Latent heat gain is the energy added to a space through moisture rather than temperature. When water evaporates from cooking, breathing, showering, or humid outdoor air leaking inside, it carries energy with it. That energy doesn’t show up on your thermostat, but it makes a room feel muggy and uncomfortable, and your air conditioner has to work harder to remove it.
How Latent Heat Differs From Sensible Heat
Buildings gain heat in two distinct ways. Sensible heat is the straightforward kind: sunlight streaming through a window, a hot oven radiating warmth, or warm outdoor air raising the indoor temperature from 72°F to 78°F. You can measure sensible heat with a thermometer.
Latent heat works differently. It involves phase changes, specifically water turning from liquid into vapor or vice versa. When water evaporates, it absorbs a surprisingly large amount of energy from its surroundings without raising the temperature at all. That energy is “hidden” in the water vapor (the word “latent” literally means hidden). The energy only reappears when the vapor condenses back into liquid, which is exactly what happens on the cold coils inside your air conditioner.
This distinction matters because cooling the air temperature (removing sensible heat) without also removing moisture (latent heat) leaves a space feeling cold and clammy. A well-designed cooling system handles both.
Where Latent Heat Gain Comes From
Every source of moisture inside or entering a building contributes to the latent heat load. The major contributors fall into a few categories.
People. Every person in a room releases moisture through breathing and perspiration. The latent heat gain from occupants scales directly with the number of people, which is why crowded spaces like classrooms and offices can feel stuffy even when the thermostat reads a comfortable number. HVAC engineers calculate this as a per-person value in watts, then multiply by occupancy.
Cooking and appliances. Boiling water, running a dishwasher, or using a clothes dryer without proper venting all push water vapor into indoor air. Cooking is one of the most significant internal sources of both sensible and latent heat in residential buildings.
Outdoor air infiltration. In most of the country during summer, outdoor air carries far more moisture than conditioned indoor air. Every crack around a window, gap under a door, or opening in the building envelope lets that humid air seep in. The latent load from infiltration depends on the difference in moisture content between outdoor and indoor air, measured in units called grains. The greater that difference, the more energy your cooling system must spend on dehumidification.
Ventilation. Fresh air brought in intentionally through a ventilation system also carries outdoor humidity. This is a necessary trade-off for indoor air quality, but it adds directly to the latent load.
Why It Takes So Much Energy
Changing water from liquid to vapor requires a large amount of energy compared to simply heating or cooling the water. When you boil a pot on the stove, the temperature stays locked at 212°F while the water turns to steam. All the energy going in during that plateau is latent heat, used to break the bonds holding water molecules together in liquid form rather than to raise the temperature. The same principle operates at lower temperatures whenever water evaporates from skin, wet surfaces, or damp materials indoors. The energy absorbed during evaporation becomes part of the indoor air’s total heat content, even though a thermometer won’t register it.
How HVAC Systems Handle It
Air conditioners remove latent heat by cooling air below its dew point. When warm, humid air passes over the cold evaporator coil, moisture condenses out of the air and drips into a drain pan. That condensation releases the latent energy the water vapor was carrying, and the system rejects it outdoors. The water you see dripping from an AC unit on a humid day is direct evidence of latent heat removal at work.
Properly sizing an HVAC system requires calculating both the sensible and latent cooling loads separately. The industry-standard method for residential buildings is called a Manual J calculation, which accounts for occupancy, appliances, infiltration rates, local climate data, and building characteristics to determine exactly how much dehumidification capacity the system needs. An undersized system, or one designed only for temperature without enough latent capacity, will leave a space damp and uncomfortable even if it hits the target temperature.
In humid climates like the Gulf Coast or Southeast, the latent load can be a significant portion of the total cooling load. In arid regions like the Southwest, it’s much smaller, and sensible cooling dominates.
The Role of Building Envelope and Vapor Barriers
Moisture doesn’t just enter through open gaps. Water vapor migrates directly through building materials whenever there’s a pressure difference between the humid side and the dry side. Every material has a permeability rating (measured in perms) that describes how easily vapor passes through it. Four inches of brick, for example, has a perm rating of about 0.8, meaning it resists vapor fairly well. Eight inches of cored concrete block rates around 2.4, letting more moisture through.
To control this migration, builders install vapor barriers on the warm side of the wall assembly. The most common residential vapor barrier is 6-mil polyethylene sheeting, which has a perm rating of just 0.06. Doubling the thickness of most materials roughly halves vapor transmission. For high-moisture buildings like indoor pools, 10-mil polyethylene is typical. In practice, though, the quality of installation matters more than the material’s rating. A carefully sealed 4-mil barrier outperforms a 6-mil sheet with gaps at seams, electrical boxes, and attic openings.
Reducing vapor migration through the building shell directly reduces the latent heat load, which means less work for the cooling system and lower energy bills.
Measuring and Calculating Latent Heat Gain
HVAC professionals use a tool called a psychrometric chart to visualize the relationship between temperature, humidity, and energy content of air. The chart plots dry-bulb temperature on one axis and humidity ratio (pounds of moisture per pound of dry air) on the other. Lines of constant enthalpy, which represents the total heat energy content of the air in BTUs per pound, run diagonally across the chart. By plotting indoor and outdoor air conditions on this chart, engineers can separate the total cooling load into its sensible and latent components.
For infiltration specifically, the latent load calculation follows a straightforward formula: multiply the airflow rate by the difference in moisture content (in grains) between outdoor and indoor air, then multiply by a constant factor of 0.69. The result is the latent heat gain in BTUs per hour that the cooling system must handle. Each source of moisture, whether people, equipment, ventilation, or infiltration, gets calculated individually and then summed to find the total latent load for the building.