How Mould Testing Actually Works

Testing sits at the end of a decision, not the start of one. The tools used in a mould assessment answer very different questions, and a few of them answer almost nothing in isolation. Knowing which is which is what stops people paying for a page of spore counts that tells them nothing they can act on.

The visual inspection does most of the work

An assessment begins with someone walking the building slowly and reading it as a water system. Under sinks and behind cisterns. The base of wardrobes on external walls. Window reveals, the shower seal, skirting boards, the ceiling cavity, the subfloor. What an inspector is looking for is history: tide marks, rippled skirting, paint that has lost adhesion, efflorescence on masonry, a hairline gap in grout that has been quietly weeping for months. If you want the wider picture first, I have written about how damp actually behaves in Brisbane homes.

Housekeeping shows up at this stage more than people expect. Mould is a moisture problem first, but dust is food, and clutter blocks airflow, so a wardrobe pressed flat against a cool external wall creates a pocket of still, cold air behind it. Clutter also hides the early signs, which is how a problem gets months of head start before anyone notices it. Moisture sets the conditions, and housekeeping shortens or lengthens the odds. Some of the corners standard cleaning skips are exactly the ones an inspector heads for first.

The limit of a visual inspection is that it only sees surfaces. Condensation forming inside a steel-framed wall cavity in a recent build produces nothing visible until it is well advanced. Mould is not mainly an old-home issue. New builds simply fail differently.

Moisture meters and what a dry-looking wall is hiding

There are 2 broad types. Pin meters push 2 electrodes into the material and measure electrical resistance between them, giving a reading at pin depth and leaving small holes behind. Non-invasive meters use capacitance or radio frequency to scan roughly 10 to 20mm below a surface without damaging it. In practice a scanner finds the anomaly and pins confirm it.

Both are relative instruments. Most are calibrated to timber, so a reading taken on plasterboard, render or tile is a comparison rather than an absolute moisture content. Foil-backed insulation, metal fixings, mesh in render and salt in old masonry will all throw a high reading with no water present at all. Method matters more than the device: the suspect area is read against a known-dry reference of the same material, in the same room, in the same conditions.

Wet material behind a dry-looking surface is the standard finding, not the exotic one. Paint dries in hours. The plasterboard, insulation and framing behind it can hold water for weeks. Given the right conditions, mould can begin growing in around 48 hours, so the question after a leak or a spill is never whether the surface felt dry the next morning. It is whether the material was dried within hours.

Psychrometrics, in plain English

3 numbers do the heavy lifting: air temperature, relative humidity and dew point.

Relative humidity is how full the air is compared with what it could hold at that temperature. Warm air holds more. So the identical amount of water vapour reads as a comfortable 55 percent at 24 degrees and a dripping 90 percent at 15 degrees. Nothing was added. The air just cooled overnight.

Dew point is the temperature at which that air has to give the water back as liquid. If the air in a room has a dew point of 19 degrees and the back of a wardrobe on an external wall sits at 17, water forms there. That is the entire mechanism behind condensation, and it is why an inspector takes surface temperatures with an infrared thermometer and compares them against the room's dew point. Surfaces sitting below dew point are where growth starts.

Most of the moisture load in a home is generated inside it: cooking, showering, drying laundry indoors, unflued gas heating, and simply breathing. Outdoor humidity matters in Brisbane, but it is not the whole story. A healthy home generally sits somewhere in the range of 40 to 65 percent relative humidity, with any water event dried within hours.

Thermal imaging shows temperature, and only temperature

A thermal camera cannot see moisture. It maps surface temperature. Evaporating water cools a surface, so damp areas often appear as cooler patches, which makes the camera a fast way to scan a large wall for spots worth investigating properly.

It will also happily show missing insulation, a stud behind plasterboard, air leaking around a downlight, and the warm ghost of afternoon sun that hit that wall 20 minutes ago. None of those are water. The camera needs a temperature differential to show anything useful at all, which is why inspections are often scheduled for early morning. An image on its own proves nothing. Anyone presenting a thermal photo as a moisture result has skipped the step that matters.

Sampling: air, tape lifts and the rest

Sampling answers what is present and roughly how much. It does not answer where the water is, which is why it belongs after the inspection rather than instead of it.

