Psychrometrics and How it Applies to Your Home

Psychrometrics is a term that most homeowners have never heard of.
It’s sad to say, but most builders aren’t familiar with this term either.

Per Wikipedia, psycrometrics, psychrometry and hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchron meaning “cold” and the metron meaning “means of measurement”

Although the principles of psychrometry apply to any physical system consisting a gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilating, air-conditioning and meteorology. In human terms, our comfort is in large part a consequence of not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor.

What we’re really trying to do here is to determine where the dew point is, in relation to the indoor environment of our homes.
Per Wikipedia:
The dew point is the temperature below which the water vapor in a volume of humid air at a given constant barometric pressure will condense into liquid water at the same rate at which it evaporates. Condensed water is called dew when it forms on a solid surface.

With the advent of more sophisticated envelope designs for houses, it has become increasingly important for residential designers and builders to understand psychrometrics – the behavior of moist air under various temperature and humidity conditions.
One basic tool is the “psychrometric chart”.

Psychrometric Chart (ASHRE)

Psychrometric Chart (ASHRE)

Most people get dizzy just looking at a complete chart and to master its use is a challenge even for many engineers.

Enter the Pshchrometric Calculator.

Psychrometric Calculator

Psychrometric Calculator

This gadget simplifies the calculation process in helping us to predict moisture condensation. (dew point)

You might be asking, “why should we want to know?”
Our understanding of Building Science tells us that it’s critical that we understand the impact of the design decisions that are made and how they will affect the performance of a variety of building components in the home.

Some common questions for houses built in the cold climate region where we build are:
1- During winter, will condensation occur on ventilation intake ducts located in a heated basement?
2- Under what conditions will condensation occur on cold water pipes?
3- If a wall is insulated with R-19 fiberglass plus R-7 exterior foil-faced foam sheathing, will condensation occur on the inner foil face of the sheathing?
And the question we hear the most from residents of homes that were built by someone with little to no understanding of Building Science:
4- Why am I having window condensation issues?

We know that over 90% of building material failures are caused by water.
If it isn’t a leak in the building envelope caused by lack of proper flashing details or poor workmanship on roofing or siding, it’s damage caused by condensation.

Condensation can really take its toll on a structure, because a good deal of it takes place in areas that are hard to access. It happens at your rim-joist, your roof system and in the cavities of your framed wall that rely on fiberglass insulation.
In addition to the structural damage caused by unchecked condensation, these areas can become a breeding ground for mold growth, which can lead to poor indoor-air quality.
Countless numbers of wood windows have been destroyed by condensation issues.
Knowing how to deal with existing issues and how to avoid them during the design phase for our new homes is crucial.

Infrared Thermometer

Infrared Thermometer

We use our understanding of Building Science and specialty tools like the infrared thermometer above, to help us gather the information we need to make a determination, so we can help owners of existing homes to develop strategies that will reduce the likelihood of condensation forming in rim joist bays, wall cavities, roof systems and windows.

The infrared thermometer will provide us with the current temperature reading of any given surface, but we need to take all things into consideration, in order to make use of our psychrometric calculator to determine where the dew point lies.

We need to take the indoor air temperature and the relative humidity of the home into consideration, if we are to determine where the dew point lies.
The thermostat below is an example of one that has the capacity to measure the relative humidity.

Thermostat w/ Relative Humidity Reading

Thermostat w/ Relative Humidity Reading

The image below is of thermometer/hygrometer device, which provides current readings on temperature and relative humidity.



We’ve heard it said that, “knowledge is power” and this holds true in this instance.
This is more than a moisture issue. The relative humidity of your home has an impact on your level of comfort as well. There is a “sweet spot” where all of these driving forces converge. This “sweet spot” is a moving target as outdoor and indoor conditions are constantly changing.

Understanding how these laws of nature come into play in your home gives us the ability to work through the problems we see in existing homes and the information we need to avoid these issues in the new homes that we build.

By specifying high-performance, super-insulated building envelope systems and super-insulated windows, we’re able to build out the inherent problems that most builders build in to their homes.
By doing so, we’re able to provide our clients with homes that are comfortable, safe, healthy, durable and energy-efficient.

If these are attributes that you find appealing, contact us. We love talking about building science, high-performance home building and how we can save you 40%-60% and more on your heating and cooling costs.

Great Lakes Carpentry is “Building Today for a Greener Tomorrow”

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