Structural Drying Equipment Needs Calculator
Estimate the number of air movers and dehumidifiers required for structural drying projects based on affected square footage, material categories, and moisture severity — following IICRC S500 guidelines.
Formulas Used
Total Wet Surface Area:
Surface = (Floor Area + Wall Area + Ceiling Area) × Stories
Wall Area = Linear Feet × 8 ft (standard wall height)
Air Movers:
Air Movers = ⌈(Surface ÷ Class Ratio) × Category Multiplier⌉
Class Ratios — Class 1: 100 sq ft/unit | Class 2: 75 | Class 3: 50 | Class 4: 40
Category Multipliers — Cat 1: 1.0 | Cat 2: 1.1 | Cat 3: 1.2
Psychrometric Capacity Factor (CF):
CF = (RH% ÷ 60) × (Temp°F ÷ 70) [clamped 0.5–1.5]
Dehumidifiers (LGR, ~100 pints/day):
Adjusted Coverage = 1,750 sq ft × CF
Dehumidifiers = ⌈(Surface ÷ Adjusted Coverage) × Class 4 Multiplier⌉
Grains Per Pound (GPP) — Magnus Approximation:
Psat = 0.6105 × exp(17.27 × T°C ÷ (T°C + 237.3)) kPa
Pv = (RH ÷ 100) × Psat
W = 0.622 × Pv ÷ (101.325 − Pv)
GPP = W × 7,000 grains/lb
Estimated Drying Time:
Days = Base Days + (max(0, RH−60) ÷ 10 × 0.5) + (max(0, 70−Temp) ÷ 10 × 0.3)
Base Days — Class 1: 3 | Class 2: 4 | Class 3: 5 | Class 4: 7
Air Changes per Hour (ACH):
ACH = (Air Movers × 1,750 CFM × 60) ÷ (Floor Area × 8 ft × Stories)
Assumptions & References
- Based on IICRC S500 Standard for Professional Water Damage Restoration (5th Edition).
- Standard wall height assumed at 8 feet; adjust for non-standard construction.
- LGR dehumidifiers rated at 100 pints/day at AHAM conditions (80°F, 60% RH); actual output varies by model.
- Air movers assumed to deliver 1,750 CFM average; axial units may vary 1,500–2,000 CFM.
- Psychrometric capacity factor (CF) is a simplified linear approximation; full psychrometric chart analysis is recommended for large or complex losses.
- GPP target of ≤ 55 grains/lb is a common structural drying goal; actual target depends on local outdoor conditions and material equilibrium moisture content (EMC).
- Drying time estimates assume continuous equipment operation, proper placement, and daily monitoring with adjustments.
- Category 3 losses require containment, antimicrobial treatment, and possible demolition per IICRC S500 before drying begins.
- Class 4 specialty drying (hardwood floors, concrete, plaster) may require desiccant dehumidifiers or directed heat systems not accounted for here.
- This calculator provides a starting estimate; a certified water damage restorer (WRT/ASD) should perform on-site psychrometric calculations and equipment adjustments.