Last Updated on August 25, 2026 by Umar Farooq
Water running down a basement window in January is not a window fault. It is your basement telling you the humidity is too high for the temperature outside, and that second half is the part almost every page gets wrong.
The most widely read general article on window condensation tells readers to keep indoor humidity between 50 and 55 percent. In cold weather that figure is roughly double what the authorities publish, and following it is a reliable way to produce exactly the problem you searched for.
The number changes with the weather outside
This is the whole thing. There is no single correct indoor humidity. There is a ceiling that falls as it gets colder outdoors, because the colder it is outside, the colder your glass is, and the less moisture the air touching it can hold.
| Outdoor temperature | Maximum indoor humidity |
|---|---|
| 20 °F to 40 °F (−7 to 4 °C) | 40% |
| 10 °F to 20 °F (−12 to −7 °C) | 35% |
| 0 °F to 10 °F (−18 to −12 °C) | 30% |
| −10 °F to 0 °F (−23 to −18 °C) | 25% |
| −20 °F to −10 °F (−29 to −23 °C) | 20% |
| Below −20 °F (below −29 °C) | 15% |
That independent check is worth quoting because it ties the number to your glazing:
The same source puts triple glazing at closer to minus forty before it starts, and recommends aiming for around 40 percent through winter generally. A federal energy programme gives 30 to 40 percent during the heating season to prevent window condensation.
So if your hygrometer reads 50 percent in a cold snap, you have found your answer without doing anything else.
Why the basement window and not the ones upstairs
Three reasons, and they stack.
Condensation always starts on the coldest surface
Basement windows are usually the oldest and worst glazed in the house. Single pane, or an early double unit, often in a steel or aluminium frame.
Basements are named as high risk
This is not an assumption. A national energy agency lists it directly, saying condensation is more likely to occur in humid areas such as the kitchen, bathroom and some basements and crawl spaces. The fenestration association separately names the basement among the locations that affect condensation potential.
The stack effect puts the inlet at your feet
Warm air rises and leaves the top of a house, which pulls replacement air in at the bottom. In building science terms, in heated buildings the air tends to flow out of the top of the building while inducing air to flow in at the bottom.
The bottom of the building is your basement. It gets the cold incoming air, and the effect gets stronger the colder it is outside and the leakier the house.
A fourth factor gets asserted everywhere and we could not find anyone who has measured it: that a window well makes the glass colder than an equivalent window above ground. It is plausible, and we are not going to state it as fact. What the well genuinely does is covered further down, and it is a different mechanism.
Look at where on the window it starts
A window is not one temperature. Ratings bodies treat it as three thermally different parts, the centre of the glazing, the edge of the glazing and the frame, and the window is rated on whichever performs worst.
| Where it appears | What it means |
|---|---|
| Bottom edge and corners first | Normal. The spacer between the panes runs around the edge, and where that spacer is metal it conducts heat straight through, so the glass is coldest there |
| On the frame rather than the glass | A conductive frame. Very common on old steel and aluminium basement windows |
| Across the whole pane | Humidity well above the ceiling for that outdoor temperature, or single glazing |
| Between the two panes, and you cannot wipe it off from either side | A different problem. The sealed unit has failed. Lowering humidity will not help; the glass unit needs replacing |
On the old metal frames specifically, the ratings body is direct. The biggest disadvantage of aluminium as a window frame material is its high thermal conductance, and in cold climates a simple aluminium frame can easily become cold enough to condense moisture or frost on the inside surfaces of window frames. The fix in newer windows is a thermal break, splitting the frame into interior and exterior pieces joined by something less conductive.
If you are replacing, the number to ask for is the Condensation Resistance rating. Higher is better on a scale the ratings body gives as 1 to 100. It is an optional rating, so plenty of labels leave it off. You will see advice to look for a specific threshold figure; we could not trace that number to any primary source, so treat it as folklore and compare products against each other instead.
Frost instead of water
Same problem, colder glass. When air contacts a cold window it loses heat and can no longer hold all its water vapour, and if the window is extremely cold the condensation appears as frost rather than liquid.
Diagnostically that is useful. Frost means the surface is not just below the dew point but below freezing, so you are further from the fix than someone with running water. Lowering humidity alone may not be enough, and you will also need to raise the glass temperature.
It is also more damaging than it looks, for a reason nobody points out. Liquid water runs off a little at a time and some of it evaporates. Frost accumulates for as long as it stays cold, so the sill stays dry and the problem looks mild, and then a thaw releases the whole accumulation at once into the sill and the reveal. That reasoning is ours rather than a published finding, but the damage pathway is documented:
Check the window well before you buy anything
Here is a basement-specific link nobody makes. If one window is far worse than the others, look outside it.
A well that holds water is a moisture source sitting against the wall at that exact window. So the window that condenses worst is often the window with the failed well, which makes it a diagnostic rather than a coincidence.
Extension guidance on doing it properly: the well should be filled from the footing to the window sill with three-eighths to three-quarter inch coarse aggregate, with a supplemental drain tile extension running from the footing to the base of the well.
What actually works, in order
1. Get the humidity under the ceiling for the weather
Use the table. This is the only fix with a number attached, and on most basement windows it is the whole answer.
