Insurance adjusters love the number 40%. They will say, “The humidity is 40%, so the room is dry.” This is a deception. Relative Humidity (RH) tells you nothing about the water trapped in your walls. To win, you must understand the true physics of drying: Psychrometry.
Stop Looking at % and Start Looking at GPP:
Content Index
1. RH vs. GPP: The “Bucket” Analogy
Why is “40% Humidity” a lie? Because Relative Humidity is Relative to temperature.
Think of Air as a Bucket:
- Cold Air is a Shot Glass (Small Bucket).
- Hot Air is a Bathtub (Huge Bucket).
If both are “50% full” (50% RH), the Bathtub holds 100x more water. We need to measure the Water, not the % full. That measurement is GPP (Grains Per Pound). It is the absolute weight of water in the air.
2. The Psychrometric Chart
Adjusters use a complex graph called the Psychrometric Chart to determine if drying is possible. If they don’t bring one, they are guessing.
The Magic Number: You want the GPP inside the house to be lower than the GPP inside the wet material. This difference is what sucks the water out.
Video: How to read the “Map of Air” that professionals use.
3. Vapor Pressure Differential (The “Push”)
Water doesn’t just evaporate because you asked it to. It must be pushed or pulled. This force is called Vapor Pressure.
High Pressure (Wet Wall) → Low Pressure (Dry Air)
To dry a house fast, you need to create a massive Vapor Pressure Differential. You do this by:
- Heating the Material: Increases the “Push” (High Pressure).
- Dehumidifying the Air: Increases the “Pull” (Low Pressure).
If the adjuster refuses to pay for heaters, they are killing the “Push.” Drying will stall, and mold will grow.
4. The “False Dry” Phenomenon
This is how claims get denied later. The contractor puts a fan on the floor for 3 days. The surface feels dry. The meter says “Dry.” They leave.
Two weeks later, the floor cupping begins.
Evaporation happens from the outside in. The “Boundary Layer” of air right next to the wet surface gets saturated instantly. If you don’t have enough Airflow (CFM) to blow that layer away, the water stops moving. You need High-Velocity Air Movers to shear that layer off.
5. The Script: Arguing Physics
When the adjuster says “It’s dry enough,” hit them with the math.
BAD ANSWER: “Are you sure? It smells damp.”
PROTECTOR ANSWER: “The RH is irrelevant. The GPP inside is 65, and the GPP outside is 60. There is no Vapor Pressure Differential. We are at equilibrium. We need to increase the temperature to 85 degrees to restart evaporation, or we will have secondary mold damage. I am demanding we continue drying until the Material Moisture Content (MC) reaches 10%.”
Conclusion
Drying a house is not about “fans.” It is about manipulating physics. If you control the GPP and the Temperature, you control the claim. Don’t let them bamboozle you with Relative Humidity percentages.