Air Movement Capacity


Let’s set the stage. Imagine yourself in a home or business after a water loss. We have developed the heating elements, now we want to establish airflow (i.e., volume, velocity), taking into consideration humidity (i.e., dehumidification, ventilation) and temperature (i.e., vapor pressure differential). Together, these elements will work towards the drying goal established in the S500. (Per current edition, S500 Ch-12.)

We need to understand the elements of airflow because it is a crucial transport mechanism for moisture, mold, fire, sewage, neglect, and the history of the area of the loss (spores, particulates, and fragments). We haven’t mentioned the things hidden within the sealed spaces and voids. As we circulate air around and over affected materials, we need to consider how air is drawn into the air movers and disbursed. The building design and construction type will influence where the air will enter or exit and how moisture and contaminants can be controlled or removed. The better we understand the mechanisms of structural science, including air ingress, egress, and passage through a structure, the more efficiently and effectively we, as remediators, will be when planning and executing a comprehensive course of action; this is all of course encompassed in Structural Science. 


We use centrifugal force as the primary air movement and occasionally axial fans as standard industry tools for air movement. Other supplemental air movement tools are available, like Air-Filtration Devices (AFDs’) and Structural Cavity Drying Systems (SCDS). AFDs’ can be staged and used in several ways: as an air scrubber or configured to be negative or positive air scrubber. SCDS can force -air in or draw moisture out, depending on the configuration we need. 

All air movement tool usage requires knowledge, awareness, and an understanding of the specific tool before, during, and after we use it/them. As we use air movement, we need to remember that, as a given volume is drawn into a space, a given volume of air must leave. Ref: IICRC and other industry educational services. (Restoration Industry, Association of American Council for Accredited Certification, EPA, and Leadership Guide for Restoration) 

Just because we set the air movement in the direction, we determine appropriate does not mean we will meet dry standards, dry goals, or optimal indoor air quality. The current edition of the IICRC S500 provides clarity on this. 

Seriously, when in doubt, go to the standards!

How much air movement?

When performing remediation, addressing the free, trapped, and/or bound moisture is the key to the task at hand and reducing or eliminating air particulates and fragments. Per the current edition, IICRC S500/520 provides answers. 

First, we need to clarify a few commonly used terms. 

  • Category (Cat) The categories of water, as defined by the current edition S500 document in Section 10.4.1, refers to the level of contamination present in water, considering both its origin source and its quality after it contacts materials present at the loss site. Time and temperature can affect or amplify contaminants, thereby affecting its category. We should consider potential contamination, defined as the presence of undesired substances. The identity, location, and quantity of which are not reflective of a typical indoor environment; and can produce adverse health effects, cause damage to structures, systems, or contents, or adversely affect the operation or function of building systems.
  • Classification (Class) We should estimate the amount of humidity control needed to begin the project. The term “Class of water” as defined in Section 10.4.3 is a classification of the estimated evaporation load and is used when calculating initial humidity control (e.g., dehumidification, ventilation). It is based on the approximate amount of wet surface area and the permeance and porosity of the affected materials left within the drying environment when humidity control or restorative drying is initiated. Restorers should consider the initial information to determine the Class of Water during the inspection process. The Classes are divided into four separate descriptions: Class 1, 2, 3, and 4. The determination of Class may be 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 procedure.
  • Condition for the current edition of the IICRC S520 Standard, Conditions 1, 2, and 3 are defined for indoor environments relative to mold. The categories of microbial, as defined by the current edition S520 document in Section 10.5, refer to the range of contamination in the indoor environment, considering its exposure, and quality after it contacts or settlement on materials present on the job site weather contaminated/cross-contaminated from condition 2 or 3. We should consider potential contamination, the identity, location, and quantity of which are not reflective of a typical indoor environment. Microbial contamination can produce adverse health effects, cause damage to the structure, systems, or contents.

Other Factors Necessary to Estimating 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 Structures, Section 12.4.2 Controlling Humidity, and Stabilization (Initial Humidity Control)

Types of Air Movers

We have four (4) types of air movement within the industry for our use. We need to understand the kind we are calculating as their value is slightly different for each. The four (4) types of air movers are: 

Centrifugal Air Movers

These are called air blowers or sometimes carpet dryers and are most commonly used to provide targeted directional air flow to dry carpets, materials and flooring. They have a multi-speed motor creating an extremely low to high flow of air when needed.

