Drying Load, Air Movers, Dehumidifiers, and Days

The evaporation load in grains per hour and pints per day from room volume and air changes, the air-mover count and the circuit load, the LGR dehumidifier count, and the drying days with the moisture standard that ends the job.

Air movers run at 1 per 16 sq ft on a Class 1 job, 1 per 12 on Class 2, and 1 per 10 on Class 3. An LGR dehumidifier covers about 3,000 cu ft, and the drying ends at the moisture standard, 1 percent of the dry reference.

—Volume, CFM, and air mass
—Evaporation load in grains per hour
—Air movers, amps, and circuits
—LGR dehumidifiers and capacity
—Drying days estimate
—Stop criteria and readings
How the math works
  • Air volume: volume in cubic feet times air changes per hour, divided by 60, is the cfm of moving air: 1,344 cu ft at 4 air changes is 90 cfm. Air mass follows the standard density of 13.33 cubic feet per pound of dry air, so 90 cfm is 6.8 lb of dry air per minute.
  • Grains and capacity: saturation in grains per pound of dry air at the room temperature (the Magnus saturation curve), multiplied by the relative humidity, is the grains the room air is holding. The difference to saturation is the drying capacity, and grains per hour is the air mass flow in pounds per hour times that difference. 7,300 grains is a pint, and 58,380 grains is a gallon.
  • Air movers (IICRC S500 classes): Class 1 fast-drying material is 1 air mover per 16 sq ft of affected area, Class 2 average-drying material is 1 per 12 sq ft, Class 3 slow-drying material is 1 per 10 sq ft, at about 2.0 amps each on a 20 amp circuit with 8 movers per circuit, and the movers aim at the wet surface at a 45 degree angle with the airflow across the surface.
  • Dehumidifier sizing: 1 LGR low-grain refrigerant dehumidifier per 3,000 cubic feet of drying volume at a nominal 20 pints per day, with an air scrubber where the air quality requires it, and a desiccant unit where the temperature is under 40 F and where the grain depression has to be below 30 grains.
  • Category handling (S500): Category 1 is clean water and the job ends at the moisture standard, Category 2 carries a significant contaminant and the porous material is removed with the cleaning sequence, Category 3 is sewage and flood water and the porous material is removed, bagged, disposed, disinfected, and the job ends at the moisture readings and the air quality.
Planning tool on the IICRC S500 water damage restoration standard and the drying equations published in the standard, with the material classes and the air-mover ratios from the standard text. The evaporation load, the drying days, and the water stored in the material are estimates: a wood species table, a gypsum core thickness, a concrete slab with a vapor barrier under it, a wall with insulation in the cavity, a hardwood floor over a slab, a wall with a vinyl coating, and a ceiling with an insulated attic above it all dry at different rates, and the in-situ relative humidity reading is the reliable answer inside a wall. Category 2 and Category 3 water, a sewage backup, a flood, a category with a health complaint, a building with a mold question, and a contents plan with a list price are all job-scope calls, and an insurer standard, a state license, a permit, and an adjuster protocol are jurisdiction and contract calls.

How many air movers does a wet room need?

1 air mover per 16 sq ft on Class 1 fast-drying material, 1 per 12 sq ft on Class 2 average material, and 1 per 10 sq ft on Class 3 slow material: a 14 ft by 12 ft room at 168 sq ft is 11 movers on Class 1, 14 on Class 2, 17 on Class 3 (IICRC S500).

What is the grains-per-hour drying equation?

Air mass flow in pounds per hour times the difference between the saturation grains at the room temperature and the grains the room air is holding at its relative humidity: 1,344 cu ft at 4 air changes is 90 cfm, 6.8 lb per minute, and at 72 F and 65 percent the air has about 38 grains per lb of room, which is about 15,500 grains per hour.

How many dehumidifiers does the job need?

1 LGR dehumidifier per 3,000 cubic feet of drying volume: a 14 ft by 12 ft by 8 ft room is 1,344 cu ft, so 1 unit, a 3,000 cu ft basement is 1 unit at the limit, and a 9,000 cu ft basement is 3 units. A desiccant unit is the choice under 40 F and where the grain depression has to be under 30 grains.

How long does a wet room take to dry?

Class 1 drywall and a sealed floor are commonly 2 to 3 days with the movers and a dehumidifier running, a gypsum wall with insulation in the cavity is 4 to 5 days, a concrete slab and a screed go to 7 days and past, and a hardwood floor over a slab is tracked over 10 to 14 days with the readings at 3 points.

Related tools

Mold containmentThe job a dry-out that runs late becomes Crawlspace encapsulationThe moisture source under the floor Paint crew productionRepainting after the wall is dry Drywall sheets, tape, and mudThe board that comes out at the reading Radon system sizingDry the crawl before the membrane goes in