How we made this guide: picks, prices and specs come from our catalog and the makers' published specs, owner quotes come from customer reviews on our product pages and every guide is checked and edited by Trevor Hall.
You create negative pressure by exhausting more air out of a sealed room than leaks back in: seal the room, set a HEPA negative air machine inside, duct its exhaust outdoors, leave a small controlled make-up air opening and confirm with a manometer that the room reads below the hallway. A machine like the Abatement Technologies HEPA-AIRE H2KM, at $1,070, does the exhausting; the manometer is what proves it worked.
Sealing is the slow part of the job, and the machine is usually running within minutes once the duct is out. This guide walks the steps in order with a reading to take at each one. If you only need the idea behind the setup, start with what a negative pressure room is.
Is this about a house or a work room?
Two different jobs share the phrase. If you are asking why a house feels negative, the cause is air leaving faster than it enters: bathroom and kitchen exhaust fans, a clothes dryer and supply ducts that leak into an attic or crawl space all pull air out, and the same volume has to come in through cracks. Fixing that is a ventilation and duct problem, and a manometer reading across the building envelope shows how far off you are.
The rest of this post is about the second job: holding a sealed work room below its surroundings so that air leaks inward and dust or debris from the job does not drift out. That is containment, and it is what a negative air machine is built for.
What you need before you start
- A HEPA negative air machine sized for the room (see the sizing step below).
- Lay-flat or flex ducting long enough to reach outdoors, with a diameter that matches the machine's exhaust collar.
- A manometer to read the difference between the room and the space next to it.
- Poly sheeting, tape and a zip door to seal the room. We do not sell poly, tape or zip doors, so buy those at a building supply store.
- A way to restrict make-up air: the ability to close or cap HVAC supply and return registers in the room.
Negative air machine
Abatement Technologies HEPA-AIRE Series Negative Air Machine H2KM
$1,070
Dual-speed 2,000 CFM unit with a 6 inch HEPA, our best-selling negative air machine.
| Item | What it does | Spec from the listing |
|---|---|---|
| Abatement Technologies HEPA-AIRE H2KM | Pulls room air through a HEPA filter and exhausts it | 2,000 CFM high and 1,300 CFM low, 115 to 120 V on a 15 amp circuit, 124 lb |
| Aerospace America Aeroclean 600EV 9160 | Smaller unit for a small room | 0 to 550 CFM variable speed, 1.67 amps, 32 lb, 8 inch side exhaust |
| Novatek Duct-2-Go ducting | Carries the exhaust outdoors | 10 or 12 inch, 25 or 50 ft, heavy duty vinyl |
| Abatement PPM3-S monitor | Reads the pressure difference | -25 Pa to +25 Pa, visual and audio alarms |
| HEPACART AnteRoom 36 | Entry module at a doorway | 39.5 x 24 x 85.5 inches, 150 lb, 120 V |
The maker's rule for negative pressure
Abatement Technologies states the rule in the manual for its PREDATOR air scrubbers. For negative pressure, the HEPA-filtered air exhausted from the area has to exceed the air entering by 10 percent or 100 CFM, whichever is greater. To get there, the air that can enter has to be restricted, which can mean reducing the HVAC supply to the area and sealing other openings.
The manual describes two ways to set it up. You can put the unit inside the area and push air out through flex duct attached at the outlet collar, exhausting outdoors or to another location within the building according to regulations. Or you can put the unit outside the area and pull air out through flex duct attached between the inlet collar and the area. The steps below use the first setup, because it keeps the machine's filters inside the work area.
Step 1: Take a baseline reading
Before you touch anything, read the pressure difference between the room and the hallway with the doors closed. Write it down with the time. A room may already read slightly positive or negative because of the HVAC, and you want to know that number before you change the room.
With the Abatement PPM3-S portable pressure monitor it shows the reading on a color touch screen. Its listed range is -25 Pa to +25 Pa, which is -0.1 to +0.1 inches. For a longer look at how these instruments work, see our post on room pressure monitors.
Pressure monitor
Abatement Technologies Portable Differential Pressure Monitor PPM3-S
$1,400
Reads the room against the hallway with visual and audio alarms and CSV logs over USB.
Step 2: Seal the room
Close the doors, windows and any openings you do not need. Cover the ones you cannot close with poly sheeting taped on every edge. Put one door or a zip door in the wall you will enter through and treat it as the only way in. Cap or block supply and return registers inside the room, because the maker's rule above depends on restricting the air that can enter.
Do not over-seal. The room needs one intended place for make-up air to come in, which you set up in step 6. A room with no path for air at all pulls the poly hard inward, makes doors hard to open and starves the machine of airflow.