*Carpet cleaners like to use these air movers as they dry the carpet more quickly after being cleaned*

*Remediators like them for the ability to direct air while elevating square feet per minute (SFPM) from its outlet.

Low Profile Centrifugal Air Mover

With current technology, low-profile centrifugal air movers are also being built to make them easier to store, carry, and move around efficiently.

Low-Pressure Air Mover

Low-pressure centrifugal air movers dry organic wood floors, kitchen, and bathroom cabinets and blow air into cavities.

Most centrifugal low-pressure air movers use 4-5 amps of electricity, good enough to make the carpet float in the air when adequately placed underneath the carpet.

High-Pressure Air Mover

Centrifugal high-pressure air movers are ideal for various systems. Most centrifugal high-pressure air movers use 10-12 amps of electricity. Much more power than standard carpet dryers.

It is important to note that a high-pressure air mover comes with a higher horsepower motor making them ideal for drying large applications.

Axial Air Movers

Axial air movers, also called a fan, work best for drying ceilings, walls, and structures with less force than a centrifugal air mover.

Axial air movers have a low amp compared to centrifugal fans, but they come in standard and high-capacity systems.

Low-Pressure Air Mover

An axial low-pressure air mover specializes in moving large volumes of air with a low amp draw and is primarily used during in-place drying.

An axial low-pressure air mover uses 1.5 to 3.0 amps and produces significantly more airflow volume than carpet dryers.

High-Pressure Air Mover

High-pressure axial air movers can also be used with various ventilation systems to dry cavities. However, they are most often used to produce significant positive or negative pressures.

Axial high-pressure air movers, as I mentioned earlier, they are commonly referred to as fans.

An axial high-pressure air mover uses 2.5 to 8.8 amps, and they are used to move mass volumes of air through ductwork.

Structural Cavity Drying Systems (SCDS)

These units are specifically designed to force air into or out of wall cavities or other interstitial spaces under flooring materials for efficient drying of otherwise inaccessible areas. This equipment is intended to dry wet walls, ceilings, and other assemblies, including complex flooring systems. The SCDS systems can be classified according to pressure ranges and by the manner in which they handle airflow through ducting and attachments.

Air Filtration Devices (AFDs)

Air scrubbers are used by restorers to assist in air quality and air improvement during remediation practices that can disturb the environment and during debris removal. Air movers range from a one-person carry unit, simple set up, to a full-blown trailer-size unit that requires a team to set up. 

When looking at AFDs’, we come across a few terms:

Air Scrubbing: AFDs’ are used to get rid of contaminants in an enclosed space. The primary function of the AFD is to collect particles and remove gases or chemicals from a particular space. If you’re looking to improve air quality indoors, an AFD is your best bet, provided the interior and exterior are clean and filters are changed regularly, and clean High Efficient Particulate Air (HEPA) filter. AFDs work by passing airborne particle through a series of filters that collect AT 99.97%. of air particles. Typically set in the middle of a room, circulating the air. 

Negative Air Scrubber: This is the industry lead use of AFDs, as they are use is to keep contaminants from escaping the work area. The machine is placed in a manner that allows the AFD to be ducted out of the work area to clean the affected area continually with air exchanges. Most workers involved in debris removal where air quality may be compromised (particulates and fragments) will use an air scrubber this way. 

Positive Air scrubber Using an air scrubber to clean the air positively is the least often used of the three (3) techniques. It is used to keep areas clear of contamination. By placing the machine near the outside of a workspace and ducting the machine into, the air scrubber cleans potentially contaminated exterior air and pushes it into the area, positively pressurizing the area. This method works well for clean places that always need to be kept clean, such as hospitals, museums, and labs. 

Let’s Talk Math: ACH, ACPH, or APH (Air Changes per Hour)

A key aspect in AFDs is understanding calculating how many air changes per hour are required to clean the air adequately. An industry standard is six air changes per hour, but there are several factors that could warrant increasing or decreasing that number.

