If you have old aluminum sliders in the Sunset or the Richmond, you probably know the routine already. Sometime in November the glass starts fogging up overnight, and by morning there’s water running down into the track. A few weeks later the corners of the frame turn black, and the paint on the sill starts to blister.
Condensation is one of the questions we get most often in winter, and the answer isn’t always “you need new windows.” Sometimes the windows are the main problem, sometimes the house is, and quite often it’s a bit of both.
Why condensation gets worse in winter
Water shows up on the glass when warm, damp air inside the house touches a surface that’s colder than it is. On a winter night in San Francisco, the coldest surface in most rooms is the window. Everything that adds moisture to the air, from cooking and showers to houseplants and a couple of people sleeping with the door closed, ends up on that glass.
Older San Francisco houses make it easy for this to happen. Single-pane glass is still common, and bare aluminum frames carry the outdoor cold straight into the room. Bedrooms are small and get shut tight at night. On the west side of the city, in Parkside, Daly City and Pacifica too, the air outside stays damp for much of the year, so the walls and window frames rarely get a chance to dry out completely.
Where the water is tells you what’s wrong
Before doing anything, look closely at which side of the glass the water is on.
When it’s on the inside surface, the room is humid and the glass is cold. This is the kind you wipe off every morning. It can be reduced from both directions, by taking moisture out of the air and by making the inner pane warmer.
Water on the outside of the glass is a different story. On newer dual-pane windows, you’ll sometimes see dew on the exterior on a cool, damp morning. That’s normal and burns off once the sun hits it. It happens because the outer pane is staying cold, which means heat from the house isn’t leaking through.
Fog trapped between the two panes is the one that can’t be fixed with a towel. It means the seal around the glass unit has failed and moist air has worked its way inside. Cleaning won’t help and it won’t go away on its own, so the glass unit or the whole window has to be replaced. If you’re finding this on more than one window, it’s worth checking the other signs in our guide on how to tell if your windows are actually failing.
Cutting down the moisture inside
For water on the inside of the glass, the first fixes are cheap and worth trying before anything else.
The bathroom fan does more than people think. Run it for the whole shower and leave it on for about twenty minutes afterward. If the mirror is still fogged when you’re done, the fan is probably too small, or it isn’t actually vented outside, which is surprisingly common in older houses.
In the kitchen, turn on the hood whenever there’s a pot boiling. Try not to dry laundry on a rack inside during the winter months. And leave the curtains open during the day: heavy drapes pulled shut all night hold a pocket of cold air against the glass, which makes the problem worse.
A few minutes with a window cracked open in the morning helps more than you’d expect, and a small dehumidifier can make a big difference in the one bedroom that’s always the worst. Whatever you do, wipe the sills dry instead of leaving the water to soak in, and clean the mildew out of the tracks before it gets into the frame.
When the windows themselves are the problem
There’s a limit to what ventilation can do with single-pane glass in an aluminum frame. On a cold night the inside of that glass is almost as cold as the air outside, so it will collect water no matter how dry you keep the room.
New windows change that. Dual-pane glass with a Low-E coating keeps the inside pane much closer to room temperature, and warm-edge spacers get rid of most of the cold band that forms around the edges of the glass. Vinyl, fiberglass and thermally improved aluminum frames don’t conduct cold the way old aluminum does. If you want to compare windows by how well they hold heat, our article on what U-factor means explains the numbers.
In fog-belt houses with old aluminum sliders, we most often replace them with vinyl windows → or thermally improved aluminum windows, usually as a retrofit into the existing frames so the stucco stays untouched. In a bathroom, it’s worth doing that together with a better fan, since the windows alone won’t clear the steam.
When to worry about mold and rot
Some mildew on the glass and in the track is unpleasant but easy to clean. It’s more serious when the wood on the sill feels soft when you press it, when paint is peeling and the wood underneath has gone dark, or when there are stains on the wall below the window. At that point water has probably been getting into the frame or the wall for a while, and someone should take a look before it spreads. Rot in the frame is also one of the most common reasons a window can’t be retrofitted and needs full-frame replacement instead.
Frequently asked questions
Is condensation on the inside of my windows normal?
A little on the coldest mornings, especially on single-pane glass, is common. If you’re wiping the glass every day, finding puddles on the sill or seeing mildew in the tracks, the room is too humid, the glass is too cold, or both.
Why is there condensation between the panes of my window?
The seal around the glass unit has failed and moist air has gotten inside. It can’t be cleaned from the outside, so the glass unit or the window has to be replaced.
Will new windows stop condensation?
They reduce it a lot, because dual-pane Low-E glass keeps the inner pane warmer. In a very humid room you may still see some on the coldest mornings, so ventilation still matters.
Is condensation on the outside of my windows a problem?
No. On newer dual-pane windows it usually means the glass is insulating well, and it clears up once the morning warms up.
If the windows are wet every morning and you’re not sure why, book a free estimate. We’ll look at the windows and the rooms around them and tell you what’s causing it.
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