Negative-air containment
Also called: containment, negative pressure containment, critical barrier, mold containment, negative air
Sealing a mold or asbestos work area in plastic and exhausting air out of it through a HEPA filter, so the pressure inside is lower than the rest of the house and air leaks in, not out. It keeps spores released during removal from spreading through the home. Standard above about 10 sq ft.
Removing moldy drywall releases spores and fragments at concentrations far above normal indoor air. Without a barrier, they ride air currents to every room and into the return ducts; the remediation of one wall becomes a contamination of the house. Containment is the fix, and negative pressure is what makes containment work — a plastic wall alone leaks at every seam, but if air is being pulled out of the work area faster than it leaks in, every leak flows inward.
A professional setup: 6-mil polyethylene sheeting sealed to floor, walls and ceiling with tape and spring poles, a zippered entry (often a two-stage decontamination chamber on larger jobs), all HVAC supplies and returns in the area sealed, and a negative air machine — a HEPA-filtered fan — ducted to the outdoors, sized to change the air in the containment several times an hour and to hold a measurable negative pressure (a manometer on the wall confirms it). Waste is bagged and wiped down inside before it leaves; workers suit up and down at the door.
A homeowner version for a wall section: the same plastic and tape, a box fan sealed into a window blowing out, HVAC registers taped over, and a HEPA vacuum inside for cleanup. It’s what the IICRC S520 standard describes for small “condition 3” areas, scaled down. The air scrubber is the other half of the equipment — recirculating filtered air inside the containment while the negative air machine exhausts it.