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

Sized at the 8 ACH standard-remediation target from the table above, so volume = CFM x 60 / 8. Re-run the division if you are working to a different ACH: 6 ACH covers a third more volume, 12 ACH a third less.

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: 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: 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: 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.