Will a Dehumidifier Stop Condensation on My Windows?

Condensation is a surface temperature event, not an air event

Air can hold a certain amount of water vapor at a given temperature, and less of it as it gets colder. Cool a parcel of air far enough and it reaches its dew point, at which the vapor it is carrying starts condensing out as liquid. Inside a heated house in winter the coldest thing in the room is almost always the window, so the window is where that happens first. The National Research Council of Canada’s building digest on the subject states it plainly: condensation “occurs on inside window surfaces whenever surface temperature falls below the dew-point temperature of the room.”

That single sentence is why two houses at the same humidity can behave completely differently. The humidity sets the dew point; the window sets the surface temperature. You only get dry glass when the second number stays above the first, and you can move either one.

The published table almost nobody quotes

The NRC digest works the arithmetic out for you and gives the maximum indoor relative humidity you can carry at 70°F indoors without condensing on the glass. We have reproduced the figures for a 15 mph wind, which is the realistic case - wind strips heat off the outer pane and makes the inner surface colder:

Look at the single-glazing column and the whole question answers itself. At 0°F outside, a single-glazed window condenses above 12% indoor relative humidity. Nobody lives at 12%, and no dehumidifier on this site will take an occupied room there - which means that in a cold snap, wet single glazing is not a humidity fault you can buy your way out of. It is the window.

Now look at the double-glazing row for 20°F: 53%. If your windows are double glazed and wetting at 20°F outside, your indoor humidity is above about 53% and that is squarely a dehumidifier’s job - and it is also above the 50% ceiling of the EPA’s ideal band, so you have a second reason to bring it down.

Which situation you are in: the two-minute test

Put a hygrometer in the room with the wet windows and read it in the morning, when both the humidity peak and the cold are at their worst. Then:

Where the moisture is coming from in the first place

A dehumidifier treats the symptom efficiently, and it is worth knowing what it is competing with. In an occupied house the moisture is mostly produced indoors: showering, cooking, drying laundry on a rack, and the occupants themselves. Removing the source is free and permanent, and the appliance only has to handle what is left. Our how to lower humidity without a dehumidifier page is the order to work through: run the extractor long enough, stop drying clothes indoors, and keep interior doors to the wet rooms shut.

The one that catches people is ventilation direction. Opening a window helps only when the outdoor air genuinely holds less moisture than the indoor air, which in a cold snap it almost always does - cold air cannot carry much. In humid summer weather the same action makes the room wetter. That is also the reason summer condensation appears on the outside of a well-insulated window, which looks alarming and means nothing is wrong.

Why morning bedrooms are the worst case

Almost every report of window condensation is about a bedroom at dawn, and the physics stacks up against that room specifically. A sleeping adult adds moisture to the air all night in a space with the door shut and the heating down. The air cools, so its relative humidity climbs even though the amount of water in it has barely changed, and it meets glass that has had eight hours to get cold. Three effects, one window.

Which is why the practical fix in a bedroom is often not a bigger machine but a quieter one running at a sensible setpoint overnight - the subject of our bedroom dehumidifiers page. Set the humidistat to 50% and let it cycle. A unit set to the bottom of its range will run all night to chase a number you do not need.

Why this matters beyond the windowsill

Wet glass is cosmetic. What it is reporting is not. The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, precisely because sustained moisture on cold surfaces is what mold grows on - and the window is only the surface you can see. The same conditions exist behind furniture on external walls, in closed wardrobes and in corners of the room that never get airflow. Our musty smell page is what happens when this goes unaddressed for a season.

What a dehumidifier will not do

• It cannot warm your glass. Everything in the single-glazing column of that table stays true no matter what appliance you run.
• It cannot beat a failed sealed unit. Moisture between the panes means the seal has gone and the cavity is drawing in outdoor air. That is a glazing repair; the room humidity is irrelevant to it.
• It will not help with outside-surface condensation in summer, which is a sign the window is working well rather than badly.
• It loses capacity in the cold, which is the exact weather you want it in. A compressor unit removes noticeably less water in a cold room and can frost its coil - look for a stated low-temperature operation figure or auto-defrost before putting one in an unheated space.
• It will not fix a draft that is chilling one pane. A cold spot is a surface-temperature problem wearing a humidity costume.

What we would actually do

Count the panes, then read the room. Double or triple glazing plus a morning reading above 55% is the case where a dehumidifier simply works, and it is the most common case in a modern house with a family in it. Single glazing plus a reading in the 40s is the case where buying one is a way of spending money on the wrong component - secondary glazing, an insulating blind or a thermal film will do more for the glass than any appliance can, and you can confirm that for the price of a hygrometer before committing to either.