The size of the area to have air movement is calculated in cubic feet. To calculate the cubic footage of a room, use the following calculation: 

cubic feet = length x width x height

Real-World Example:

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

Air movement should be set up to provide continuous airflow across all affected wet surfaces (e.g., floors, walls, ceilings, framing). In order to achieve this, it is recommended that we as restorer’s position air movers to:

  • Ensure adequate circulation of air throughout the drying environment to include interstitial spaces;
  • Direct airflow across the affected areas;
  • Account for obstructions (e.g., furniture, fixtures, equipment, and structural components), if their presence prevents sensible airflow across the affected surfaces; 
  • Deliver air along the lower portion of the affected wet wall and edge of the floor;
  • Point in the same direction with the outlet almost touching the wall; and
  • Deliver air at an angle (e.g., 5° to 45°) along the entire length of affected walls.
  • Upon initiating the restorative drying effort, we should install one air mover in each affected room. In addition, add one air mover:
  • For every 50-70 SF (4.5-6.5 M2) of affected wet floor in each room (to address floors and lower wall surfaces up to approximately 2 feet);
  • For every 100-150 SF (9.3-14 M2) of the affected wet ceiling and wall areas above approximately 2 feet (60 cm); and
  • For each wall inset and offset more significant than 18 inches (45 cm).

Within the ranges stated above, the number of air movers needed can vary between projects depending upon the build-out density, obstructions to airflow, and amount and type of wet affected materials. 

In circumstances where water migration has primarily affected lower wall sections and limited flooring (e.g., less than 2 feet or 60 cm of migration out into the room or area), restorers should install a total of one air mover for each 14 affected linear feet of the wall. This calculation is independent of the above SF calculation and is not meant to be used in the same room or area.

A single air mover may be adequate in small rooms (e.g., closets, pantries under ~25 SF or 2.3 M2), especially if upper walls and ceilings are not affected. However, when any calculation for a room or space results in a fraction, the indicated number of air movers should be rounded up. 

In Class 4 intrusions involving significant water absorption into low evaporation materials and assemblies, vapor pressure differential should be increased once free water has been evaporated (e.g., increase the temperature of wet materials; reduce the humidity of the surrounding air; or a combination of both). In these circumstances, it can be beneficial to decrease the velocity of airflow.

After the initial installation, we should inspect and make appropriate adjustments (e.g., increase, decrease, reposition) to the number, type, and placement of air movers based on materials’ moisture readings. The first of these inspections to monitor and make adjustments should be performed no later than the day following the initiation of restorative drying. The frequency of subsequent monitoring should be daily until drying goals have been met, and may be adjusted by the agreed-upon scope of work, potential for secondary damage, job site accessibility, or by agreement between the materially interested parties. The drying process can aerosolize soil and particulates present in the environment. As water evaporates from surfaces and materials such as carpets, more particles can aerosolize, creating possible health, safety, comfort, and cleanliness issues. 

Before we implementing the restorative drying effort (e.g., rapid air movement), we should evaluate then clean materials within the work area as needed. Where necessary, we should clean visible debris, dust, and soil from materials and surfaces to reduce the amount of soil or particulates that can become aerosolized. We should employ cleaning methods that minimize aerosolizing particulates.

Where cleaning cannot sufficiently remove soil or particulates, when airflow is directed through interstitial spaces, or there are high-risk occupants, it is recommended we install one or more air filtration devices (AFDs). Per the current IICRC S500 12.5.3:


  1. Calculate the volume of the room in cu. ft. (L x W x H).
  2. Calculate SCFM, air change (volume ÷ 60mins).
  3. Calculate CFMs’ or Select AFD capacity required (SCFM x ACH).
  4. Area to be treated in cu. ft. and the number of AFDs’. 

Detailed Method

The exact method requires the understanding of several factors related to the structure:

  • Weather
  • Security
  • Public Awareness

Electrical / Heat / Energy Terminology:

  • 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

Real-world example: Use no more than two five-amp air movers per 100 ft., 12-gauge extension cord

Air Movement Conclusion

As remediators, we know airflow is one ingredient in the recipe for properly drying a structure. The number of air movers, their velocity, and additional air movement equipment used to move air into and behind the walls, under floors, and various other cavities are critical for a successful job.



  • The current edition of the IICRC S500 Water Restoration Standard (2021)
  • The current edition of the IICRC S520 Microbial Remediation Standard (2015)
  • Leadership Guide In Restoration


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