• Air sampling: a calibrated pump draws a measured volume of air, commonly 75 to 150 litres, through a spore trap cassette. Everything captured is counted under a microscope, alive or dead, because dormant spores and dead fragments still matter. Whether mould is alive was never the measure of whether it is a problem.
• Tape lifts: clear tape pressed onto a visible surface, lifted, mounted and examined. This is what separates actual growth from soot, dirt or a stain that has been argued about for months.
• Swabs and bulk samples: a swabbed area or a physical piece of material, useful when growth is suspected inside or underneath something.
• Culturable plates: samples grown in a laboratory. Only a minority of what is present will grow, so a culture tells you what was viable, not what was there.

Colour is close to useless as evidence. The same organism produces different colours on different materials in the same room, and some moulds are effectively transparent. Black mould is a colour, not a species. Identification requires a laboratory.

Chain of custody and accredited laboratories

A sample is only as good as the paperwork travelling with it. A chain of custody form records who took the sample, where in the building, at what time, the volume drawn, the conditions at the time, who received it and when it was analysed. A break anywhere in that chain makes the result unusable in a dispute, and disputes with builders, landlords and insurers are one of the main reasons people pay for sampling at all. In Australia, laboratory competence is signalled by NATA accreditation. Most reports identify to genus rather than species, and species-level identification rarely changes what needs to be done about the building.

Why an indoor result without an outdoor control is not a result

Fungal spores are outside right now, in enormous numbers, and always have been. Normal indoor air is essentially a filtered, slightly diluted version of the outdoor air. An indoor count in absolute terms therefore carries very little information.

The comparison is what carries it. A control sample is taken outdoors on the same day, in the same weather, with the same equipment, and the indoor results are read against it. What raises a flag is an indoor genus present at a materially higher concentration than outdoors, or one that barely appears outdoors at all. That pattern suggests amplification, meaning something is growing inside. Water-damage-associated genera such as Chaetomium and Stachybotrys carry particular weight, because they need sustained wetting rather than ordinary humidity.

Australia has no legal indoor mould exposure limit to test against. Interpretation is a judgement built from the outdoor comparison, published guidance and the physical evidence in the building. A spore count with no building story attached is trivia.

What a good report lets you do

The point of paying for an assessment is to be able to make a decision. A report should tell you where water is entering or forming, which materials are affected, how far the growth extends, what can be cleaned and what has to come out, and what needs fixing so the same thing does not repeat next summer. If it lists spore counts and stops there, the useful half is missing.

When testing is worth the money, and when it is not

Visible growth on a hard, non-porous surface you can reach does not need a laboratory. It needs removing. Killing mould is the wrong goal in any case, because dead growth still matters. Physical removal is the objective, and on non-porous surfaces a cloth, a suitable detergent and genuine effort will do it. Porous materials are a different question, because growth extends below the surface, and how far it has gone determines whether the material can be saved or has to come out.

Bleach deserves its own sentence. It removes the colour while leaving the growth in place on anything porous, and it is mostly water, so it adds moisture to the exact spot you are trying to dry out. Poor choice, well marketed.

• Reasonable reasons to pay for an assessment: buying a property, a dispute with a landlord, builder or insurer, recurring growth with no obvious source, clearance after a flood or a significant leak, or suspected condensation inside a wall cavity in a newer home where nothing is visible.
• Poor reasons: confirming what you can already see with your own eyes, or using a report as a substitute for fixing the water.

Worth saying plainly: mould does not have to be visible and it does not have to smell. Most significant problems have no odour at all, and nose blindness sets in within days of living with one that does. No smell is not reassurance.

Where I fit, and where I do not

I am not an assessor and I am not a remediator. I do not take moisture readings, I do not take samples, and I do not carry out mould assessments. Full remediation is specialist work involving containment, negative pressure and clearance testing, and it should stay with the people equipped for it. What I do is clean surface mould where it is safe to do so as part of a clean, and during a Home Health Review I walk the whole home and flag what I can see: the wardrobe hard against the cool external wall, the laundry drying in a closed bedroom, the bathroom fan venting straight into the ceiling cavity, the dehumidifier nobody has cleaned since the day it was unboxed. That last one matters more than people think, because a neglected dehumidifier becomes a moisture and microbial problem in its own right.

I am currently studying mould and moisture assessment (unit NAT11429003), which is why I can explain how the field works in this much detail. Studying is not certification, and I do not offer testing. If your home needs an assessment, it needs an assessor.

The 3 things mould requires are moisture, food and time. None of them can be eliminated. All 3 can be influenced: less dust, less clutter, fewer surplus soft furnishings sitting in cold corners, and above all a much shorter gap between something getting wet and being properly dry. Most of what protects a house is unglamorous and repeatable.