Dehumidification Capacity in Structural Drying


Picture this: we are at a loss, and we need dehumidification capacity. Before we start installing and setting up, we need to do a little math and area observation. Many answers to matters like this can be found in the Institute of Inspection Cleaning and Restoration (IICRC) current edition of the S500 10.4.3 & Appendix B. The standard will provide an understanding of how to calculate capacity.

Dehumidifiers are the primary source of dry air with low relative humidity (RH), but we also have other means of supplemental heating. There are potential hazards associated with the use of supplemental heat-producing equipment, such as power sources (electrical or fuel). We need to consider the impact supplemental temperatures put on building components (wood, metal, etc.) and structural components (HVAC, plumbing, insulation, electrical, etc.) When we consider the use of the heat on a structure and its contents, we also need to think about shrinkage, bowing, crooking, kinking, cupping, and twisting. Manufacturers’ instructions and safety precautions as well as recommendations should be followed to reduce potential exposure to heat hazards and other potential occupational safety & health issues.

Using supplemental heating from a direct-fired source should not be used unless adequate ventilation is available; when using fuel-operated systems, we need to consider the by-products (i.e., carbon monoxide).

When using equipment producing combustion by-products, restorers should monitor the air space for carbon monoxide.

It’s worth noting that just because we set the dehumidifier, route the drain line, power up the unit, and set appropriate air movement/movers does not mean we will meet dry standards and dry goals.

How Many Dehumidifiers?

When performing remediation, addressing the free, trapped, and bound moisture is the key to the task at hand. So, how do we know how much dehumidification is needed? The IICRC has created a way to calculate how many dehumidifiers are necessary for a job (current S500 Appendix B: Example Dehumidification Formulas Approximating a Minimum for Humidity Control (Mickey Lee)).

First, we need to clarify a few dry goals/terms and dehumidifier language.

Class of Water Intrusion

IICRC current edition of the S500 section 10.4.3 states restorers should estimate the amount of humidity control needed to begin the project.

The term “class of water intrusion” is the estimated evaporation load used when calculating the initial humidity control (e.g., dehumidification, ventilation). The designated classification is based on the approximate amount of wet surface area, the permeance (P=VAt∆p), and the porosity of affected materials in the drying environment when humidity control or restorative drying is initiated. The restorer should gather information needed to determine the class of water intrusion during the inspection process. The classes are divided into four separate descriptions: Class 1, 2, 3, and 4.

Determining the class of water intrusion may become dependent upon the restorability of wet materials and access to damp substrates.

Depending upon the project, this determination may occur at different points of the initial restoration procedures.

Other Factors Necessary to Estimate Humidity Control

The current edition of the S500 section 10.4.4 Other Factors Necessary to Estimate Humidity Control can impact the environment. When estimating the humidity control needed to prevent additional damages or begin the drying process, we should understand and consider these factors. These factors are discussed in Section 12.3.5: Humidity Control in Contaminated StructuresSection 12.4.2 Controlling Humidity and Stabilization (Initial Humidity Control)Section 12.5.2: Using Mechanical Dehumidification to Promote Drying.

Types of Dehumidifiers

We have three types of dehumidifiers within the industry for our use. We need to understand the type we are calculating, as their value is slightly different for each. The three types of dehumidifiers are:

  1. Conventional – a series of dehumidifiers that rely on cooling coils to induce condensation. Conventional dehus condense moisture vapor by passing air over refrigerated coils. A heating element (pipe) designed to prevent frost from forming on the coils. Conventional dehumidifiers are not as effective or efficient at removing water and producing drier air in environments with large volumes of water as LGRs.
  2. Low-grain refrigerant (LGR) – LGRs can dry a space to a lower humidity than the conventional dehumidifier series. LGRs provide a tremendous amount of power to remove water from the air. LGRs have a double cooling system that creates a process that lowers the temperature of moisture-filled air as it passes through the machine. LGRs creates more condensation that forms on the device’s coils. The condensation being processed means less moisture in the returned air.
  3. Desiccant – Desiccant dehumidifiers work on a different process entirely from their siblings. As humid air passes through a desiccant, moisture is removed by a chemical attraction process. A desiccant dehumidifier uses chemical attraction instead of condensation to remove moisture from the air. The most crucial piece of the machinery is the desiccant wheel, which is a sort of honeycomb tubing of small airways lined with silica gel.

