Question 1

How do you calculate the dew point?

Accepted Answer

With the Magnus formula. First determine the saturation vapour pressure E = 6.112 hPa · exp(17.62 · T / (243.12 + T)) and multiply it by the relative humidity. You get the dew point by solving the formula for temperature: dew point = 243.12 · γ / (17.62 − γ) with γ = ln(RH/100) + 17.62 · T / (243.12 + T). At 20 °C and 50% this gives about 9.3 °C, at 20 °C and 60% about 12.0 °C.

Question 2

At what humidity does mould become a risk?

Accepted Answer

What matters is not the room humidity but the humidity directly at the wall surface. If it stays at 80% or more for several days, mould can grow — that is the criterion of the German standard DIN 4108-2. At 20 °C and 50% indoor humidity this limit is reached once the wall is colder than about 12.6 °C. At 60% indoor humidity the limit is already around 15.5 °C.

Question 3

What humidity is ideal in a home?

Accepted Answer

Germany's Environment Agency (UBA) recommends a relative humidity between 40 and 60%. During the heating season and with poorly insulated exterior walls the lower part of this range is safer, because wall surfaces are colder. Whether your wall becomes critical at your humidity is shown by the calculator's maximum indoor humidity for your wall.

Question 4

What is the difference between relative and absolute humidity?

Accepted Answer

Absolute humidity states how much water vapour is actually in the air, usually in grams per cubic metre. Relative humidity relates this amount to the maximum possible amount at the given temperature. Air at 20 °C can hold about 17.2 g/m³, air at 0 °C only about 4.8 g/m³. That is why relative humidity rises when air cools, even though the amount of water stays the same.

Question 5

Is airing worthwhile when it rains or is foggy outside?

Accepted Answer

In winter, usually yes. Cold outdoor air holds little water even at 90% relative humidity. When warmed to 20 °C indoors its relative humidity drops sharply — from 85% at 0 °C outside to only about 22%. In summer it can be the other way round: warm, humid air can deposit moisture in cooler rooms such as cellars. The calculator's ventilation check compares exactly that.

Question 6

What does the temperature factor fRsi mean?

Accepted Answer

The temperature factor fRsi describes where the inner surface temperature lies between outdoor and room temperature: fRsi = (θsi − θe) / (θi − θe). The German standard DIN 4108-2 requires at least 0.70 for thermal bridges. At 20 °C inside and −5 °C outside this corresponds to a surface temperature of 12.5 °C, This matches the mould-critical temperature of 12.6 °C at 50% indoor humidity; the limit of 0.70 is rounded from it.

Question 7

Why do my windows fog up on the inside?

Accepted Answer

Because the glass surface is colder than the dew point of the indoor air. Older glazing and especially the glass edge get so cold in winter that water condenses. If the panes fog up regularly, the humidity is too high for these windows: air the room after showering, cooking and in the bedroom in the morning, and wipe water off the window sill.

Question 8

How accurate is the U-value estimate?

Accepted Answer

It applies to the undisturbed wall area at steady temperatures. In room corners, window reveals, ceiling junctions and behind wardrobes the surface is often several degrees colder. For a reliable result, measure the wall temperature at the coldest spot, for example with an infrared thermometer, and use the calculator's “Measured myself” mode.

Guide: Dew Point Calculator

How to Calculate the Dew Point: Formula, Table and What It Says About Mould

Ventilating Correctly in Winter: How Often, How Long and Why Cold Air Dries

Mould on Exterior Walls: Causes, Thermal Bridges and the 80% Criterion

Ideal Bedroom Humidity: Values, Temperature and Ventilation

Condensation on Windows: Why Panes Fog Up and What You Can Do

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