Air scrubbers & negative air: how to size containment (ACH)
During mold, sewage, fire or renovation work, HEPA air scrubbers and negative-air machines keep contaminants out of the rest of the home. They are sized by air changes per hour (ACH). Here is the identity, the formula, and a worked example.
The short answer
Size an air scrubber from the room volume and a target number of air changes per hour. For a 15 × 20 × 9 ft room (2,700 cu ft) at 4 ACH, you need 180 CFM of filtration — one typical ~500 CFM scrubber covers it. Size your own space in the air scrubber & negative-air CFM calculator.
The identity and the formula
Airflow, room size and air changes are linked by one identity:
ACH = CFM × 60 ÷ room volume (cu ft)
Rearranged to find the airflow you need:
required CFM = ACH target × room volume ÷ 60
scrubbers = ceil(required CFM ÷ scrubber CFM)
The 60 converts between per-hour and per-minute (CFM is cubic feet per minute). Containment and IAQ work usually targets 4–6 ACH. These are labeled planning conventions; a certified technician confirms the plan on site.
Worked example
A room 15 ft × 20 ft × 9 ft has a volume of 15 × 20 × 9 = 2,700 cu ft. At a 4 ACH target:
required CFM = 4 × 2,700 ÷ 60 = 180 CFM.
With ~500 CFM scrubbers, units = ceil(180 ÷ 500) = 1. Push to 6 ACH for tougher containment and you need 270 CFM — still one scrubber, with headroom. The air changes & CFM table lists common room volumes and targets.
Air scrubber vs. negative air
The same HEPA machine does two jobs depending on how it is ducted. As an air scrubber, it recirculates and filters the room air to lower the particle count. As a negative-air machine, its exhaust is ducted outside the containment, so the sealed work area is kept at slightly lower pressure than the rest of the home — air flows into the containment, never out, so spores or dust cannot escape to clean areas. Negative pressure is what makes mold, asbestos and sewage containment actually contain.
Why ACH, not just “a fan”
Filtration only works if you move enough of the room’s air through the HEPA filter often enough. Expressing it as air changes per hour scales the requirement to the room: a big room at the same ACH needs proportionally more CFM. Under-size it and the particle count never drops; over-size it and you waste equipment. The ACH target reflects the risk — higher for active mold or asbestos, lower for general IAQ polishing.
Where it fits in a restoration
- Mold — negative-air containment during removal; pair with the mold containment cost calculator.
- Sewage and biohazard — scrubbing and containment while contaminated material is removed.
- Fire and smoke — scrubbing airborne soot and odor particles.
- Post-remediation IAQ — final air polishing before clearance; see the indoor air quality cost calculator.
- Drying — scrubbers complement air movers and dehumidifiers; see how many air movers and dehumidifiers.
Reading the ACH target off the risk
The right ACH is a judgment about how aggressively the air must be cleaned. General indoor-air-quality polishing after a minor event might sit at the low end around 4 ACH; active mold or asbestos containment, where escaping particles are the whole concern, pushes toward 6 ACH or higher. Higher ACH means more CFM, which means either a larger scrubber or more units. Setting the target too low leaves the particle count stubbornly high; setting it needlessly high just burns rental days. Matching it to the risk is the judgment a certified technician brings.
Filters, ducting and make-up air
A scrubber is only as good as its filter train and how it is set up. True HEPA filtration captures the fine particles that matter, usually behind a pre-filter that catches the coarse dust so the HEPA lasts. For negative pressure, the exhaust is ducted outside the containment, and you have to allow controlled make-up air to enter, or the machine simply starves and the pressure differential collapses. A single small gap in the containment barrier can undo the whole negative-air setup — sealing matters as much as CFM.
Scrubbing is not drying
It is worth being clear about the division of labor. Air scrubbers and negative-air machines manage airborne particles and pressure; air movers and dehumidifiers manage moisture (see how many air movers and dehumidifiers). A containment often runs both at once — scrubbers keeping spores out of clean areas while the drying equipment pulls water from the structure. Sizing them is two separate calculations, which is exactly why this hub keeps a separate tool for each.
Documenting containment for clearance
On mold, asbestos and other regulated jobs, the containment is not just set up — it is documented, because clearance depends on proving it worked. Technicians record the scrubber CFM and the number of units, verify negative pressure (often with a manometer reading the pressure difference across the barrier), and note the target ACH for the space. That paper trail is what lets an independent clearance test at the end mean something: it shows the air was actually being turned over at the planned rate while contaminated material was disturbed. It also protects you, by making it clear whether the plan matched the risk. When you size a job in the air scrubber CFM calculator, think of the number as the start of that record, not the end — the certified technician confirms it on site, adjusts for real conditions, and signs off only when the readings and the clearance test agree. Under-sizing to save a rental day is a false economy here, because a failed clearance means redoing the containment and the test.
Bottom line
Multiply the room dimensions for the volume, pick an ACH target for the risk, divide by 60 for the CFM, and round up to the number of scrubbers. The calculator does it instantly and shows the identity. These are planning typicals; a certified technician confirms the containment plan on site.