A Few Words about Humidity

Ask most people which is heavier and denser, dry air or humidified air. Most folks will wrongly respond that the humidified air is heavier and denser. The general public uses the logic that water is heavy, and the difference between the dry air and the wet air is the addition of the heavy water. The logic is good, but no cigar for this line of thought.

The truth is that as we add humidity to a sample of air, the air becomes lighter and less dense. How does this happen? Let’s talk about what water is. H2O is the water molecule composed of two hydrogen atoms combined with an oxygen atom. If one looks at a periodic table, one will notice that the lightest of all the elements is hydrogen.  Oxygen is also a relatively light gas and is already present in dry air. The more humidity we add to the air, the more hydrogen we add to it to take up the available space by displacing the heavier oxygen and nitrogen atoms in the existing space. With this view, at the molecular level, it is clear that humidified air is less dense and lighter than dry air. Whether it is on the job or at a cocktail party, wow them with this little HVAC tidbit.

Humidity is important in a number of ways when it comes to comfort, health, and even the conditions of the spaces we occupy. The psychrometric chart shows us the sweet spot for human comfort. In cooling months this sweet spot is roughly from 72°F with a relative humidity of no greater than 70% all the way to 80°F with a relative humidity of a little less than 20%. Anything outside of these temperature/relative humidity ranges is not considered comfortable to human beings.

Likewise, in the heating season, the psychrometric chart shows a slightly different sweet spot for human comfort. In heating mode we can be as cool as 67.5°F if the relative humidity is around 82%. The upper end of this human comfort range would be a temperature of 76.5°F with a relative humidity of around 22%.

In the south where cooling is king and heating is almost just an afterthought, we are pretty unconcerned about moisture levels in the heating season. In cooling season we know that if our air conditioning system is sized correctly and operating as designed we will be dehumidifying as we remove heat.

Up north the reverse is true. Heating is king up yonder. Winters are long and dry. Heating systems without humidifiers will cause nosebleeds for people and wooden furniture and building materials begin to crack. So the addition of humidity is very important to heating systems in the colder climates.

For many who have grown up in the south and cut their teeth on air conditioning, the addition of humidity is almost sacrilege.  Down here we dehumidify. But in the land of the frozen tundra, humidity is our friend and it lightens things up a bit.

Rob Merrifield

ACCA Board of Directors

4/25/2007