Psychrometry
The science of air and water vapor, temperature, humidity, and vapor pressure, that structural drying is built on.
The short version
- Drying works by vapor pressure difference: water leaves a wet material only when the surrounding air is drier than the material.
- The working measurements are temperature, relative humidity, dew point, and grains per pound of moisture in the air.
- Warmer air holds more water vapor, which is why drying plans manage heat as deliberately as they manage airflow.
- Relative humidity alone is misleading, because the same relative humidity at two different temperatures means very different amounts of water.
- Readings are logged at every visit so the drying can be proven, not assumed.
What it actually means
Psychrometry is the reason a professional drying plan looks like more than a few fans in a room. Evaporation happens when the air next to a wet surface holds less moisture than the material does. Air movers create that difference at the surface by pushing the saturated boundary layer away; dehumidifiers create it in the room by pulling moisture out of the air so that the airflow has somewhere to send it. Take away either half and the drying stalls.
The four readings that matter are dry-bulb temperature, relative humidity, dew point, and grains per pound. Relative humidity is the one people know and the one that misleads most, because it is a percentage of what the air could hold at its current temperature. Sixty percent relative humidity at 90 degrees is far more water than sixty percent at 70 degrees. Grains per pound is an absolute measure, which is why restorers use it to compare the air going into a dehumidifier with the air coming out and confirm the machine is actually working.
This is also why an air conditioner is not a drying plan. Air conditioning cools and incidentally dehumidifies, and in a South Florida summer it is fighting outdoor air at very high grain levels every time a door opens. A dedicated dehumidifier removes far more water per day, discharges dry warm air back into the space, and can be metered.
What it looks like on a real job
On a drying job, readings are taken at three places on every visit: outside air, unaffected indoor air, and the affected space, plus the intake and discharge of each dehumidifier. Falling grains per pound in the affected space and a wide split across each dehumidifier mean the system is working. Flat readings mean something is wrong: a machine has filled or failed, the containment is leaking, or there is more wet material than the plan accounted for.
Those logs are also the claim documentation. Daily psychrometric readings and material moisture readings, taken at the same points each day, are what support the number of equipment days on the estimate and what demonstrate at the end that the structure reached a dry standard rather than merely feeling dry to the touch.
Questions we get asked
- Can I just run my air conditioning and some box fans?
- For a small, shallow, clean-water spill caught immediately, sometimes. For anything that soaked absorbent materials it is not enough. Fans without dehumidification move moisture around the house rather than out of it, and in Florida humidity that can push wet air into unaffected rooms and create a second problem.
- Why is the house warm while the equipment runs?
- Refrigerant dehumidifiers discharge warm dry air, and mild added heat raises the air’s capacity to hold water vapor, which speeds evaporation. It is deliberate, though a good plan keeps temperatures within the range the standard describes rather than simply making the space as hot as possible.
- What is dew point and why do restorers watch it?
- Dew point is the temperature at which the air becomes saturated and moisture starts condensing on surfaces. Drying air that is pushed against a cold surface below its dew point makes new wet spots, so keeping the dew point well below surface temperatures prevents secondary damage.
Official sources
Everything on this page rests on the documents below. Read them yourself, they are all free.
Last reviewed against these sources on August 26, 2026. These definitions are general information about restoration work and property claims in Florida, not legal advice or a coverage opinion. Your policy language and the facts of your loss control what is owed. Swift Response Restoration Inc is a licensed restoration contractor, not an insurer, a public adjuster, or a law firm.
Not sure how this affects your loss?
Send us the estimate, the photos, or just tell us what happened. We will walk you through what you are looking at and what the next step is. Free inspection, no obligation, and someone answers the phone at any hour.
Related terms
Grains Per Pound
An absolute measure of how much water vapor is in the air, used to prove a dehumidifier is actually removing moisture.
LGR Dehumidifier
A low grain refrigerant dehumidifier, the workhorse drying machine that can keep pulling water out of air that is already fairly dry.
Air Mover
The high-velocity fan that drives evaporation by stripping the saturated layer of air off wet surfaces.
Dry Standard
The documented moisture level a material has to reach for the drying to be finished, taken from unaffected material in the same building.