Step By Step

Mold Remediation Process: IICRC S520 Steps, Negative Air and Clearance

Updated October 20, 2026

This is the general procedure described in the ANSI/IICRC S520 standard (current edition S520-2024), the industry reference for professional mold remediation. It exists because mold that is simply wiped off a visible surface, without fixing the moisture source or containing the disturbed spores, reliably comes back. Fixing the water comes before stage one and the rebuild comes after stage six; the six stages are the mold work in between.

1. Inspection and classification

Nothing useful happens before the moisture source is explained. That means a moisture meter rather than a guess, readings across the affected wall, floor or framing compared against a known-dry reading elsewhere in the building, and being willing to open an inspection hole where the smell says there is growth behind a finished surface. That map decides where demolition stops and is the baseline for the drying endpoint.

S520 then sorts the area into three conditions. Condition 1 is a normal fungal ecology, comparable to a clean indoor space. Condition 2 is an area contaminated by spores or fragments from a Condition 3 source. Condition 3 is actual growth, visible or hidden. The goal of remediation is to return the structure and contents to Condition 1, and the classification sets the scope: a small Condition 2 area behind a leaking dishwasher does not need the same containment as a Condition 3 basement wall.

Before anything is cut you should have the scope in writing: the moisture source, what is being removed, what is being cleaned, what containment is being used, how drying will be verified, and what the rebuild does and does not include.

2. Containment and negative air

Physical barriers and negative air pressure isolate the work area from the rest of the building. Disturbing contaminated material without containment is how a problem in one closet becomes an HVAC-wide problem. The HVAC system is normally shut down and affected supplies and returns are sealed. The EPA's remediation guidance, written for schools and commercial buildings and widely used for homes, describes two levels:

The area thresholds are a planning aid, not a rule. The EPA says the remediation manager applies professional judgment, and S520 sets the level by condition and risk, not square footage alone.

The negative air machine

A negative air machine (NAM), also called an air filtration device (AFD), is a fan with a pre-filter, a secondary filter and a HEPA filter, which captures at least 99.97 percent of particles at 0.3 microns. It pulls air out of the containment, through the filters, and exhausts it outdoors through a duct, so the work area sits at a slightly lower pressure than the house and every gap in the poly leaks inward. Sheeting bowed inward means it is working.

Sizing is arithmetic: CFM needed = room volume (cubic feet) x air changes per hour / 60. A 15 by 20 foot basement room with an 8 foot ceiling is 2,400 cubic feet, and at 6 air changes per hour that is 240 CFM. Industry practice commonly targets around 4 to 6 air changes per hour or more, and the rating has to hold with a loaded filter, not a clean one. Many contractors confirm pressure with a manometer, with a commonly used target of about negative 0.02 inches of water column (roughly negative 5 pascals) relative to the adjacent space.

The duct exhausts outdoors, away from windows, doors and air intakes, never into an attic or garage, and the machine runs continuously, including overnight, until final cleaning is finished. An air scrubber that only recirculates air inside the room filters it but creates no pressure difference, so it is useful after the work and no substitute for exhausting outside during removal. The full explanation is in containment, negative air and HEPA scrubbers.

3. Removal

Porous material that has absorbed growth generally cannot be cleaned to Condition 1 and is removed: paper-faced drywall, fiberglass insulation, carpet and pad, ceiling tile, particleboard, some wood. It is cut out with a margin beyond the visible growth, to the first clean, dry material, because on a porous surface the growth extends past what you can see, and it is bagged inside containment before it leaves. Non-porous and semi-porous material such as tile, metal, masonry and solid framing can usually be cleaned and kept.

4. HEPA cleaning

Cleaning follows a repeating pattern: HEPA vacuum to remove loose material, damp wipe or abrade the surface, then HEPA vacuum again. A standard shop vacuum is not a substitute, because its filter passes the smallest, most easily inhaled spores back into the air. Solid framing with surface growth is often cleaned by wire brushing, sanding or media blasting, with the dust captured inside containment.

The EPA does not recommend routine use of biocides, including bleach, although professional judgment may call for one in a specific case. Bleach is mostly water, does not reach below a porous surface, and adds moisture to a space that already has too much. Where an antimicrobial or sealing coating is used, it goes on only after cleaning and drying, and never over growth that was not removed. An encapsulant has a legitimate role on cleaned structural wood that cannot be replaced. Painting or fogging over growth is not remediation.

5. Drying

The area is brought down to a moisture level that will not support new growth before containment comes down. Drying is complete when the material is back to its normal moisture content, not when it feels dry. For wood framing that is generally at or below about 15 percent, compared against a dry reference reading from the same building. Dehumidifiers and air movers run inside containment, and readings are logged with dates. The EPA's rule of thumb for water damage is to dry wet materials within 24 to 48 hours. This is the step most often skipped in DIY attempts, and closing a wall over a damp cavity is why mold reappears weeks or months later.

