Building Science

Humidity illiteracy

An open letter to the editor of Indoor Environment Connections:

Carl Grimes denounced illiteracy and innumeracy with great elocution in his article “A Serious Problem- Innumeracy” (Volume 11, Issue 11). My good friend Mr. Grimes would have made a more convincing point had he not exhibited illiteracy and innumeracy himself when describing absolute humidity. What he described in his article as absolute humidity (“grains of moisture per pound of dry air”) is actually humidity ratio. He should have expressed absolute humidity as grains of moisture per cubic foot of air (or some other volume). ASHRAE defines absolute humidity as the ratio of the mass of water vapor to total volume of the sample1.

I normally overlook this common misunderstanding, but when considering the article’s subject matter, it was too ironic to ignore.

Ian Cull, the Indoor Air Nerd

ps. Carl encouraged me to write this!

  1. 2009 ASHRAE Handbook-Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, page 1.2 []
Building Science

Introduction to Psychrometrics, Part 3 of 3

In the first two installments of “Introduction to Psychrometrics” I covered concepts such as air, evaporation, temperature, condensation and dew point. I strongly encourage you to read Part 1 and Part 2 before reading this final installment where I’ll be explaining relative humidity, humidity ratio and a few other concepts.

The amount of humidity in the air will affect the indoor air quality, therefore it is important to measure it. Unfortunately, there are four common terms used to quantify humidity: relative humidity, humidity ratio, absolute humidity, and specific humidity. I’ll cover all four concepts in this post.

As a quick refresher, humidity is a measure of the water molecules in the air that have escaped the surface of liquid water. I’ll be using the term “water vapor” to describe these molecules. Water vapor is the result of evaporation (see the word “vapor” hidden in there?).

Relative humidity, or “RH”, is the most commonly used expression for humidity. It also happens to be the least understood.   Relative humidity is the ratio of water vapor in the air compared to fully saturated air at the same temperature.  In other words, there is a certain amount of kinetic energy in a system to free water molecules.  RH looks at how much of the system’s kinetic energy has been used to free molecules.  When I use the term “system” I am referring to the air + any liquid water that may be present.

If a room has a relative humidity of 40%, it still has a lot of unused energy (60%).  Put a cold glass of water in that room and the kinetic energy in all gas molecules (nitrogen, oxygen, water vapor etc.) will transfer heat to the cold water.  When the water molecules heat up, that increases their kinetic energy and ability to escape the liquid surface.

Building Science

Introduction to Psychrometrics, Part 1 of 3

Psycho-metrics is a measure of how psycho you are.  That can be a helpful measurement when dealing with clients who are driving you crazy.  Today, I want to give a brief introduction to something different, called psychrometrics (notice the “r”).  Psychrometrics is the study of the physical and energy related (thermodynamic) properties of air-water vapor mixtures.

The first response I get when teaching psychrometrics is, “Why do I need to know this stuff?”  With an understanding of psychrometrics, we can better predict where condensation may form, causing water damage and leading to indoor air quality concerns.  By understand these concepts, you’ll be better able to look at a wall assembly and identify common problems.

Some of the key variables of psychrometrics include temperature, relative humidity, humidity ratio, and dew point temperature.  Before we get too deep, we need to answer the question, “What is air?”


Controlling Dust Mites

I often hear people say, “I’m allergic to dust.”  They really mean to say, “I’m allergic to dust mites.”  Or, if you’re an indoor air nerd, you tell the people they are allergic to “terrestrial invertebrate arachnids known taxonomically as Dermatophagoides pteronyssinus, Dermatophagoides farinae or Euroglyphus maynei.”

Dust mites are a concern because they can trigger an allergic reaction in sensitive individuals.  The Third National Health and Nutrition Examination Survey found that 27.5% of the population had a positive skin test response for dust mite sensitivities.  Now for the gross part… the allergic reaction is from allergens in the dust mite feces.  95% of the mite allergen is in the feces which has a mean diameter of 22 microns ± 6 μm (with range of 10 to 40 μm)1.

People come in contact with dust mite allergen via the air or a surface.  Because of the fecal pellet’s size, it isn’t airborne for very long.  Fluffing a pillow with a large number of dust mites will make the allergen airborne, but just for a few minutes before it settles out via gravity.  Dust mites like the ecology of dust, especially where there is a high percent of skin scales (think skin cells from dandruff).  Pillows, mattresses and sofas in front of the TV are good habitats.

There are a lot of ways to control dust mites.  

  1. Tovey ER, Chapman MD, Platts-Mills TAE. Mite feces are a major source of house dust allergens. Nature. 1981;289:592–593 []