My Windows Started Dripping Every October. The Humidity Was Only 50% — Dew Point Explained Why
Every October, the same thing happened in our house: the first properly cold night arrived, and by morning the bedroom windows were streaming with water. The sills were damp, a dark line of mildew was creeping along one frame, and I was convinced the house was "too humid." So I bought a hygrometer. It read 50% — a perfectly normal, even textbook-healthy number. The windows were still dripping the next morning. The number I'd been watching was the wrong one. What I needed was the dew point, and once I understood it, the fall condensation problem stopped being a mystery.
This guide is for homeowners and renters who see foggy windows, damp sills or musty corners when the weather turns cold. By the end you'll know how to calculate your dew point, compare it with your window temperature, and decide whether you need a dehumidifier or just a bit of ventilation.
⚠️ Disclaimer
This article is for informational purposes only and does not constitute professional building or health advice. If you have persistent mould, structural dampness or respiratory symptoms, consult a qualified building inspector or medical professional.
What Is Dew Point and Why Does It Beat Humidity?
Dew point is the temperature to which air must be cooled, at constant pressure, for water vapour to start condensing into liquid. Any surface colder than the dew point of the air touching it will collect water — which is exactly what a cold window pane does on an October night.
Relative humidity, by contrast, is only a percentage of what the air can hold at its current temperature. Warm air holds more water than cold air, so relative humidity rises and falls as the temperature moves even when the actual amount of moisture hasn't changed. Dew point doesn't have that problem: it is an absolute measure of moisture. The U.S. National Weather Service treats dew point as the most reliable single gauge of how humid the air really is.
In plain terms: relative humidity tells you how full the air is. Dew point tells you how cold a surface has to be before it starts to sweat. For a homeowner, the second question is the one that matters.
How Do You Calculate Dew Point?
Meteorologists and most weather stations use the Magnus formula (the August-Roche-Magnus approximation), which is accurate to within about 0.35°C between −40°C and 60°C. You work in Celsius, where T is air temperature and RH is relative humidity as a percentage:
You almost never need to do this by hand. Enter a temperature and humidity into the Dew Point Calculator and it handles the Celsius conversion and the logarithm for you, in °F or °C.
Key Takeaway
Dew point depends only on how much moisture is in the air. If a surface — a window, a cold water pipe, an uninsulated wall — is colder than that number, water will form on it, no matter how "normal" your humidity percentage looks.
A Worked Example: Why My Windows Dripped
Here's the maths for my bedroom. The thermostat was set to 70°F (21°C) and my hygrometer showed 50% relative humidity.
- US units: 70°F at 50% RH gives a dew point of about 50.5°F.
- Metric units: 21°C at 50% RH gives a dew point of about 10.2°C.
Now the outside temperature drops to the low 30s°F (around 0°C). The inner surface of an older double-glazed window doesn't sit at the room temperature — it sits somewhere in between, and on that first cold night mine was roughly 45°F (7°C). That is below the 50.5°F dew point, so the air touching the glass was being cooled past its condensation point. Water formed on the pane. My "healthy" 50% humidity was perfectly capable of wetting the glass.
Here is how much a modest humidity drop changes the picture at the same 70°F room temperature:
| Room Humidity | Dew Point (°F) | Dew Point (°C) | Glass at 45°F (7°C)? |
|---|---|---|---|
| 60% | 55.5°F | 13.0°C | Heavy condensation |
| 50% | 50.5°F | 10.3°C | Condensation |
| 40% | 44.6°F | 7.0°C | Just clear |
Dropping from 50% to 40% was enough to take the dew point below the glass temperature. That 10-point shift is what a dehumidifier or better ventilation buys you.
⚠️ Note
The glass temperature in my example was an estimate. A cheap infrared thermometer pointed at the window gives you the real figure; compare it with your calculated dew point and you know instantly whether condensation will form.
The same logic explains outdoor fog. When the overnight low is within about 5°F (3°C) of the dew point — what pilots call a small "dew point spread" — fog or frost is likely. A 50°F night with a 48.6°F dew point (95% humidity) has a spread of only 1.4°F, so expect a foggy morning commute.