Step 3: Size the machine and place it
Size by air changes per hour. Abatement's PREDATOR air scrubber manual gives the formula: the number of units is the room volume times the design air changes per hour, divided by the unit's rated CFM times 60, rounded up to the next whole number. Its example uses a 20 by 30 by 12 foot area and 6 air changes per hour as a general target for construction areas when none is specified, then adds a safety factor for filter loading and duct losses. OSHA's asbestos standard sets at least 4 air changes per hour in a Class I negative pressure enclosure, and the manual notes most users target at least 6 when none is specified.
Here is a small worked example, our calculation from the H2KM's rated figure. A 12 by 15 foot room with an 8 foot ceiling holds 12 x 15 x 8 = 1,440 cubic feet. At the H2KM's rated 1,300 CFM on low, air changes per hour = 1,300 x 60 / 1,440 = about 54. That is the rated figure; real airflow is lower with a duct run and a loaded filter, which is why a single unit has plenty of margin in a room this size and why you run it on low. For a room this small, the Aeroclean 600EV 9160 at 550 CFM gives about 23 air changes per hour by the same formula (550 x 60 / 1,440) and is easier to hold at a gentle reading. For the full method, see our post on calculating air changes per hour.
Place the machine inside the room, near the far end from the entry, so incoming air sweeps across the work area on its way to the machine. Keep it a few feet from walls so the inlet is not starved. Check the amp draw on the listing before you plug in: the H2KM is listed for a 15 amp circuit, and a second tool on the same circuit can trip it.
Step 4: Duct the exhaust outdoors
Attach the ducting to the exhaust collar and run it to a window, a vented opening or a door cover panel that leaves the building. Keep the run short and straight. Long runs, sharp bends and kinks all cost airflow. Abatement's PREDATOR manual lists the ways to keep losses down: use the shortest run with the fewest turns, use sweeping radiused bends, keep flexible duct taut so it does not flatten and size any louver or damper to the full area of the duct. Support the duct so it cannot sag or fold, and seal the opening around it with poly and tape.
Match the duct diameter to the machine's collar from the maker's sheet. The Novatek Duct-2-Go comes in 10 or 12 inch diameters and 25 or 50 foot lengths, priced from $212.75. Its listing says the 10 inch size fits the Novair 700, Novair 1000 and Novair 1000 Variable Speed, and the 12 inch size fits the Novair 2000. Check the H2KM's or 600EV's collar size (the 600EV has an 8 inch side exhaust) before ordering, because an adapter may be needed.
Ducting
Novatek Duct-2-Go Ducting with Carry Case
From $212.75
Safety orange vinyl ducting that packs into its own carry case.
Step 5: Start low and step up
Turn the machine on at its lowest speed with the filters installed. Abatement's PREDATOR air scrubber manual says not to operate the unit unless the pre-filters and HEPA filter are installed and the access door and panel are closed; check your own machine's manual for the same rule. Watch the poly: it should pull slightly inward and the zip door should flutter inward. Take a manometer reading after the first few minutes.
If the reading is not yet negative, go to the next speed. The H2KM has a low and a high setting, and the Aeroclean 600EV 9160 has a dial from 0 to 550 CFM on the standard model, so a small room can be tuned finely. Stop when the reading holds, not when the machine is at maximum.
Small room unit
Aerospace America Aeroclean 600EV 9160 Variable Speed HEPA Air Scrubber
From $880.60
Variable speed up to 550 CFM at 1.67 amps in a 32 lb cabinet, with an 8 inch side exhaust.
Step 6: Adjust make-up air
Negative pressure only exists when some air can still come in. If the room reads too negative, the poly is straining or the door is hard to open, allow a little more make-up air: open a controlled slot or a filtered opening on the side away from the machine. If the room is not negative enough, the cause is usually leaks: look for gaps at the ceiling, outlets and the door seal before you add more fan speed.
Use the manometer to guide each adjustment. Change one thing at a time, wait a minute and read again. This is the part of negative air machine setup most people skip, and it is what separates a room that looks sealed from one that reads negative.
Step 7: Read, log and set the target
Targets come from the rules that apply to your job, not from the machine. One published figure comes from OSHA's asbestos construction standard, 29 CFR 1926.1101: for Class I asbestos work in a negative pressure enclosure it requires a minimum of -0.02 column inches of water pressure differential, relative to outside pressure, shown by manometric measurements (source: osha.gov). In pascals that is about -5 Pa (our conversion). OSHA states that figure for asbestos work; other jobs have their own requirements, and we give no asbestos or lead abatement instructions here.
