Psychrometric Drying Logs Explained: GPP, Dew Point, and Why They Matter for Water Damage Claims

June 23, 2026 · RemediDocs Team

Plenty of good water damage techs learned drying by doing it. You can read a room, place equipment by feel, and get materials dry. Then an adjuster asks for the psychrometric drying log and wants to see GPP trending, and suddenly the part you do by instinct has to be explained in numbers. This is the explainer nobody gave you on the job: what the psychrometric numbers mean, why they matter, and how they protect the claim.

If you understand the air, you can prove the drying. That's the whole point of a psychrometric drying log.

Relative humidity lies, and that's the core problem

Relative humidity is the number everyone knows, and it's the most misleading one in the room. RH is a percentage of how much moisture the air is holding compared to the most it could hold at that temperature. The trap is in those last three words. As air warms, it can hold more moisture, so the same amount of water in the air reads as a lower RH simply because the room got warmer.

So if you walk in tomorrow, the heater's been running, the room is warmer, and the RH dropped from 60% to 50%, did you dry anything? Maybe. Maybe not. RH alone can't tell you, and it can't tell an adjuster either. That's why a log that's just a column of RH percentages is weak evidence. You need a number that measures the actual water in the air regardless of temperature.

GPP: the number that doesn't lie

GPP stands for grains per pound: the weight of water vapor in each pound of dry air. (A grain is a tiny unit of weight, 7,000 of them to a pound.) The reason GPP matters is that it measures the moisture itself, not a temperature-dependent percentage. Warm the room up and the RH drops, but the GPP doesn't move unless you actually added or removed moisture from the air.

That's exactly what makes GPP the metric adjusters trust. When your log shows GPP in the affected area falling day over day while the unaffected area holds steady, you've demonstrated that your dehumidifiers removed real water from the environment. The temperature can do whatever it wants; the GPP trend tells the true story of the drying.

A healthy drying log usually shows the affected area starting high in GPP and trending down toward the unaffected, normal area. When those two numbers converge, you've made your case that the environment is dry.

Dew point: the temperature where water shows up

Dew point is the temperature at which air becomes fully saturated and moisture starts condensing onto surfaces. It's another way of expressing how much water is in the air, and it's useful on the job for two reasons.

First, condensation control. If a surface is colder than the dew point of the surrounding air, water condenses on it. That's how you get secondary damage, mold on a cold exterior wall, or a window dripping in a room you thought was drying. Watching the dew point keeps you from accidentally creating new problems while solving the original one.

Second, like GPP, dew point moves with actual moisture, not just temperature, so a falling dew point is more honest evidence of drying than a falling RH.

Vapor pressure and the engine of drying

You don't have to do the math by hand, but the concept is worth knowing because it explains why your equipment placement works. Water moves from wet materials into drier air because of a difference in vapor pressure: high pressure in the saturated material, lower pressure in the dry air you're maintaining. The bigger that difference, the faster the material gives up its moisture.

This is why dehumidification matters as much as airflow. Air movers pull moisture off the surface of materials, but if the surrounding air is already loaded with water, there's nowhere for that moisture to go. Your dehumidifiers keep the air dry, which keeps the vapor pressure difference high, which keeps the materials drying. A psychrometric log that shows low, falling GPP is showing that you maintained that gradient.

How the numbers fit together on a daily log

For each visit, a complete psychrometric drying log captures temperature and relative humidity in several locations, then derives GPP and dew point from them:

Affected area — the space you're drying
Unaffected area — a comparable dry space in the same building, your baseline
Dehumidifier outlet — proof the unit is actually removing moisture (its output should be drier than the room)
Exterior — context for what's coming in when you ventilate

Read the same points every visit, and the trend over the job becomes the argument. Affected GPP falling toward the unaffected baseline, dehumidifier output reliably drier than the room, dew point staying below surface temperatures. That's a drying log an adjuster can't pick apart.

Why this matters for the claim, not just the science

Under the IICRC S500 framework, drying is a measured process and the documentation is expected to show it. A psychrometric log is the difference between "trust me, it's dry" and "here's the data showing it's dry." When the readings tie to specific locations, carry timestamps, and trend in the right direction, the adjuster has nothing to dispute. When they don't, the claim is an argument waiting to happen, and arguments cost you line items.

How RemediDocs handles the psychrometry for you

You shouldn't have to run a psychrometric chart in your head in a wet basement. In RemediDocs, you log temperature and relative humidity, and the app derives GPP and builds the trending log automatically. Readings are timestamped and pin to a floor plan, so every value has a room, a material, and a time. The AI Meter Scan reads your moisture meter display through the phone camera, so the number in the log matches the number on the meter even in bad lighting.

When the job closes, the psychrometric drying log prints as part of a formatted IICRC S500 daily drying report. One PDF to the adjuster, and the science speaks for itself. It all works offline, because the basement is where the readings happen.

RemediDocs is a documentation tool and doesn't replace your obligations under IICRC or your insurer agreements.

Prove your drying with the numbers that hold up

If your logs are all RH and no GPP, you're handing adjusters room to argue. See the plans at our pricing or start free at a free account and build a psychrometric drying log on your next loss. No credit card, cancel anytime.