Mould fogging, sometimes called dry fogging, cold fogging, or antimicrobial fogging, is a treatment method that uses a mechanical fogger or ULV (ultra-low volume) machine to disperse a biocidal solution as fine airborne droplets throughout a space. It is one of the tools in a professional mould remediator’s kit, but it is often misunderstood: fogging is not a standalone mould treatment in the same way that physical cleaning is, and it does not eliminate the need to address moisture sources. This guide explains exactly what fogging does, what it does not do, and when it is the right choice for an Illawarra home. For full fogging service details, see our mould fogging and antimicrobial treatment service.
Quick Answer (BLUF)
Mould fogging uses a fine aerosolised biocide to reach surfaces that physical cleaning cannot access, inside wall cavities, behind furniture, in HVAC ducting, and in roof voids. It is most effective as a supplementary treatment after physical remediation, not as a standalone fix. It significantly reduces airborne and surface spore counts throughout a space. It does not fix moisture problems or physically remove mould colonies.
What Fogging Does
A fogging machine generates droplets in the 1-50 micron range, fine enough to remain suspended in air for several minutes and to settle onto all surfaces in a room, including vertical surfaces, ceiling faces, and the inside of enclosed spaces that receive air from the treated room.
The biocidal agent in the fog, typically a quaternary ammonium compound (quat), hydrogen peroxide (H2O2), or chlorine dioxide, comes into contact with mould spores and mould cell walls, disrupting cell membrane function and killing or inactivating the spores and surface mould colonies it reaches.
What fogging achieves:
- Dramatically reduces airborne spore counts in treated spaces, studies show reductions of 90-99%+ in spore counts immediately post-fogging
- Deposits antimicrobial agent on all surfaces the fog contacts, including soft furnishings, wall surfaces, ceiling cavities, and HVAC registers
- Treats areas that physical cleaning would require destructive access to reach (e.g., inside stud wall cavities via the register or ceiling void)
- Provides a residual antimicrobial coating on surfaces, inhibiting re-establishment of surface mould for some period after treatment
What fogging does not achieve:
- It does not physically remove mould colonies, the dead spores and mould bodies remain on surfaces until physically cleaned or disturbed
- It does not penetrate deeply into porous materials, mould growing within the body of plasterboard, timber, or masonry may not be reached by the fog
- It does not address the moisture source, fogging in a room that is still damp will be followed by mould regrowth
- It is not a substitute for physical remediation of heavily contaminated surfaces
Types of Fogging Systems
Thermal foggers: Generate aerosol by heating the solution. Less common in indoor mould remediation due to heat and the need to use solvent-based formulations.
Cold (ULV) foggers: Use mechanical atomisation to generate fine droplets at ambient temperature. The most common system used in residential mould treatment in Australia. Allow use of water-based biocidal formulations.
Dry fog systems: Proprietary systems (e.g., Dry-Fog) that use high-pressure air to generate extremely fine droplets (1-5 microns) that behave more like gas than liquid, they do not wet surfaces measurably. Marketed for use inside electronics, vehicle interiors, and sensitive contents. Premium cost.
For most Illawarra residential jobs, a high-quality ULV cold fogger with a quaternary ammonium or hydrogen peroxide formulation is appropriate.
When Is Fogging the Right Choice?
Appropriate uses of fogging in mould remediation:
1. As a final step after physical remediation Once physical cleaning, HEPA vacuuming, and surface treatment are complete, a whole-room or whole-building fogging treatment reduces residual airborne spore counts to baseline and provides surface antimicrobial coverage on areas that may have received minor spore deposition from the remediation activity itself.
2. When access to contaminated areas is not practical Wall cavities, ceiling voids, and roof spaces that are difficult or expensive to access for physical treatment can receive supplementary treatment via fogging, either through existing ventilation openings or by making small access holes that are later sealed.
3. For HVAC systems Fogging into supply air registers treats ductwork and coil surfaces that would otherwise require complete dismantling. This is used in addition to, not instead of, coil cleaning for heavily contaminated systems.
4. For soft furnishings and contents Fabric, soft furnishings, clothing, and books can be treated with a fogging session that would be impractical to clean surface-by-surface. This is a common component of post-flood and post-remediation content treatment.
5. For preventive treatment in coastal Illawarra properties Some Illawarra homeowners with persistent mild mould issues (particularly in holiday homes, caravan parks in Kiama, and beachside rentals) use periodic fogging as a preventive maintenance measure, reducing the ambient spore count before it develops into an active mould problem.
When fogging is not appropriate:
- As a sole treatment for visible active mould colonies, the mould bodies must be physically cleaned first
- When the moisture source has not been addressed, the mould will re-establish within weeks
- As a substitute for waterproofing, ventilation improvement, or structural drying
Safety Considerations
During fogging, the building must be vacated by all occupants and pets. Fogging agents, even hydrogen peroxide-based formulations, should not be inhaled. The dwell time after fogging before re-entry is typically 1-4 hours depending on the product and ventilation, but your remediator will specify the exact re-entry time.
Food, food prep surfaces, and aquariums should be covered or removed from the treatment area. Smoke detectors may need to be temporarily covered during fogging and uncovered before re-entry.
FAQs
How long does a fogging treatment take?
The fogging itself takes 20-60 minutes for a typical residential home, depending on size and the number of zones. Add dwell time (1-4 hours) and ventilation time before re-entry. Most treatments are complete and the home is accessible within 4-6 hours of the job starting.
How often should fogging be done as a preventive measure?
For a coastal Illawarra home with persistent mild mould pressure, an annual fogging treatment (typically entering winter when humidity is rising) can be a cost-effective preventive measure. For holiday homes that are unoccupied for extended periods, treating before the wet season is worthwhile.
Is fogging effective in the subfloor?
Yes, fogging can be directed into subfloor spaces via ventilation openings. However, access for physical inspection and HEPA vacuuming of the subfloor is still recommended before fogging in heavily contaminated subfloor spaces. See our subfloor and roof void mould service.
Will fogging get rid of the mould smell?
Fogging significantly reduces airborne spore counts and kills surface mould, both of which contribute to the musty smell. However, if the odour is coming from mould deep within materials (plasterboard, timber, carpet underlay), fogging alone may not fully resolve it, the affected materials need to be replaced or physically treated.