The dew point: the invisible line between dry and wet
Fogged-up windows in the morning, a dark patch in the corner of a room, a cold bottle that instantly “sweats” in summer — all of these share the same physical principle: the dew point. Once you understand it, you can explain mould on walls, ventilate more effectively and judge whether your home has a moisture problem. This guide explains what the dew point is, how to calculate it and why mould can grow at temperatures well above the dew point.
You can reproduce every figure in this article with your own values in the dew point calculator — including a mould-risk traffic light for your exterior wall and a ventilation check.
What is the dew point?
Air always contains some water vapour. How much it can hold at most depends strongly on temperature: warm air can carry far more water than cold air. When humid air cools, the amount of water initially stays the same, but the air gets closer and closer to its saturation limit. The temperature at which this limit is reached is called the dew point or dew point temperature. If the air cools further — for example on a cold window pane — the excess water condenses and becomes visible as condensation.
The dew point is therefore a property of the air itself, not of the surface. It says: every surface colder than this value will get wet. For typical indoor air at 20 °C and 50% relative humidity, the dew point is about 9.3 °C. A wall, window frame or water pipe that is colder will fog up.
Relative and absolute humidity
To understand the dew point you need to separate two terms. Absolute humidity states how much water vapour is actually contained in one cubic metre of air, usually in grams per cubic metre (g/m³). Relative humidity relates this amount to the maximum possible amount at the current temperature. 50% therefore means the air is half saturated.
Some rounded values show how strongly the maximum amount depends on temperature: at 0 °C one cubic metre of air can hold about 4.8 g of water, at 10 °C about 9.4 g, at 15 °C about 12.8 g, at 20 °C about 17.2 g and at 25 °C already about 23.0 g. Indoor air at 20 °C and 50% therefore contains about 8.6 g/m³. If this air cools, its relative humidity rises — at about 9.3 °C those 8.6 g are exactly the saturation amount. That is the dew point.
This is exactly why relative humidity alone is often misleading. The same air can feel pleasantly dry in a warm living room and be almost saturated at the cold exterior wall of the bedroom.
The formula: calculating the dew point with the Magnus formula
For everyday temperatures, the saturation vapour pressure is usually calculated with the so-called Magnus formula. Commonly used are the coefficients by Sonntag (1990), which are also used in the guide of the World Meteorological Organization (WMO-No. 8) and by Germany's national weather service (DWD). Over water they apply from roughly −45 to +60 °C:
E(T) = 6.112 hPa · exp(17.62 · T / (243.12 + T))
Here E is the saturation vapour pressure in hectopascals and T the air temperature in °C. The actual vapour pressure is obtained by multiplying E by the relative humidity: e = RH/100 · E(T). For the dew point you solve the formula for temperature:
γ = ln(RH/100) + 17.62 · T / (243.12 + T)
Dew point = 243.12 · γ / (17.62 − γ)
Worked example
1. Indoor air: T = 20 °C, RH = 50%.
2. 17.62 · 20 / 263.12 = 1.3393; ln(0.5) = −0.6931; so γ = 0.6462.
3. Dew point = 243.12 · 0.6462 / (17.62 − 0.6462) ≈ 9.3 °C.
At 60% relative humidity the same calculation gives about 12.0 °C. Ten percentage points more humidity raise the dew point by almost three degrees — one reason why even small changes in everyday life make a noticeable difference.
Absolute humidity follows from the vapour pressure via the gas law for water vapour: ρ = e / (Rv · T) with Rv = 461.5 J/(kg·K) and T in kelvin. For 20 °C and 50% this gives the roughly 8.6 g/m³ mentioned above.