Finding the moisture is the other half. A basement dryer venting indoors, laundry drying down there, or an uncovered sump pit will each swamp everything else you do. Vent appliances that produce moisture to the outside where possible.
2. Let air reach the glass. This is free.
- Open blinds and curtains so air can circulate freely over the windows.
- Direct heat towards exterior walls and windows.
- Take the clutter off the sill. Where condensation at window sills is a persistent problem, removing objects on the sill matters because they prevent air circulating and removing the moisture. In a basement that sill is usually holding paint tins.
- Move furniture and shelving so it is not touching outside walls, which improves circulation around the cooler wall.
3. An interior storm panel or insert
This is the fix with the most impressive measured result, and it appears on no other page about basement windows. A federal building programme published a thermal image of an interior storm window showing the interior panel at 50 °F while the existing glass beside it sat at approximately 0 °F.
That is the mechanism doing exactly what you need: the surface you are looking at is now far warmer, so the air touching it is nowhere near its dew point. The same guidance notes that low-emissivity storm windows reduce the potential for interior condensation problems on the window system.
The caveat is the important part and nobody mentions it:
A leaky insert does not stop the condensation. It moves it somewhere you cannot see it, which is worse. Seal the edges properly or do not bother. Plastic film kits work on the same principle, and a federal energy programme credits them with reducing moisture buildup on the window due to condensation.
4. Replace the window
Triple glazing is the step change, on the numbers above. If you are buying, look for a non-metal or thermally broken frame and a warm edge spacer, since windows with warm edge technologies and non-metal frames are less likely to have condensation on the frame or at the edge of the glass.
The dehumidifier problem nobody mentions
Every page about this recommends a dehumidifier. Almost none of them mention that a cold basement in winter may be below the temperature at which a normal one works.
Sixty-five degrees Fahrenheit is about 18 °C. Plenty of unheated basements sit below that all winter, which means the machine can be running, humming, costing money and removing nothing.
The guidance is to consider a product specified for use at lower temperatures if the space typically falls below that, and notes that some units have an anti-frost sensor that shuts the unit off below a set point. The reason this limit exists is the cold coil, so desiccant machines, which do not use one, are not subject to that particular failure. You will find precise cut-off temperatures quoted for them online. We could not find a manufacturer datasheet or independent test behind any of those figures, so check the specification of the actual model rather than trusting a number from a review site.
Three things you will be told, corrected
“Never open a window in winter”
This one is usually stated as a rule and it is wrong. Cold air is dry in absolute terms, so letting it in genuinely lowers indoor humidity. Two authorities recommend it. An extension service advises temporarily opening windows just a little in winter to allow warm, moist air to leave the home and cold, dry air to come in, and the environmental regulator lists increasing ventilation, if outside air is cold and dry, among its moisture controls.
The sensible qualification for a basement: ventilate briefly, and preferably not through the window that is already condensing, since holding that one open chills the glass and frame further and admits air right at the stack effect’s inlet.
“Turn the heating up”
Partly true, and worth understanding rather than dismissing. Heat raises the glass surface temperature and lowers relative humidity, and directing heat towards exterior walls and windows is recommended. What it does not do is remove any moisture. If the source keeps running, the window condenses again as soon as the surface cools.
“Put a tub of moisture absorber on the sill”
Look at the units. Passive desiccant tubs are sold by the ounce of lifetime capacity. Dehumidifiers are rated in pints per day. Those are not the same order of magnitude, and the unit of measurement tells you that before any test does.
We are not going to quote you a specific absorption rate, because every figure we found for these products traces back to affiliate review sites rather than to a manufacturer specification or an independent test.
A sensible order of work
- Read the hygrometer and compare it against the table for today’s outdoor temperature. Most people stop here, because they are 15 points over.
- Find the moisture source. Laundry, an unvented dryer, an open sump pit, a wet floor.
- Clear the sill and open the blinds. Free, and often visible within days.
- Check the window well from outside, especially if one window is worse than the rest.
- Check your dehumidifier can work at the temperature down there before concluding it is not helping.
- Then consider an insert or new glazing, sealed properly at the edges.
FAQ
Is condensation a sign my window has failed?
Only if it is between the panes. Wipe it. If it comes off, it is surface condensation and a humidity problem. If it is trapped inside the sealed unit and cannot be reached from either side, the seal has gone and the unit needs replacing.
What humidity should I keep the basement at in winter?
It depends on the outdoor temperature, which is the point of the table. Around 40 percent when it is near freezing outside, dropping toward 30 percent and below in a serious cold snap.
Should I put a cover on the window well?
Possibly, and the honest answer is that the evidence is entirely commercial. Every source arguing either way sells covers or waterproofing. What is documented is that a badly drained well is a moisture source against the wall. A cover that keeps rain and snowmelt out of a well addresses that. A cover over a well that still drains badly does not.
Why is only one window doing it?
Usually because that one is coldest, has the least air movement, or has a wet well outside it. Compare the frames, look at what is stacked in front of it, and look outside before assuming the window is faulty.
Will insulating the basement walls fix it?
It helps the walls and can make the window slightly worse in relative terms, because the glass is then even more clearly the coldest surface in the room. It is still worth doing for the walls. Just do not expect it to solve the window.