Desiccants range from a one-person mobile setup to a full-blown trailer-size unit that requires a team to set up and maintain.

Pints-per-Day (PPD)

It is also important to consider pints-per-day (PPD), which is the amount of moisture a dehumidifier can remove from the air in 24 hours. When comparing the PPD efficiency of dehumidifiers, we as restorers need to know if the PPD rating is for AHAM (American Home and Appliance Manufacturers) or Saturation conditions.

AHAM vs. Saturation

When looking at dehumidifiers, AHAM and Saturation define the dehumidifier testing conditions. The AHAM acronym is widely used because the group recommends specific testing conditions.

-AHAM is 60% humidity and 80°F / 26.7°C

*Why 60% and 80°F / 26.7°C? According to AHAM, they best represent conditions dehumidifiers will be used in*.

-Saturation is 90% humidity and 90°F / 32.3°C.

Most dehumidifiers will never face these conditions. This testing aims to determine the maximum amount of moisture that can be removed in one day.

Size of the Area to be Dehumidified is represented in Cubic Feet

To calculate cubic footage of a room, use the following calculation:

cubic feet = length x width x height

Restoration Egg: Consider the cubic feet instead of square feet; the wrong number will impact our calculations.

Initial Dehumidification Recommendations

IICRC S500 has the following dehumidification recommendations.

Note: Current edition of the IICRC S500 Appendix B: This chart has figures used to determine an approximate minimum initial dehumidification capacity for the purpose of humidity control. 

They may change based on psychrometric readings and the types of materials presents. As with any method for determining humidity control requirements, verification of efficacy and adjustment through the use of appropriate instruments will be necessary. Current edition of the IICRC S500 12.3.5 After the initial installation, appropriate adjustments (e.g., increase, decrease, reposition) in dehumidification capacity should be made based on psychrometric readings in order to achieve or maintain the targeted conditions set in the drying plan.

Calculate the Required Number of Dehumidifiers

 Dehumidifiers: (Conventional and LGR)

  1. Calculate the volume of the room in cu. ft. (L x W x H).
  2. Calculate AHAM, the volume of the room cu. ft. divided by the appropriate number of dehumidifier types and Classes.
  3. Divide (÷) the dehumidifier’s rating to arrive at the number of dehumidifiers required.


  1. Calculate the volume of the structure in cu. ft. (L x W x H).
  2. Calculate SCFM, air change (volume ÷60 mins).
  3. Calculate the desiccant capacity required (SCFM x ACH).

Detailed Method

The exact method requires the understanding of several factors related to the structure and surrounding weather conditions, such as building science.

Below is a description of the factors to consider:

Build-Out Density: Impacts the ability to create lower vapor pressure air in all areas of the space, as well as the amount of affected wall material that may need to be addressed.

  • Very open: As in a factory, warehouse, convention center, large ballroom, sports complex, box store, or theater;
  • Fairly open: As in a school with large classrooms or open office areas (e.g., open space with cubicles), department store;
  • Average: As traditional office buildings or hotels; and
  • Very dense: As in an executive office suite with many small (e.g., 10′ x 10′ / 3mt x 3mt) offices and few open common areas, medical offices, or dormitory.

Building construction and finishes: Impacts the evaporation characteristics of the structure and contents:

  1. Standard: Standard material and construction, such as primarily carpet/pad over concrete or plywood subfloor or commercial glue-down, single-layer drywall, little to no insulation in interior walls and construction is standard; either wood or metal framing, mostly painted walls, and builder-grade wood or vinyl baseboards.
  2. High-end/Complex: High-end materials and complex construction, such as extensive carpet over heavy pad, multiple layers or high-density wall assemblies, insulation and/or sound-attenuation may be present in interior walls, construction includes some fire-rated walls, complex assemblies (e.g., multiple layer flooring systems, chase walls) and higher-end finishes (e.g., vinyl wallcoverings, architectural-grade paneling, and wood trim details).

