Condensation on the Inside of Windows

Water on the inside of a window in January means one thing: the glass surface is colder than the dew point of the air in the room. That is not an opinion or a defect claim, it is a measurement.

Dew point is the temperature at which air has to release moisture because it cannot hold any more. Air at 70 degrees and 50 percent relative humidity has a dew point around 50 degrees. Any surface in that room colder than 50 degrees will collect water. In a Spokane winter with outdoor temperatures in the twenties, plenty of surfaces qualify.

The window shows it first because glass is the least insulated part of the wall and because you can see through it. Everything else that is equally cold is behind drywall or insulation.

What else is getting wet at the same time

This is the reason the page exists. If your windows are streaming, these are almost certainly collecting water too, and none of them are visible:

• The rim joist, the band of wood at the top of the foundation, behind whatever insulation is stuffed in front of it. Reliably the wettest hidden surface in a cold climate house. See rim joist condensation.
• Exterior wall corners, where three surfaces meet and the framing creates a thermal bridge with less insulation than the cavity beside it.
• The inside of closets on exterior walls, which are unheated, unventilated and full of organic material. See mold in a closet on an exterior wall.
• Attic sheathing, where indoor air leaking upward deposits moisture on the cold underside of the roof deck.
• Behind furniture pushed against an exterior wall, where a headboard or a sofa blocks room air from warming the wall surface.
• Uninsulated ductwork running through unconditioned space.

The rule that matters: colder outside means drier inside

This is counterintuitive and it is the single most useful thing on this page. The colder it gets outside, the lower your indoor humidity has to be to avoid condensation, because your window and wall surfaces get colder along with the weather.

Rough guidance for a typical double glazed window, assuming 70 degrees indoors. These are starting points to aim at, not code:

Single pane or metal framed windows need lower numbers than this, because the glass runs colder. If you have storm windows over single glazing, the interior surface is warmer and you get more margin.

The practical failure is a furnace humidifier set once in October for comfort and never adjusted. That setting is fine in November and wrong in January, and the homeowner does not connect the fogging windows to the dial in the basement.

Where the indoor moisture comes from

Showers, cooking, dishwashers, laundry, houseplants, aquariums, firewood stored indoors, and people simply breathing. A family of four puts several gallons of water a day into the air of a house without trying.

Then the sources people do not count: a furnace humidifier, a crawl space with bare soil evaporating ground moisture into the house through stack effect, an unvented gas appliance, and drying laundry on a rack indoors.

A newer, tighter house makes all of this worse, which is worth saying clearly because it sounds backwards. An old leaky house exchanged its air constantly and dumped moisture outside without anyone noticing. A well sealed modern house retains it. That is why newer construction with no mechanical ventilation can have worse winter humidity problems than a drafty 1920s bungalow, and why a bath fan that actually runs matters more now than it used to.

What to do

1. Measure. A hygrometer costs about fifteen dollars. Compare your reading against the table above for the current outdoor temperature.
2. Turn the furnace humidifier down or off and see what happens over a week. In a lot of houses this is the entire fix.
3. Run bath fans during and for twenty minutes after every shower, and confirm they actually vent outside rather than into the attic. A sheet of tissue held to the grille should stick when the fan runs.
4. Use the kitchen range hood, and check that it ducts outdoors rather than recirculating through a charcoal filter.
5. Cover bare crawl space soil with polyethylene. This removes a continuous source most people never consider.
6. Open curtains and blinds during the day and move furniture a few inches off exterior walls, so room air can reach those surfaces.
7. Wipe the sills. Unglamorous, but standing water on a painted wood sill is what turns a physics problem into a rot problem.

Condensation between the panes is a different thing

Fog or haze trapped between two panes of glass that you cannot wipe off means the insulated glass unit has lost its seal. That is a window repair, not a humidity problem, and it does not respond to anything you do with a dehumidifier. It also does not cause mold in the house, because the moisture is sealed inside the glass assembly.

When it becomes a testing question

Condensation by itself does not need us. Growth does, and so does water that has been there long enough to get into materials.

Call when there is visible growth on the sash, frame, sill or the drywall return around the window. When the drywall or plaster around a window is soft, stained or crumbling, which means water has been getting into the wall rather than just sitting on the glass. When you have corrected the humidity and growth keeps coming back, which usually means water is entering the assembly from outside rather than condensing on the inside. When there is growth in the closets or corners as well, which indicates a whole-house humidity condition rather than a window issue. Or when someone in the house is reacting.

Pricing

Why an independent tester

Neither Washington nor Idaho licenses mold assessment or mold remediation. The Washington Department of Health states plainly that there are no certification requirements and that anyone can call themselves a mold specialist. New York made it illegal for one company to both assess and remediate the same job under Labor Law Article 32. Nothing like that exists here, which means the company telling you how bad it is and the company selling the fix are frequently the same company.

We test only. No remediation, no reconstruction, no referral fees from the companies that do that work. The report reads the same whether the answer is that your house is fine or that something needs to be opened up.

The one sentence version

Your windows are a free, continuously updating readout of whether the humidity in your house is appropriate for the weather. When they fog, something you cannot see is getting wet too, and the answer is almost always to put less water into the air rather than to buy different windows.

Frequently asked questions

Are foggy windows a sign of bad windows? Not usually, if the moisture is on the room side and wipes off. That is a humidity and surface temperature issue. Fog sealed between the panes that cannot be wiped is a failed glass unit and is a window repair.

What indoor humidity should I run in winter? Lower as it gets colder outside. Roughly 40 percent at 30 degrees, 35 percent at 20, 30 percent at 10, and 25 percent or less near zero, for typical double glazing at 70 degrees indoors. Single pane windows need lower numbers.

My new windows fog more than the old ones did. How? Usually because the new windows sealed up air leakage that had been drying the house out for free. The moisture that used to escape now stays inside and raises the relative humidity. It is not a defect, it is a ventilation gap that the tighter envelope exposed.

Is the black stuff on my window sill dangerous? It is typically growth feeding on dust and paint film in a repeatedly wet spot, and on a hard surface it is usually straightforward to clean. What matters more is whether the same conditions have produced growth in places you cannot clean, like the wall cavity or a closet.

Should I buy a dehumidifier for winter? Usually not the first move. Winter indoor humidity in a heated house is normally best managed by reducing the sources and by exhausting moisture at the point it is produced, which costs far less to run. A dehumidifier is a summer and basement tool more than a winter tool.

Can condensation rot the window frame? Yes, over time. Standing water on a painted wood sill gets under the finish, and repeated cycles lead to soft wood at the corners of the sash and the sill. That is the practical reason to wipe them rather than let it go all winter.