6. Verification

Post-remediation verification confirms the work achieved Condition 1 and the moisture problem is resolved. S520 separates two steps: the remediator does a post-remediation evaluation of their own work, and an independent indoor environmental professional does the verification. The independence is the point, since the person paid to finish the job should not be the one who certifies it. The check covers visible growth, odor, moisture readings against a dry reference and, where the scope calls for it, air or surface samples compared with an outdoor control. There is no universal numeric pass threshold for indoor spore counts, so the report should say what was compared.

Sampling happens after final cleaning and drying but before containment comes down and before the wall is closed. If the area fails, it is recleaned and retested, and the contract should say before work starts who pays for the retest. A third-party assessment typically adds $300 to $1,000 (see cost), and for a larger or health-sensitive job it is worth asking about if anyone in the home has respiratory sensitivities. What a clearance report contains is in post-remediation verification.

After verification: source repair, teardown and rebuild

The water path is corrected, which may be another trade. Containment is then broken down carefully, with poly folded inward and bagged and surrounding surfaces wiped, and the room is put back. Rebuilding before the source is fixed is the most expensive ordering mistake available, and rebuild work is normally priced separately.

How long it takes

What changes in older Gettysburg houses

The process is the same, but older housing adds work. In Gettysburg borough the median home dates to about 1942 and 87.2 percent of homes were built before 1980 (American Community Survey, 2020 to 2024). In practice:

• Lead paint. Homes built before 1978 may have lead-based paint. Under EPA's Renovation, Repair and Painting Rule (40 CFR Part 745, Subpart E), firms that disturb painted surfaces in pre-1978 housing must be certified and use lead-safe work practices. Demolition for mold counts.
• Asbestos. Older floor tile, pipe wrap, vermiculite insulation and some plaster and joint compound can contain asbestos. Suspect materials should be tested before they are disturbed, which changes the scope.
• Fieldstone and brick walls. Poly does not stick to rubble masonry the way it does to drywall, so containment is often framed with studs or sealed to a prepared strip, and takes longer to set up.
• Low ceilings, tight stairs and plaster and lath limit where the machine and exhaust duct can go, and plaster is heavier and dustier to demolish than drywall.
• Attached homes. About one in six homes in the borough shares a wall with a neighbor, so exhaust location and notice to the neighbor matter.

Our historic homes guide covers why these buildings hold moisture.

Do you have to move out?

Usually not. Containment is what makes it possible to stay, and for a single room in a basement most households carry on normally. The situations worth taking seriously are a large affected area, work in a space that cannot be sealed off from the living area, and a household member with asthma or who is immunocompromised. See staying home during mold remediation, and have the conversation before the job starts, not on day two.

What we will tell you that other people might not

• If the affected area is under about 10 square feet, the EPA's own guidance is that a homeowner can often handle it, and we will say so
• If a lab test will not change what gets done, it is not worth paying for. See testing vs remediation
• If the real fix is a downspout and twenty feet of grading, that is the quote we would rather give you
• If it is efflorescence rather than mold, it is not mold, though it does prove water is moving through that wall
• If the moisture source needs a plumber or a roofer, that is their job and we will say so rather than quoting it

Related: signs of mold, what it costs, keeping it from coming back.

Common questions

What are the steps in the mold remediation process?

Six stages under ANSI/IICRC S520: inspection and classification, containment with negative air, removal of contaminated material, HEPA cleaning, drying, and verification. The water source is fixed first and the rebuild comes last.

Do I need a negative air machine for a small job?

Not always. EPA guidance lists no containment as required for an area under 10 square feet. Once demolition is involved, the area is over roughly 10 square feet, or anyone in the home is sensitive, a HEPA negative air machine exhausting outside is standard practice.

What is the difference between an AFD and a NAM?

Both are HEPA-filtered air filtration devices. Run in scrubber mode, an AFD recirculates and cleans air in the room. Run as a negative air machine (NAM), it is ducted to exhaust outdoors and creates the pressure difference that contains the work area.

What is post-remediation verification?

A check by someone independent of the remediator that the work area is back to Condition 1 under ANSI/IICRC S520: no visible growth, no musty odor, normal moisture readings, and air or surface samples consistent with a normal fungal ecology where sampling is part of the scope.

Who should do the clearance test?

An independent indoor environmental professional, not the contractor who did the work. Their opinion is more useful in a home sale, insurance claim or tenant dispute because they have no stake in the result.

What if the area fails verification?

It is recleaned and retested. The contract should say before work starts who pays for the retest and how long the contractor has to correct it.

Sources

• ANSI/IICRC S520 Standard for Professional Mold Remediation (4th edition, 2024)
• EPA, Mold Remediation in Schools and Commercial Buildings (EPA 402-K-01-001)
• EPA, A Brief Guide to Mold, Moisture and Your Home
• EPA, Renovation, Repair and Painting Rule (40 CFR Part 745, Subpart E)
• U.S. Census Bureau, American Community Survey 2020 to 2024 (Gettysburg borough)