Stack Effect = the movement of air into, throughout, and out of buildings.

  1. As a remediator, we all know that warm air rises. That, after all, is what gives a hot-air balloon its buoyancy. Warm air rises inside facilities by the means of stairwells and elevators/lifts, and lowers the air pressure at the base of the structure. Air from outside the facility is drawn in by the lower pressure though doors, windows, and vents in the lower part of the building.
  2. As interior air is warmed within the facility, it too rises, and the process repeats in a continuous cycle. This is called the stack effect because it is the same process which makes a chimney effective in expelling smoke and other gases from a building.
  3. This phenomenon, sometimes also called the chimney effect, is a good thing as it ventilates the facilities, but it has unwanted side effects. The lowest part of many facilities is the crawl space and/or the basement or the lower portion of the first floor.

HVAC Impact

The HVAC can impact the project if the system is present, operable, and can assist in controlling humidity and maintaining conditions in the environment.

  • Beneficial: The system is present, operable, and will help maintain conditions.
  • Non-Beneficial: The system is not present, not operable, or will not assist in maintaining conditions.

Prevailing weather: Impact will vary significantly from one climatic region to another and from one season to the next.

Such variations may require us the restorer to use different equipment and techniques when controlling humidity in similar wet structures during different times of the year, or different regions.

  1. Estimate the expected impact. Examples provided are approximate:
  • Favorable: Anticipated to aid humidity control (e.g., less than 40 gpp or 43°F DP); (e.g., less than 5.7 gpk or 6.1°C DP)
  • Neutral: Anticipated to have minimal impact on humidity control (e.g., between 40 and 60 gpp, or 43°F and 53°F DP) (e.g., between 5.7 gpk and 8.5 gpk, or 6.1°C and 11.6°C DP); and
  • Unfavorable: Anticipated to hinder humidity control (e.g., above 60 gpp, or 53°F DP) (e.g., above 8.5 gpk, or 11.6°C DP).
  1. Estimate the building envelope’s ability to keep the outside conditions from influencing humidity control (i.e., infiltration):
  • Tight: Humidity can be controlled without significant influence by the outdoors;
  • Moderate: humidity will be influenced somewhat by the outdoors; and
  • Loose: humidity will be significantly influenced by the outdoors.

Note: Some of the overall considerations for choosing tight, moderate, or loose would be:

  • Number of occupants and tradespeople on-site (e.g., opening doors, windows, work processes);
  • Damage to the building’s envelope (e.g., windows, roof, outer sheeting);
  • General construction (e.g., barriers, insulation, age); and
  • Outdoor wind speed (e.g., higher wind speeds increase filtration rates).

Electrical / Heat / Energy: (ref:

  • Amperes (amperage or “amps”) – the amount of electricity (current) flowing in a circuit
  • Voltage – the force of electricity flow in a circuit
  • Watts – the amount of electricity an electrical device uses when operating
  • British Thermal Units (BTUs) – heat generated by an electrical device
  • Formula – amps x volts x 3.4 = British Thermal Units (Btu) per hour
  • HVAC– unit removes 12,000 Btu per ton
  • Residential – residential 15 amp
  • Commercial – commercial 20 amp
  • 220 splitters – use where there is limited amperage or fuses
  • Power consumption formula – volts x amps x 24 hours = watts ÷ 1000 = kW x cost per kW per day

Dehumidification Consideration

IICRC current edition of the S500 section 10.4.3 Dehumidifiers are part of the equation; another part that is just as important is the influences of surface evaporation. Surface evaporation is essential during the drying stage, which is airflow velocity.

Check out this article about air movement/mover’s (centrifugal fans, axel fans, and air filtering devices (AFDs’)).

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Joshua Woolen

Josh Woolen is the owner of EGA-Commissioner (EGA-Cx), which works as a third-party on behalf of the contractor and only the contractor, unlike most of the other third-parties who work for the benefit of the property owner, property manager, or insurance company. The Commissioning (Cx) Process is a systematic process for the contractor to ensure they have representation during specific points of the project or the entirety of the project. This gives the documentation for the work neededwork being performed, and the work achieved after site departure that requires third-party validation.

(570) 664-7947 &

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