HEPA Filtration Sizing for Remediation

Why this matters

HEPA filtration is the engine of mold remediation: it removes airborne spores from the work area and maintains negative pressure within containment. The right size air scrubber for the work area produces effective filtration and proper negative pressure; an undersized one fails to control spores; an oversized one wastes money. Knowing the math (CFM, air changes, room volume) lets the contractor size appropriately for each project.

What HEPA means

HEPA (High-Efficiency Particulate Air) is a standard for filtration:

• 99.97 percent of particles 0.3 micron and larger captured
• Larger and smaller particles also captured (HEPA's worst case is the 0.3 micron range)
• The standard used for medical, industrial, and remediation work

For mold remediation:

• Mold spores typically 1 to 10 microns
• Well above HEPA's worst case
• HEPA effectively captures mold spores

Air scrubber types

Standalone HEPA air scrubber

• Self-contained unit
• HEPA filter inside
• Fan motor
• Inlet and outlet
• Common brands: Phoenix, Drieaz, Drymatic, BlueDri

Portable negative air machine

• Air scrubber configured for negative pressure
• Inlet inside containment
• Exhaust to outside (typical) or to clean room
• Pulls air through containment

Stationary HVAC-mounted HEPA

• Permanent installation in HVAC system
• Standard for high-end commercial buildings
• Not typical for residential remediation

Sizing methodology

Calculate room volume

Volume (cubic ft) = length × width × height

Example: 12 × 12 × 8 ft room = 1,152 cubic ft

Determine target air changes per hour (ACH)

For different contamination levels:

Calculate required CFM

CFM (cubic feet per minute) = (volume × ACH) / 60

Example: 1,152 cubic ft × 8 ACH / 60 = 154 CFM

So an air scrubber providing 200 CFM or more is appropriate for this room.

Common air scrubber sizes

For typical residential remediation (one or two rooms), 1,000 to 2,000 CFM air scrubber.

Air scrubber placement

For containment:

• Place inside the containment
• Inlet (filter side) facing the contaminated area
• Exhaust through wall or window to outside (preferred)
• Exhaust to clean air zone (acceptable if outside not feasible)

For non-contained cleaning:

• Place in the room being cleaned
• Allow air to circulate naturally
• Run during and after cleaning

Negative pressure considerations

For containment, the air scrubber:

• Pulls air through containment
• Discharges outside (or to clean zone)
• Creates negative pressure inside containment

Negative pressure verification:

• Use manometer at the critical barrier
• Target: -0.02 to -0.05 inches water column negative
• More negative = air more strongly pulled into containment from clean zones
• Positive pressure = air pushing out of containment (BAD; spreads spores)

Adjusting negative pressure

• Air scrubber speed
• Containment seal quality
• Make-up air opening (sometimes a small opening allows fresh air)

Filtration capability

A HEPA air scrubber's filter:

• HEPA filter: 99.97 percent at 0.3 micron
• Pre-filter (typical): captures larger particles; extends HEPA life
• Carbon filter (some): captures odors

Filter life:

• Pre-filter: replace every 1 to 5 days during heavy use
• HEPA filter: replace every 200 to 1,000 hours
• Replacement cost: a modest per HEPA filter

Verify filter condition during work; replace as needed.

Air sampling for verification

After remediation:

Post-remediation verification (PRV)

• Air sample from inside the cleaned area
• Compared to outside air sample (control)
• Both samples sent to lab
• Lab counts spores per cubic meter

Target for clearance:

• Indoor spore count similar to or lower than outdoor control
• Indoor spore count not exceeding industry benchmark
• No specific target species above threshold

Sampling cost: a moderate per pair of samples (indoor + outdoor).

Capacity calculation worksheet

For each remediation project:

1. Measure containment dimensions
2. Calculate cubic volume
3. Choose target ACH (typically 6 to 8 for typical work)
4. Calculate CFM needed: (volume × ACH) / 60
5. Choose air scrubber that meets or exceeds this CFM
6. Verify the air scrubber will achieve negative pressure
7. Plan exhaust location

Multiple air scrubbers

For large containment:

• Multiple smaller scrubbers may be more effective than one large
• Better air distribution
• Easier to maintain
• Redundancy if one fails

Calculate combined CFM matches the target.

Practical considerations

Air scrubber noise

• HEPA scrubbers are loud (typical 70 to 85 decibels)
• Noise affects worker comfort
• Customer's neighbors may complain
• Consider duty cycle

Power consumption

• Typical air scrubber: 700 to 1,500 W
• Operating cost: pennies per hour at typical residential tariffs; multiply machine wattage by the customer's kWh rate
• Multi-day rental can add up

Heat generation

• Air scrubber generates heat
• Containment temperatures rise
• May require additional cooling or ventilation

Sequence of operation

During remediation:

• Set up containment
• Install and start air scrubber
• Verify negative pressure
• Perform remediation
• Continue running air scrubber for 4 to 8 hours post-work
• Test air for PRV
• Stop air scrubber after PRV passes

Rental vs ownership

For occasional remediation:

• Rental: a modest per day
• Multiple scrubbers may be rented per project

For active mold remediation business:

• Owned scrubbers: meaningful per-unit capital cost (varies by CFM, brand, and dealer)
• Multiple owned for typical operation
• Maintenance and filter replacement ongoing

Break-even: typically 20 to 50 days of use per year favors ownership.

Common HEPA sizing errors

Undersized for the containment

• Inadequate air changes
• Negative pressure not maintained
• Worker comfort compromised
• Spores not adequately captured

Oversized for small jobs

• Wasted money on equipment that's unused
• Excessive noise
• Power consumption

Filter not replaced

• Reduced airflow
• Negative pressure drops
• Less effective filtration

References

• IICRC S520 (Standard for Professional Mold Remediation).
• ASHRAE Filter Standards (Minimum Efficiency Reporting Value, MERV; HEPA filters are MERV 17+).
• EPA Guidance on indoor air quality.
• ANSI / ASA Standard on aerosol filtration.
• Manufacturer literature: Phoenix, Drieaz, Drymatic, BlueDri.
• Manuall internal: Mold Remediation Containment Barrier Setup, Mold Remediation IICRC S520 Standards Summary.