What Do Different Dew Points Feel Like?
Based on National Weather Service guidance, here is how dew point maps to comfort:
| Dew Point (°F) | Dew Point (°C) | How It Feels |
|---|---|---|
| Below 50°F | Below 10°C | Dry, crisp |
| 50–59°F | 10–15°C | Comfortable |
| 60–69°F | 16–20°C | Sticky |
| 70°F and above | 21°C and above | Oppressive |
Indoors in fall and winter, a dew point of roughly 40–50°F (4–10°C) is a sensible target: comfortable to breathe, but low enough that most window glass stays dry. Once the indoor dew point creeps toward 55°F (13°C) in cold weather, expect trouble. Mould growth generally takes off when surfaces sit at or above roughly 70–80% relative humidity for extended periods, and a cold corner of a wall can reach that locally even when the room average looks fine.
In summer the same number works the other way. A dew point above 65°F (18°C) is when most people start to describe the air as muggy, which is why it pairs naturally with our Heat Index Calculator when you're judging how hot a day will feel.
How Do You Stop Fall Condensation?
Once you can calculate the dew point, you have three levers: lower the moisture, raise the cold-surface temperature, or both.
- Cut indoor moisture at the source. Run bathroom and kitchen extractor fans during and after use, put lids on pans, and vent the tumble dryer outside.
- Ventilate in short bursts. Opening windows for 5–10 minutes swaps humid indoor air for cold, drier outdoor air. Cold outdoor air has a low dew point, even if the weather app says 90% humidity.
- Use a dehumidifier in sealed rooms. In a tightly sealed modern home, a small unit can pull the indoor dew point down several degrees. If you're worried about running costs, our Electricity Cost Calculator shows what a 300-watt unit costs per month.
- Warm the cold surface. Better insulation, heavy curtains held away from the glass, and secondary glazing keep surfaces above the dew point. If you suspect a thin attic or wall layer is part of the problem, check it with the Insulation Calculator.
- Match your heating to the weather. Cranking the heat in a humid house warms the air but doesn't change its dew point, so it won't stop condensation on a cold pane. If you are weighing heating options, the BTU Calculator helps size equipment properly.
💡 Pro Tip
Check your windows on the coldest morning of the week, not a mild one. Condensation that only appears once or twice a year usually isn't a problem; condensation every cold morning that leaves mould on the frames is a sign your indoor dew point is too high.
Your situation will differ. A bathroom, a basement and a well-ventilated old farmhouse each behave differently, and single-glazed windows will sweat at much lower humidity than modern triple glazing. The point of the dew point is that it gives you a number to compare against, rather than guessing.
Frequently Asked Questions
Is dew point better than relative humidity?
For condensation and comfort, yes. Relative humidity changes with temperature even when the moisture content stays the same, while dew point is an absolute measure of moisture. Use relative humidity for a quick check and dew point to predict whether water will form on a surface.
What indoor humidity should I aim for in winter?
Many building guides suggest 30–50% relative humidity indoors, lowering toward the bottom of that range in very cold weather. The right figure depends on your windows, so compare your indoor dew point with your coldest glass temperature.
Why do I get fog on cold mornings even when it hasn't rained?
Overnight, air cools toward its dew point. When the temperature falls to within a few degrees of the dew point, moisture condenses into tiny droplets near the ground, forming fog. Clear, calm nights make this more likely.
How accurate is the Magnus formula?
The August-Roche-Magnus approximation is accurate to within about 0.35°C for temperatures between −40°C and 60°C and humidity above 1%. That is more than precise enough for home and weather use.
Can a dehumidifier fix window condensation on its own?
Often, yes, if it lowers the indoor dew point below your coldest surface temperature. In very cold climates or with single glazing, you may also need better insulation or secondary glazing.
Try It Yourself
Relative humidity tells you how full the air is, but dew point tells you whether the glass will get wet. Take your indoor temperature and humidity, run them through the Dew Point Calculator, and compare the result with your coldest window. If the dew point is higher, you've found your drip. For the other side of the humidity story, try the Heat Index Calculator and the Wind Chill Calculator.
