Before answering, let’s review a few basic concepts.

Relative humidity (RH), expressed as a percentage and displayed on household sensors or weather stations, represents the amount of water vapor in the air relative to the maximum amount the air can contain at that temperature (100%). The term “relative” is no coincidence: the value depends on temperature (and also on pressure, although its effect is negligible for our purposes). During our training courses, we use the analogy of a sponge: air is like a sponge that becomes increasingly “squeezed” as it gets colder. If we pour water onto a dry sponge, it will absorb the water without dripping. The amount of water we pour in represents absolute humidity. There are two ways to make the sponge start dripping.

1. Add more water. We increase the absolute humidity until, at a certain point, the sponge can no longer absorb any more water and starts dripping. We have reached saturation, corresponding to 100% relative humidity. Similarly, water vapor in the air condenses on the floor or on cold windowpanes.
2. Squeeze the sponge without adding any water. This is where the analogy becomes interesting: cooling the air is equivalent to squeezing the sponge. We are not adding anything, but we are reducing the air’s capacity to hold water vapor. By cooling the air, we are effectively “squeezing the sponge,” causing water vapor that was previously suspended in the air to condense.

In this second case, the absolute humidity does not change; what changes is the air’s capacity to hold water vapor, and therefore its relative humidity. We can go from, for example, 60% relative humidity to 100%. This explains a phenomenon that may seem strange at first: condensation can form on a radiant floor even when it is neither raining nor foggy outside.

Imagine outdoor air at 30°C with 60% relative humidity (relative to 30°C, remember!). This air enters the house and cools down without any change in the amount of water vapor it contains… we are squeezing the sponge. When will it start dripping? As the air cools, at what temperature will condensation form? At the temperature known as the dew-point temperature, or simply the dew point.

In our example, with air at 30°C and 60% RH, the dew point is 21.4°C. If that outdoor air reaches our 20°C floor without being dehumidified, condensation will form. Note: if you search for “Dew Point Calculator” on Google, you will find many websites that let you perform these calculations and experiment with different values.

So, which parameter should we use to control the system? Absolute humidity or relative humidity?

It depends on what we want to achieve. In the next posts, we will explore the different scenarios in which we need to measure and control humidity and assess, case by case, which parameter is the right one to consider—possibly with a few surprises along the way.