Understanding Psychrometrics: How Air and Water Play Together in Buildings
Have you ever noticed how a cold glass of iced tea gets “sweaty” on a summer afternoon? Or wondered why your bathroom mirror fogs after a warm shower?
Believe it or not, you are observing a science called psychrometrics—pronounced sigh-crow-MET-ricks.
Psychrometrics is simply the study of moist air: its temperature, the water vapor mixed with it, and how the two behave as conditions change. Although that may sound like something only a scientist or engineer would study, psychrometrics is an important part of Building Science. It helps us design buildings that are comfortable, healthy, energy-efficient, and durable.
Air and Water Vapor Share the Same Space
The air around us is a mixture of gases, mostly nitrogen and oxygen. Water vapor is another gas mixed into that same space.
It is common to say that warm air can “hold” more moisture than cold air, but air does not actually hold water vapor like a sponge or bucket. The water vapor is simply mixed with the other gases in the air.
Here is what really happens:
At warmer temperatures, water can remain in its vapor form at a higher vapor pressure without condensing.
As moist air cools, the amount of water vapor may initially remain unchanged, but the conditions under which it can remain entirely as vapor change.
If the air cools far enough, it becomes saturated. Any additional cooling causes some water vapor to condense into liquid water.
So, rather than imagining air as a sponge that grows and shrinks, imagine air and water vapor as gases sharing the same room. Temperature changes the rules governing whether the water remains an invisible vapor or becomes liquid water.
Three Key Terms to Know
When building scientists examine moist air, they focus on three important properties:
Air Temperature—Dry-Bulb Temperature
This is the ordinary air temperature measured by a standard thermometer.
Relative Humidity
Relative humidity compares the water vapor pressure actually present with the highest water vapor pressure that can exist in equilibrium over a flat surface of pure water at the same temperature. It is expressed as a percentage:
At 50% relative humidity, the water vapor pressure is one-half of the saturation vapor pressure at that temperature.
At 100% relative humidity, the air is saturated.
If saturated air is cooled further, some of its water vapor must condense into liquid water.
This is why relative humidity is called relative: it describes the water vapor present relative to the saturation condition at the same temperature.
Temperature matters greatly. If we warm a sample of air without adding or removing water vapor, its relative humidity decreases. If we cool that same air without changing its moisture content, its relative humidity increases.
The air did not gain or lose moisture during either process. Only its temperature—and therefore its saturation vapor pressure—changed.
Dew-Point Temperature
The dew point is the temperature to which air must be cooled, without adding or removing water vapor, for it to become saturated.
If the air or a surface becomes colder than that dew-point temperature, some water vapor can condense into liquid droplets.
Unlike relative humidity, dew point directly indicates the amount of water vapor present. A higher dew point means more moisture in the air; a lower dew point means less.
The Cold Iced Tea Experiment
Let’s return to that cold glass of iced tea on a humid day.
The glass does not leak, and the droplets on its outside do not come through the glass. They come from water vapor already present in the surrounding air.
When moist air contacts the cold glass, the air immediately next to the surface cools. If the glass is at or below the air’s dew-point temperature, the water vapor near the surface reaches saturation and begins changing from vapor into liquid water.
Those droplets are condensation—the same basic process that can occur on windows, pipes, ductwork, walls, and other cold building surfaces.
How Does This Apply to Buildings?
Cold surfaces in buildings can behave much like the iced-tea glass.
During Winter
Windows, wall sheathing, framing, and other building materials can become cold because of outdoor conditions. If warm indoor air containing sufficient water vapor reaches a surface below its dew point, condensation can form on that surface—or remain hidden inside the wall.
During Summer
Warm, moisture-laden outdoor air can enter an air-conditioned building through cracks, openings, or ventilation systems. If that air encounters a surface colder than its dew point—such as cold ductwork, piping, diffusers, floors, or walls—condensation can occur.
When building materials such as wood, drywall, or insulation remain wet for too long, the result may be mold growth, wood decay, corrosion, material damage, and poor indoor air quality.
How Building Scientists Control Moisture
Building scientists use psychrometric charts, instruments, calculations, and computer models to predict how temperature and moisture will behave. We manage moisture by:
Ventilating appropriately: Removing moisture generated by showers, cooking, and other indoor activities—while recognizing that outdoor ventilation air may also require dehumidification during humid weather.
Dehumidifying: Removing water vapor from the air and lowering its dew point.
Insulating: Helping keep interior surfaces warmer and less likely to fall below the surrounding air’s dew point.
Air-sealing: Limiting the movement of moisture-laden air into walls, ceilings, and other concealed spaces.
Controlling vapor movement: Using properly selected materials and assemblies to manage moisture diffusion through the building enclosure.
Managing surface temperatures: Keeping cold pipes, ducts, walls, and other surfaces properly insulated and above the surrounding air’s dew point whenever possible.
The most important idea is simple:
Air does not “hold” moisture. Water vapor coexists with the other gases in the air, and temperature determines the vapor-pressure relationship we call relative humidity. When a surface falls below the air’s dew point, condensation can begin.
That is psychrometrics—and it is happening all around us every day.