Dew point table for living spaces
The following values were calculated with the Magnus formula and rounded to one decimal place:
16 °C room temperature: 40% → 2.4 °C · 50% → 5.6 °C · 60% → 8.2 °C · 70% → 10.5 °C
18 °C room temperature: 40% → 4.2 °C · 50% → 7.4 °C · 60% → 10.1 °C · 70% → 12.4 °C
20 °C room temperature: 40% → 6.0 °C · 50% → 9.3 °C · 60% → 12.0 °C · 70% → 14.4 °C
22 °C room temperature: 40% → 7.8 °C · 50% → 11.1 °C · 60% → 13.9 °C · 70% → 16.3 °C
24 °C room temperature: 40% → 9.6 °C · 50% → 12.9 °C · 60% → 15.7 °C · 70% → 18.2 °C
Why mould appears before the dew point
A common misconception is: as long as the wall is warmer than the dew point, nothing can happen. In fact, mould does not need liquid water. Permanently high humidity directly at the surface is enough, because wallpaper, plaster and dust absorb that moisture. The German standard for minimum thermal insulation, DIN 4108-2, therefore sets the criterion that relative humidity at the component surface should stay below 80%.
For indoor air at 20 °C and 50%, this 80% limit is reached as soon as the surface is colder than about 12.6 °C — more than three degrees above the dew point. At 60% indoor humidity the mould-critical surface temperature is already around 15.5 °C. The standard translates this into the temperature factor fRsi: at 20 °C inside and −5 °C outside, the inner surface of thermal bridges must reach at least 70% of the way from the outdoor to the indoor temperature (fRsi ≥ 0.70), which corresponds to a surface temperature of 12.5 °C.
Read more about causes and typical weak spots in the article Mould on exterior walls.
How cold is your wall?
The surface temperature of an exterior wall depends on its insulation. For the undisturbed wall area, a simplified rule applies: θsi = θi − Rsi · U · (θi − θe). Here U is the thermal transmittance of the wall and Rsi = 0.25 m²·K/W is the internal surface resistance the standard uses for the mould assessment.
Two examples at 20 °C inside and −5 °C outside: an uninsulated old wall with U = 1.4 W/(m²·K) only reaches about 11.3 °C on the inside — below the 80% limit at 50% humidity. A well-insulated wall with U = 0.24 W/(m²·K) reaches about 18.5 °C and is uncritical. In room corners, window reveals or behind wardrobes, however, it is often several degrees colder than on the open wall area. If you want to know for sure, measure the coldest spot with an infrared thermometer.
Ventilation: absolute humidity is what counts
Airing replaces humid indoor air with outdoor air. Whether this dries your home depends not on relative but on absolute humidity. In winter it almost always does: outdoor air at 0 °C and 85% contains only about 4.1 g/m³. Warmed to 20 °C indoors, its relative humidity is only about 22%. In summer, by contrast, warm humid outdoor air can deposit moisture in cool cellars.
How to ventilate effectively in winter without wasting energy is explained in the article Ventilating correctly in winter.
What humidity is recommended?
Germany's Environment Agency (Umweltbundesamt, UBA) recommends a relative humidity between 40 and 60% for living spaces. A simple hygrometer is enough to keep an eye on it. If your humidity is permanently higher or your exterior walls are poorly insulated, check the calculator: it shows up to which indoor humidity your wall stays below the 80% limit. Bedrooms have their own peculiarities, covered in the article Ideal bedroom humidity.
What to do about high humidity or cold walls?
There are two levers: lower the humidity of the indoor air or make the surfaces warmer. In the short term, regular burst ventilation, evenly heating all rooms and leaving space between furniture and exterior walls help. In the long term, insulating the exterior wall raises the surface temperature permanently. You can roughly estimate what such a renovation costs with the renovation cost calculator; if you are thinking about your heating anyway, the heat pump calculator is a good starting point.
Limits of the calculation
The dew point itself can be calculated very precisely. The surface temperature derived from the U-value, on the other hand, is a simplified steady-state estimate: it does not account for thermal bridges, furniture in front of the wall or fluctuating temperatures. The calculation does not replace an expert assessment. Visible mould should be removed professionally and its cause identified — according to the UBA, heavy mould growth can affect health, for example through an increased risk of respiratory problems.