Compare two air masses in grains per pound. Effective drying needs the dehumidifier’s exhaust drier than the room — a positive GPP depression.
GPP depression: is the equipment winning?
Drying works when the dehumidifier’s exhaust is drier than the room — a positive GPP depression. If the depression is zero or negative, the drier air is not dry enough to move moisture and the equipment is fighting itself. Aim for a 20–40+ GPP depression to dry aggressively.
The condensation check
If the room’s dew point rises to or above the temperature of a surface — a window, a slab, an exterior wall — water condenses there and causes secondary damage. Enter the coldest surface temperature to see how much margin you have and whether you need to lower the room’s GPP or warm the surface.
Frequently asked
What is GPP depression?
GPP depression is the difference in grains per pound between the room air and the dehumidifier’s exhaust. A positive number means the exhaust is drier than the room and will pull moisture; restorers aim for 20–40 or more to dry quickly.
How do I know if condensation will form?
Compare the room’s dew point to the temperature of the coldest surface. If the dew point is at or above that surface temperature, moisture condenses on it. Keep the dew point several degrees below the coldest surface to prevent secondary damage.
Is this psychrometric calculator IICRC S500 aligned?
Yes — it uses standard psychrometric relationships and the GPP/dew-point logic taught for S500 structural drying. Treat the outputs as starting estimates and verify against on-site readings.
More free tools
This knowledge, built into the software
RestoredOS runs the whole job on the same field knowledge behind these tools — drying logs, moisture mapping, scope, and billing.