durable repair, and which usually reuses the existing frame and sash. Defogging services that drill, clean, and vent the cavity will clear the view, but the unit is no longer sealed or gas-filled afterward, so you lose the insulating performance and, with it, the surface temperature that kept the edges dry.

Does a dehumidifier help with window condensation?

In summer and in damp basements, yes. In cold weather it is usually the wrong tool, because a standard refrigerant dehumidifier loses capacity as the room gets cooler, and the underlying issue is a cold surface rather than a warm humid room. In winter, exhaust ventilation, turning off humidifiers, and air sealing do more.

Why is there mold around the window but not on the glass?

Because glass is smooth and non-porous and the frame is not. Paint, wood, caulk, and the dust that settles in the track hold water and provide nutrients. The WHO dampness and mould guidelines, summarized in a document archived by CDC, state that "the dust and dirt normally present in most indoor spaces provide sufficient nutrients to support extensive microbial growth." The glass just delivers the water.

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