If you only do one thing: buy a $12 hygrometer, put it in the main living area, and look at it. Water on your windows is a humidity reading, and once you have the actual number this whole problem becomes a dial you turn instead of a mystery.
The first hard cold snap arrives, and the next morning there is water running down the inside of the glass, puddled on the sills, maybe a little frost in the corners. People call us about the windows. It is almost never the windows.
Your house holds a certain amount of moisture. The glass is the coldest surface in the room. Moisture leaves the air at whatever it touches first, and the first thing is always the window. The glass is not the problem, it is the gauge.
What the number should be
The University of Minnesota published the table everybody in this trade still uses, and it answers the question directly: at 70 degrees inside, how humid can the air get before your windows start running? It depends almost entirely on what kind of windows you have.
Maximum indoor relative humidity before condensation, at 70 degrees indoors:
| Outdoor temperature | Single pane | Single pane plus a storm window | Double pane |
|---|---|---|---|
| 40 degrees | 43% | 58% | 63 to 68% |
| 30 degrees | 32% | 48% | 54 to 58% |
| 20 degrees | 24% | 40% | 47 to 50% |
| 10 degrees | 18% | 32% | 40 to 43% |
| 0 degrees | 12% | 27% | 32 to 37% |
Read one row and the whole thing clicks. On a 20 degree Northern Virginia night with double pane windows, anything above about 47 percent indoor humidity and the glass will run. Below it, it will not.
And notice what the table does to the target. The EPA’s general guidance for a house is 30 to 50 percent, kept below 60. That is a year-round number. In January it is the ceiling, not the goal, and on a really cold night even 40 percent may be too much for older windows.
Single pane owners, in Alexandria, Arlington, Falls Church and the older parts of every town here: your windows will sweat at humidity levels that are genuinely too dry to be comfortable. That is not your fault and it is not fixable with a dehumidifier. It is what single glazing does.
Why it always starts in the corners
Look at where the water actually is. It is at the bottom edge and up the sides, in a band, before it is anywhere near the middle of the pane.
That is the frame and the spacer. The edge of an insulated glass unit has metal or composite running through it joining the two panes, and the frame around it is in contact with outdoor air. Those edges run colder than the center of the glass, so they reach the dew point first. The Minnesota table is built around that: run the pure physics for the middle of the pane and you get numbers several points more forgiving than the table gives. The difference is the edges, and the edges are where you will see it.
Practical consequence: a band of condensation at the bottom of the glass on the coldest morning of the year is close to normal. Water across the whole pane, or water that is still there at noon, is not.
Where the water is coming from
It got into the air somewhere, and the list is short.
People. Breathing and sweating. Four people put a meaningful amount of water into a house every day just by living in it, and a full house over the holidays is a humidity event.
Showers and cooking, especially with the bath fan not running or not actually vented to the outside. Pull the grille and look, because a fan that dumps into an attic is a common and expensive mistake.
The dryer, if the vent is disconnected, crushed, or packed with lint. This one is worth checking on its own merits.
A humidifier doing exactly what you told it to. If you have a whole house humidifier, it has a control with a number on it, and most of them are set once and never touched. It needs to come down as the weather gets colder, which is the entire point of the table above. Some have an outdoor sensor that does this automatically. Many do not.
The crawlspace or the basement. This is the big one people miss. A damp crawlspace with bare dirt pushes moisture up into the house all winter, and no amount of adjusting a humidifier fixes it, because the humidifier is not what is running. That is its own post, here.
A house that got tighter. New windows, new siding, air sealing, all good things. They also mean the moisture you generate has fewer ways out than it used to. A house that never had a condensation problem and suddenly does, right after an efficiency project, has not developed a fault. It has changed.
When it stops being cosmetic
Most window condensation is a nuisance. Here is where it is not.
Water pooling on wood sills every morning. That finish will fail and then the wood will. Wipe it and fix the cause.
Dark spots in the corners of the frames, or a musty smell in a closed room. Sustained high humidity on a cold surface is what biological growth needs, and the EPA’s advice is to keep indoor humidity under 60 percent for exactly this reason. Get the number down before you clean anything, or you will be cleaning it again.
Water on the walls, not just the windows. Usually an exterior wall, often a north-facing one, often behind furniture. If the wall is cold enough to sweat, you have both a humidity problem and an insulation problem.
Frost inside the glass that does not melt all day, or ice at the bottom of the sash. That is well past the dew point and into freezing, and it means the humidity is considerably too high for the weather.
Condensation on the inside of the toilet tank and the cold water pipes at the same time. That is a whole-house humidity level, not a window issue.
The one that is actually a window problem
If the fog is between the panes, where you cannot wipe it off from either side, that is different. The seal on that insulated glass unit has failed and the space between the panes is no longer sealed. It is not a humidity problem, it is a window problem, nothing you do to the air in the room will change it, and the fix is a new glass unit or a new window. Glass companies do that, we do not.
What to actually do
Get the number first. A hygrometer costs about twelve dollars. Put one in the main living space and one in the worst bedroom. Everything below depends on knowing where you are starting.
Turn the humidifier down before you do anything else. Find its control, which is usually on the unit itself or on the ductwork beside the furnace rather than on the thermostat, and bring it down. Use the table. When it warms back up, you can bring it up again.
Run the bath fan for twenty minutes after a shower, not five. A cheap timer switch does this for you and is one of the best small purchases in a house.
Use the range hood, if it actually vents outside rather than recirculating through a filter.
Open the curtains during the day. Heavy drapes closed on a cold window trap moist room air against the coldest surface in the house. That is why the worst condensation is often behind the nicest curtains.
Move furniture a couple of inches off exterior walls so air can get behind it.
Crack a window for a few minutes on the coldest days. It is counterintuitive and it works. Air that is 15 degrees outside holds almost no water, so a short exchange drops indoor humidity noticeably and costs very little to reheat.
Deal with the crawlspace or the basement if that is the source, because everything else on this list is management and that is a fix.
Now the other direction, because most houses here have the opposite problem
We should be honest that condensation is the less common complaint. Far more Northern Virginia houses are too dry in January than too wet: static shocks, cracked furniture, bloody noses, a piano that will not hold tune, and everyone in the house with a sore throat.
The same table is the answer to that too. It tells you the ceiling, and comfort sits just below it. If you are at 22 percent on a 30 degree day with double pane windows, you have room to add moisture, and a whole house humidifier is a reasonable purchase. Ours run $651 to $1,816 installed for a bypass or fan-assisted unit, with most landing between $1,025 and $1,299, and $1,256 to $4,000 for a steam unit. Whether it is worth it, and which of the three kinds you want, is here.
The mistake is installing one and setting it to a single number for the whole winter. A humidifier set at 45 percent is comfortable in November and running down your windows in January.
Do this week
Buy the hygrometer. Find your humidifier’s control and learn where it is set. Screenshot the table above, or just remember one row: double pane, 20 degrees out, stay under about 47 percent.
This is one of ten in the Cold Season Field Guide. If the dry air is your problem rather than the wet, winter indoor air is here.
SwiftPro does indoor air and humidity work across Northern Virginia. (703) 997-9757, or book a time online.
