How to stop condensation on windows

Condensation on the inside of a window is not a window fault, a ventilation fault or a heating fault on its own. It is a meeting of two conditions, and the EPA states the mechanism in one sentence: “When warm, humid air contacts a cold surface, condensation may form.”

That gives you the whole solution space. The glass is colder than the dew point of the air in the room. Raise the glass temperature above the dew point, or lower the dew point below the glass temperature. Everything else — wiping, towels, window vacuums, bowls of salt — is managing the symptom.

The EPA publishes a single band in two separate documents, with the same figures:

• “Indoor relative humidity (RH) should be kept below 60 percent — ideally between 30 percent and 50 percent, if possible.”
• “Keep indoor humidity low. If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.”

That band exists for mold rather than for your windows, but the two problems share a cause and so they share a fix. The EPA is explicit about the link: “Mold is frequently found on walls in cold corners behind furniture where condensation forms.” Wet glass is the visible version of what is also happening behind the wardrobe on the north wall.

It also sets the clock on acting: “If wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow.” A window sill that is wet every single morning never gets its 24 hours.

This is free or nearly free, and the EPA’s own list of actions to reduce humidity puts it before buying equipment. In rough order of how much water each one puts into a house:

1. The clothes dryer. The guidance is to “vent appliances that produce moisture, such as clothes dryers, stoves, and kerosene heaters to the outside where possible”. An unvented or badly vented dryer is the largest single moisture source in a house, and drying clothes on an airer indoors is the same water with no appliance attached. How to dry laundry indoors without fogging the house.
2. Showers. “Run the bathroom fan or open the window when showering.” The published guidance is to keep the fan running at least 20 minutes after each use, which almost nobody does. Why 20 minutes, and what it costs.
3. Cooking. “Use exhaust fans or open windows whenever cooking, running the dishwasher or dishwashing.” Lids on pans is the free half of this.
4. Unvented combustion. A kerosene or bottled-gas heater produces water as a combustion product, in quantity, directly into the room it is heating. The EPA names them alongside dryers for venting.
5. People, plants and fish tanks. Smaller, constant, and not worth doing anything about except knowing they are there.

The other half of the mechanism gets less attention because it is less satisfying, but the EPA lists both: “Increase air temperature” and “Cover cold surfaces, such as cold water pipes, with insulation”.

• Heavy curtains make window condensation worse, not better. They insulate the glass from the warm room, so the pane runs colder overnight and collects more. The same is true of a blind pulled tight to the frame.
• Furniture against an external wall does the same thing to the wall behind it, which is exactly the cold-corner case the EPA describes mold appearing in.
• Air movement across the glass raises its surface temperature. The guidance says to “increase ventilation or air movement… Use fans as needed”. A small fan pointed along a cold wall is a genuinely effective and very cheap intervention. Circulators, which are built for exactly this.
• Single glazing will not be fixed by any of this. A single pane in cold weather sits close to the outdoor temperature, and its dew point requirement is beyond what a normal, comfortable indoor humidity can meet. Secondary glazing or insulating film raises the surface temperature; a dehumidifier chasing it will run forever. What window film does, measured.

Nothing about a house changes in November except the temperature of its surfaces. Outdoor air in winter holds very little water, so when it leaks in and gets heated, the relative humidity of the room falls — which is why the same house that fogs its windows in January can also feel dry enough to crack furniture.

Both things are true at once, and it confuses people into the wrong purchase. The room air can be moderately dry by the measure that matters for comfort and still be far too wet for a pane of glass at 40 F. The glass is not reporting on the whole house; it is reporting on the coldest surface in it.

Which is why the first thing to buy is not a machine. It is a way to read the number the EPA publishes, because “condensation can be a sign of high humidity” is a sign and not a measurement, and the band you are aiming for is 30 to 50 percent. Whether you need to add moisture or remove it turns entirely on that reading.

Dehumidifiers are rated on IEF — integrated energy factor, liters of water removed per kilowatt-hour. That makes the cost of solving this problem unusually easy to state. Divide the rate by the IEF and you have the price of a liter.

The useful consequence: the whole certified field is within about three cents a liter of itself. Choosing the right capacity and the right technology for a cold room matters far more than squeezing the last point of efficiency, which is why the desiccant unit earns a place here despite publishing nothing.