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 []

The House of Horrors

I set aside an hour to write a blog post before I headed off to Michigan for Thanksgiving.  45 minutes of that hour was taken up speaking to an acquaintance with a residential construction nightmare story.  I’ll share his quick story in the 15 (now 14) minutes I have before my wife picks me up for our family trip.

This individual had plans drafted up for an energy efficient home with promised good indoor air quality.  The builder, it appears, didn’t reference those plans too carefully.  A ground source heat pump (a.k.a. geothermal) was installed to heat and cool the home located in the Chicagoland area.  This can only work if the walls are extremely well insulated.  Although the plans called for rigid foam insulation on the exterior of the sheathing along with superior cavity insulation, all he got was some blown-in cellulose in 2×4 (not 2×6) walls.

That missing rigid foam insulation is causing two problems.  The first is related to heat transfer.  The foam insulation is there to help prevent thermal bridging.  When you have insulation only between studs, heat will often be transferred through the wood studs.  Remember: drywall is in direct contact with the wood framing.  The wood framing is in direct contact with the sheathing.  The sheathing is in direct contact with the cold weather.  Thermal bridging is when all that great insulation gets bypassed, leaving cold spots on the wall in the winter time.  Thermal bridging also occurs in the summer time, but in the opposite direction.

The second problem caused by the lack of rigid foam insulation on the exterior of the sheathing is that the ground source heat pump was sized based on the additional R value it provides.  Without it, the home is colder in the winter, and warmer in the summer.  Now he is looking for ways to supplement the heating and cooling because the geothermal system isn’t cutting it.

Time doesn’t permit me to go into all of the other problems we discussed.  Here are some of my favorites:

  • Bathroom exhaust fan dumping humid air into attic, not the outdoors
  • Exhaust fans allowing cold attic air to dump down into the bathroom
  • Improper weeps at the base of the brick facade
  • Only a feeble attempt at zoning the HVAC system that greatly missed the mark
  • Batt insulation with the kraft paper facing the wrong direction (visible mold growth discovered)
  • No humidifier installed, which is needed in the winter because of the air leakage
  • R-13 walls where R-30 was in the design

Well my time is up.  Have a great Thanksgiving!  I know I have so much to be thankful for!

The answer is blowin’ in the wind

I’m surprised that many indoor air quality assessments do not include any measurement of pressurization.  The difference in pressure between two rooms will dictate the direction of airflow.  Air will move from high pressure to low pressure.

How do you measure the pressure?  The quantitative way of measuring pressure is with a differential manometer.  Two small rubber tubes are attached to the manometer and run into the two areas to be measured.  The manometer will display the pressure difference in inches of water gauge, Pascals, or other various units.

The problem with these devices is that the tubing doesn’t always fit nicely in the doorways or openings separating the areas being measured.  Imagine trying to pass a tube through a doorway without any undercut below the door.  You need to keep the door open to run the tubing through, but that changes the pressurization.

On most jobs I prefer to measure pressurization qualitatively.  I’m usually more interested in knowing where air is moving than knowing the exact Pascal pressure differential.  To know which way air is moving, we need a tool to help us “see” the air.

For most of my career I have used ventilation smoke tubes from MSA.  After crushing two hermetically sealed glass ampoules, a chemical reaction occurs resulting in the generation of buoyant smoke.  The black aspirator bulb pushes air through the tube to puff the smoke out.  Although this isn’t purposefully irritating (like an irritant smoke tube), it still will make you cough if inhaled.

I’ve always been on the look out for a better smoke tube.  I’ve tried powders, wicks, and even the 6 STP1 method.

This week while teaching a class, a student Dean Lobas recommended the Wizard Stick.  I had heard of this product in the past, but I always dismissed it because it was a toy, literally.  They probably realized building scientists were buying these in greater numbers than kids, so they recently repackaged the toy as the Wizard Vapor Air Flow Indicator.   The smoke is made of distilled water, glycerin and propylene glycol so it can be considered non-toxic.  I will be buying one soon.  Here is a video:

If you know of any other great products to measure airflow, please leave a comment on this blog post here.

  1. 6 STP stands for “six squares of toilet paper”!  I learned that joke from Don Herrmann. []

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.

Continue reading

Introduction to Psychrometrics, Part 2 of 3

In my first installment on psychrometrics, I covered the basics of air, humidity and evaporation. Here I’ll cover temperature, kinetic energy, attractive forces and condensation.  If you haven’t already, I strongly suggest you read Part 1 first.

We’re all familiar with the general concept of temperature. Temperature ends up being quite complex if you dig deeper (and deeper we shall dig).  Technically, temperature is related to the average energy of motion, known as kinetic energy. To use our illustration from the last blog post, the faster the billiard balls move on the table, the greater the temperature. Continue reading

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?” Continue reading