Multi-Floor Office Water Loss Desiccant Deployment

Why this matters

A burst riser on floor 14 of a Class A tower that ran through the night will saturate the carpet, gyp board, ceiling tile, raised access flooring, and concrete slab from floor 14 down through floor 8 in an hour. The cubic-foot volume is in the millions; refrigerant dehumidification (LGR) cannot pull the grain depression needed inside an occupied envelope still feeding outside air through the AHU. Desiccant rigs are the only practical answer above roughly 100,000 cu ft of contiguous water-affected envelope, and the deployment math (rig sizing, duct routing, condensate management, exhaust path, AHU coordination) is what separates a dry-in-7-days project from a structural drying failure with mold to follow. This tech manual is the desiccant sizing and deployment framework for multi-floor commercial water loss.

Why desiccant, not LGR

Refrigerant LGR units (Phoenix R250 LGR, Drieaz LGR 7000XLi) operate efficiently at 60 F to 90 F and 40% to 90% RH. Below 40% RH or below 60 F the coil frosts and efficiency drops. In a partially-conditioned occupied envelope you want grains per pound (GPP) below 40 inside the work zone, which means RH below 25% at 70 F. LGR cannot reach that range; you stall at 35 GPP and gyp board never dries below 16%. Desiccant rotors (silica gel impregnated) drive process-air output to 5 to 20 GPP regardless of inlet RH. Drymatic Boost, Phoenix DryMAX XL, and Aramsco D6000 are field-standard rigs in the 6,000 to 15,000 cfm range.

Sizing math

Step 1: calculate water-affected cubic feet. Sum the affected envelope by floor, using the floor-to-deck dimension if the ceiling plenum is wet (almost always). A 25,000 sq ft floor plate with a 12 ft floor-to-deck is 300,000 cu ft per floor. Seven floors wet = 2.1 million cu ft.

Step 2: calculate required air changes per hour (ACH). IICRC S500 Section 13 recommends drying-environment ACH based on classification. Class 3 (saturated porous materials over a large area) targets 60 to 100 ACH at the affected surface. Use 60 ACH as the design point for desiccant zones; the high process-air dryness compensates for slightly lower volumetric turnover vs. an LGR-and-air-mover combination.

Step 3: required process-air cfm = (cu ft x ACH) / 60. For one 300,000 cu ft floor at 60 ACH = 300,000 cfm peak, which no single desiccant rig delivers. The realistic field approach is to zone the floor into containment cells of 30,000 to 50,000 cu ft (typically by demising walls and stairwell partitions), each cell served by a dedicated rig of 6,000 to 10,000 cfm. A 6,000 cfm rig serving a 36,000 cu ft cell delivers 10 ACH continuous; combined with high-velocity air movement (one axial fan per 250 sq ft per S500), the equivalent surface ACH reaches the 60 ACH design point.

Step 4: heat output. Desiccant regeneration burns the moisture off the rotor and dumps heat (and reactivation exhaust air) at the unit. A Drymatic Boost outputs roughly 100,000 BTU/hr of heat to process air. In a partially-conditioned tower you must coordinate with building engineering on AHU rebalance or you cook the occupants on the floor above.

Trailer-mounted vs. portable

Above 10,000 cfm process air, the rig is trailer-mounted (Phoenix 6500D-MV, Aramsco PolarMax 100). Trailer rigs stay at street level; you run 16 in to 24 in lay-flat duct up the freight elevator shaft or through a stairwell to the affected floors. Lay-flat duct is rated to roughly 2,500 cfm per 16 in run at 2 in static pressure; for 10,000 cfm you parallel four runs. Plan freight elevator reservation with the building before mobilization; a fire-rated stairwell cannot be the permanent duct route without an AHJ variance under IBC Section 1023 (stair enclosure).

Portable indoor rigs (Drymatic Boost, Drieaz DriTec Pro 65) ride the freight elevator to the affected floors and sit inside containment. They are simpler to deploy but consume conditioned space and add heat where you do not want it. Standard deployment is trailer rig for floors 2 and below the loss source, portable rigs for the source floor and one floor above (where ceiling plenum is wet from splash-back).

Containment and exhaust path

Floor-by-floor containment with 6 mil poly on a stud or pole frame is the minimum; for a multi-week dry, hard-wall STARC-style panels are worth the extra mobilization cost because the negative-pressure seal holds. Each cell needs:

1. Process-air inlet (desiccant rig supply duct in).
2. Process-air return path back to the rig.
3. Reactivation exhaust routed outside the cell (and ideally outside the building) to dump the moisture-laden reactivation stream.
4. Continuous data-logging thermo-hygrometer (Tramex Hygromaster, FLIR MR176 with iPad logger, Drymatic Smart Tracker).

The reactivation exhaust is the single most common deployment failure. Dumping reactivation back into the cell or the building return defeats the desiccant; the unit pulls the moisture it just removed back across the rotor. Reactivation must vent outside the building envelope; on an upper floor this is an operable window or a temporary curtain-wall penetration sealed and weatherproofed nightly.

Daily monitoring documentation

S500 Section 13 documentation per equipment-day. For each cell, log:

1. Outside air temperature, RH, GPP (control reading).
2. Cell ambient air temperature, RH, GPP (effectiveness reading).
3. Process-air supply temperature, RH, GPP at the rig outlet (rig performance).
4. Reactivation exhaust temperature (verifies the rotor is regenerating).
5. Moisture content of each material class (gyp board, framing, concrete) at three points per cell, using Tramex Compact for non-destructive scan and Tramex Heavyduty pin meter for verification.

The cell is dry when material moisture content reaches the customer's documented dry standard (typically gyp board less than 12%, framing less than 16%, concrete less than 4% wood-moisture-equivalent or below the manufacturer's flooring substrate spec, e.g., RH less than 75% per ASTM F2170 if hardwood is going back on the slab).

References

1. IICRC S500-2021, Standard for Professional Water Damage Restoration, Sections 10 (psychrometry) and 13 (drying environment).
2. ASTM F2170-19, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.
3. NFPA 54-2024, National Fuel Gas Code (regen burner fuel routing).
4. IFC 2024, Chapter 13 (Special Operations) and IBC 2024 Section 1023 (stair enclosures).
5. Drymatic Boost Desiccant Dehumidifier Technical Data Sheet (process-air output, regen heat output).
6. Phoenix DryMAX XL Desiccant Technical Bulletin (sizing and duct sizing tables).
7. Aramsco PolarMax 100 Trailer Desiccant Specification Sheet.