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The Psychrometric Chart: 7 Minutes of BS (Building Science)

October 16, 2018


This chart explains how moisture behaves in the air as temperatures rise and fall—rainstorms, flooding, and glaciers in wall cavities are distinct possibilities.


Today, Jonathan Smegal joins the show from RDH Building Science Laboratories in Waterloo, Ontario to clarify some of the confusion about the psychrometric chart.


What it is:

psychrometric chart | sī·krə¦me·trik ′chärt | n.

Psychrometrics is the physics which deals with the process and properties of moist air, a combination of water vapor and dry air.

It’s a four-syllable word for how moisture and temperature affect the air around us. It turns out; it is not an abstract concept, an old guy a long time ago drew a picture of the three-way relationship.

In 1905 Willis Carrier created the first version of the psychrometric chart. He made a graphical representation to simplify the process of using all of the data tables when designing HVAC systems.

Because everyone knows that charts are more fun than tables—unless you’re talking about the psychrometric chart.


How the psychrometric chart works

JS: The full psychrometric chart can be very daunting and intimidating due to their complexity and they have several axes of data.

It is usually simplified to exclude the lines not central to the point being expressed.

JS: Generally speaking, for our purposes, we can often simplify the psychrometric chart down to just three variables.

Temperature, the moisture content of the air, and relative humidity.

JS: On the x-axis along the bottom of the chart is always temperature. Technically, it is called the dry bulb temperature, but it is simply the temperature you’d read with a normal temperature sensor or thermometer.

The y-axis on the right side of the graph is always the moisture content of the air. This can have different units, depending on what you’re comfortable with, most with pascals, or kilopascals, or grams of water per kilograms of dry air.

Sometimes it is expressed as grains of moisture per pound of air, which seems even less intuitive than the other options.

JS: Next we have the curved lines of the graph, which represent relative humidity. The leftmost curved line, defines the edge of the graph and is the 100% relative humidity line.


Why the psychrometric chart matters to building professionals

This 100% relative humidity line is also the dew point line because, at 100 RH, water vapor becomes liquid. That’s where the usefulness of the chart comes into play for building professionals.

It allows you to see— at a glance—the conditions for water condensation and hence the chances of moisture problems at different times of year or parts of the country.

JS: Probably the most common use for the table in the building industry is to determine the dew point of interior air, or the temperature that is required on a surface for condensation.

For example, to use a psychrometric chart, let’s say I’m inside my house in a cold climate and I want to know how cold the windows need to be to result in condensation.

With my hand-held meter, I can easily measure the temperature and RH in my house, which happens to be 20 degrees Celsius and 40% RH.

From that point, you can determine the temperature at which vapor will condense into a liquid. Or, how cold your windows need to be before they start rotting themselves.

JS: So from the spot that I located on the chart, I move in a horizontal line, toward the left on the graph

Crossing all of the other Relative humidity curves…

Until I reach the 100% RH line.

Dropping back down to the x-axis from that 100% relative humidity point gives you the temperature.

JS: From the graph, it looks to be about 6 or 7 degrees Celsius. Now, if my interior RH was lower, as a result of ventilation, and I did the exact same exercise for an RH of approximately 30%, I would find that the temperature for condensation would be closer to 2 degrees Celsius.

This explains in a very mathematical way why it’s better in colder climates to have a lower interior relative humidity because there is less chance for condensation.

The point is that roof and wall sheathing will be colder than the dew point for during winter in cold climates. When moist air gets into the roof and wall cavities, it can condense to liquid, frost, or ice, depending on how cold it is in there.

JS: This is the reason that air control is so important in cold climates.

Because moisture rides on air currents.

JS: So in cold climates, we know that the sheathing in most wall systems is approximately the outdoor temperature.

Because the sheathing is outdoors.

JS: If we look at the psych chart, and we know that the outdoor temperature is in the range of -5 to -10 C, or around 20 F, we know that that’s going to be the sheathing temperature.

Now, if we have interior conditions—even recommended interior conditions— of 20 degrees Celsius and 30% relative humidity, we know that that air when it moves left, across the graph is going to hit the 100% RH line before it reaches the -5 or -10 degrees sheathing.

It’s going to rain somewhere inside your walls.

JS: So at this point that the line meets the 100% relative humidity line, around 3C, it’s going to start to condense water. And it’s going to keep condensing all the way down, until the interior air reaches the sheathing temperature.

And that kind of rain causes floods. Or glaciers that become floods come springtime.

JS: and it will eventually cause moisture damage to the sheathing.

In hot, humid climates, the science is the same, but the migration of moisture is from outside to in, because, if you have AC, it is cooler and dryer inside than it is outside.

JS: For example, let's say we have air temperature outdoors that’s 35 Celsius and approximately 70% RH.
For those of you keeping score, that would be 95 degrees F and still 70% RH

And as that air cools, because the interior conditions are 20 degrees Celsius

68 degrees F

JS: It’s going to start to condense, right around 27 degrees C.

80 degrees F

JS: So it can’t reach the interior temperatures without condensing most of the moisture out before it gets there.

Another situation where Making It Rain isn’t a good thing. 
Again, if you can stop the air movement, you can stop the rain.

And no rain, no pain.

JS: A secondary fix to some of these problems is ventilation or pressurization of the enclosures.

Pressurizing sections of a building is more common in commercial and industrial construction than residential because mechanical systems in houses tend to depressurize (read exhaust fans) where commercial and industrial buildings tend to have systems capable of pressurizing.

However, slightly pressurizing homes in hot, humid climates and slight negative pressure in homes in cold climates can add a suspenders layer to your building science belt.

So there’s another info tool to stick in your bag of tricks. Because you get paid for what you do and what you know.

Now you know that, so do this: subscribe to this podcast through iTunes, SoundCloud, and The Google, and while you’re there give us a five-star rating and a positive review; it helps us stand out in the algorithms.


—7 Minutes of BS is a production of the SGC Horizon Media Network.


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