Whatever the target, compare it to a calibrated reading and keep a record. The PPM3-S listing says it archives logs in CSV format, with a USB port for downloads. It has visual and audio alarms. The reading should stay steady across the shift. For how containment fits each job, see our mold remediation process and asbestos abatement process posts.
| Reading | Pascals | Inches of water column |
|---|---|---|
| OSHA minimum differential for an asbestos negative pressure enclosure | about -5 | -0.02 |
| PPM3-S listed range, negative end | -25 | -0.1 |
| PPM3-S listed range, positive end | +25 | +0.1 |
The pascal figures are our conversion from inches of water column.
What to do when the reading drifts
- Reading rises toward zero. Check for a loose door seal, a new gap in the poly, a register someone reopened or a loaded filter.
- Reading swings. A door being opened is the usual cause. An anteroom at the entry keeps the room from losing its pressure every time someone walks in.
- Duct flattens or kinks. Pull slack flexible duct taut and replace sharp turns with sweeping bends; Abatement's PREDATOR manual says flexible duct must be kept as taut as possible to avoid flattening.
- Alarm sounds. Stop, find out why and read the manometer before you continue.
An anteroom at the door
Every time someone opens the entry, the room loses some of its negative pressure. A transition space helps. The HEPACART AnteRoom 36 is a mobile isolation module that fits recessed 36 inch doors, with a Magnehelic gauge mounted internally and a flex-duct adapter that connects to a negative air machine. HEPACART publishes it for ICRA levels I to V alongside a separate negative air machine. Prices run from $6,429 with the standard handle, with the door hardware options priced apart. It suits occupied buildings and healthcare spaces, covered in our ICRA guide.
Entry module
AnteRoom 36 Mobile Isolation Anteroom
From $6,429
Mobile anteroom for 36 inch doors that ducts to a separate negative air machine.
Negative air machine setup versus a plain air scrubber
The same box can be run two ways. As a recirculating air scrubber it cleans room air and returns it to the room; ducted outdoors with the room sealed, it becomes a negative air machine. Our post on air scrubber versus negative air machine goes through the difference. For HEPA ratings, what a HEPA filter is explains what the 99.97 percent at 0.3 micron figure means as the maker rates it.
The machine does not clear a job on its own; it holds air direction while the work happens. Dust control on a renovation follows the same pattern, covered in dust control in construction. Our mold remediation equipment list shows where negative air fits in a full kit.
Where to find the equipment
Browse all negative air machines, the wider range of HEPA air scrubbers, the mobile containment line or everything from Abatement Technologies. A basic ducted rig of one machine, ducting and a monitor comes to under $3,000 at today's prices, before poly and tape.
Common mistakes
- Skipping the baseline reading, so there is no proof of what changed.
- Leaving HVAC supply open into the room, which feeds air the machine then has to beat.
- Running at full speed with no make-up air path, which bows the poly inward and starves the machine.
- Exhausting into a space that is not allowed. Abatement's PREDATOR air scrubber manual says to exhaust according to regulations.
- Relying on rated CFM and ignoring duct and filter losses.
- Taking one reading at the start and never checking again.
Frequently asked questions
What is the point of a negative pressure room?
It keeps air flowing into the room instead of out. Any leak becomes an inflow of outside air rather than an outflow of room air, as Abatement's manual describes it. That is why the setup is used when the work area must not leak into the rest of the building.
What is positive air pressure in a room?
It is the opposite: the room sits above its surroundings, so leaks flow outward. Abatement's manual describes a positive pressure mode where the filtered air supplied exceeds the air exhausted by 10 percent or 100 CFM, whichever is greater.
How do I know if my house has negative air pressure?
Measure it. A manometer reading between inside and outside, or between rooms, gives a number in pascals or inches of water column. Exhaust fans, dryers and supply ducts that leak into an attic or crawl space are the usual contributors, so check the readings with those running and with them off.
How much pressure is in a negative pressure room?
It depends on the rule that applies. OSHA's asbestos construction standard, 29 CFR 1926.1101, sets a minimum differential of -0.02 column inches of water for a negative pressure enclosure for Class I asbestos work, about -5 Pa by our conversion. Other settings set their own targets, so check the requirement for your job first.
Can a negative air machine alone make the room negative?
No. The room has to be sealed enough that the machine can pull a difference, and make-up air has to be restricted and controlled. The manometer, not the machine's speed setting, tells you whether it worked.
Does one machine do both negative and positive pressure?
Abatement's PREDATOR manual lists negative pressure, recirculation and positive pressure as modes of operation for its air filtration units, set by where the unit sits and which collar the flex duct attaches to. Check your own machine's manual before you plan to switch modes.
Hold the room negative
Related guides
- What Is a Negative Pressure Room? How It Works, Pressure Targets and Monitoring
- Room Pressure Monitors for Containment: How Differential Pressure Monitoring Works
- Air Scrubber vs Negative Air Machine: Same Box, Different Setup (and When You Need Each)
- Air Changes per Hour Formula: How to Calculate ACH and Size an Air Scrubber
