Guide

Containment, Negative Air and HEPA Scrubbers

The principle: make the air go the wrong way on purpose

Demolition liberates material. Cutting drywall with growth on the back of it, pulling wet insulation, and cleaning framing all put particulate into the air of the work area. The goal of containment is that none of that reaches the rest of the house.

You cannot achieve that with a seal alone, because no seal in a real building is perfect and someone has to walk through it repeatedly. What you can do is make every imperfection leak in the right direction. Hold the work area at lower pressure than the house, and every gap becomes an inlet rather than an outlet.

What proper containment looks like

• Sheeting sealed to walls, floor and ceiling with tape, not draped or stapled loosely
• A negative air machine running, with a duct or hose exhausting outside through a window or door
• Visible inward deflection of the sheeting, which is the pressure difference made visible
• HVAC shut down, with supplies and returns in the work area sealed off
• A zippered entry, or better a small airlock chamber, rather than a flap
• Floor protection along the whole route from the work area to the exit
• Debris bagged inside containment and sealed before it leaves
• Equipment left running continuously rather than switched off at the end of the day

The sheeting deflection is the easiest thing for a homeowner to check. Sheeting hanging slack means either the machine is off or the seal is so leaky that it cannot build a difference. Sheeting sucked inward means the system is doing its job.

Filtration, plainly

A HEPA filter is defined by its efficiency at a particular particle size, and it is a meaningful specification rather than a marketing word. Machines run a staged setup: a coarse pre-filter to catch the large debris, often a secondary filter, then the HEPA element. The pre-filters exist to stop the expensive element clogging in a day.

Two things are worth knowing. HEPA filters particulate, including spores and fragments, but it does not remove gases, so the musty smell can persist while the air is genuinely being cleaned. And a filter only works if the air actually goes through it, which is why a machine sitting in the corner of an unsealed room is doing very little.

Scrubber versus negative air

	Air scrubber mode	Negative air mode
Air path	Recirculates within the space	Exhausts outside through a duct
Filters the air	Yes	Yes
Creates a pressure difference	No	Yes
Contains the work area	No	Yes
Typical use	Cleanup after work, or where exhausting is impossible	During active removal

During removal you want negative air. Scrubbing without containment cleans the air in a room that is still connected to the rest of the house.

Why the HVAC gets shut down

An air handler is a machine designed to take air from one part of a house and deliver it to all the others. During demolition that is precisely what you do not want. The returns in the work area get sealed and the system gets shut off, and on a longer job the filters get changed at the end.

In extreme weather this is a genuine inconvenience and it is worth discussing when the schedule is set, because it can be the deciding factor in whether a household stays. See staying home during remediation.

Containment theatre, and how to spot it

• Sheeting hung across a doorway with no machine running behind it
• A machine running but recirculating, with no exhaust duct going anywhere
• Sheeting taped only at the top and hanging loose at the sides and floor
• HVAC still running during demolition
• Debris carried through the house in open bags or a wheelbarrow
• Equipment switched off overnight because of the noise
• Fogging or spraying presented as a substitute for removal

That last one is worth naming. A treatment applied to a surface is not remediation. Remediation is physical removal of the growth and the material it is on. A coating or an antimicrobial applied over uncleaned or damp material seals the problem in.

Proportionality

Not every job needs full containment. Surface growth on a bathroom tile is a cleaning task, and the EPA's own rough threshold for a homeowner handling mold themselves is around 10 square feet. A contractor proposing full negative-air containment for a patch of shower grout is overselling as surely as one proposing a plastic curtain for a gutted basement is underselling.

The scope should say which is being used and why. See our process and choosing a contractor.

Common questions

Is plastic sheeting enough on its own?

No. Sheeting without negative air is a barrier that leaks in both directions, and every time somebody opens it, air carrying material moves out into the house. The pressure difference is the mechanism, not the plastic.

What is the difference between an air scrubber and a negative air machine?

Physically they are often the same device. Used as an air scrubber it recirculates and filters air within the space. Used as a negative air machine it exhausts filtered air outside, which both filters and depressurises. The job usually calls for the second.

Why does the HVAC have to be off?

Because a running air handler pulls air from the work area through the return and distributes it to every room in the house. It is the most efficient way possible to turn a one-room problem into a whole-house one.