Moisture, Mould and Indoor Air Quality
Mould in a building is a moisture problem with a delay built in. It appears years after the decision that caused it, in a location that looks arbitrary, and by then the person who made the decision has usually moved on.
For a builder, that delay is the whole problem. Callbacks arrive well past practical completion, the cause is invisible behind linings, and liability discussions do not go well.
What mould needs
Three things: moisture, a food source and time.
Food is unavoidable. Timber, paper-faced plasterboard, dust and most organic building materials all qualify. Time is a given. Moisture is the only variable you control.
Keep the moisture out and mould does not grow, regardless of what the wall is made of.
Where the moisture comes from
Interstitial condensation. Vapour reaching a surface below dew point inside the assembly, condensing there. Invisible until it presents on the internal surface, by which point the assembly has been wet for years.
Air-carried moisture. Humid air moving through gaps in the envelope into cavities where it cannot dry. This delivers far more moisture than diffusion does.
Surface condensation. Interior air condensing on cold internal surfaces, typically at thermal bridges, corners and window reveals. This is the visible version and the one occupants report.
Bulk water. Rain ingress at openings and penetrations, plumbing failures, and construction moisture that never dried out.
Occupant moisture load. Cooking, showering, laundry, drying clothes indoors, respiration. Significant every day, and if ventilation does not remove it, it condenses somewhere.
The construction moisture problem
Buildings are wet when they are built. Concrete, screeds, wet trades and rain during construction all put water into the structure.
An assembly that is sealed up before it has dried, with a vapour barrier on one side and a low permeance membrane on the other, cannot dry in either direction. The water stays in the wall.
This is a real cause of early-life mould in otherwise well built houses, and it argues for allowing drying time and for assemblies that can dry in at least one direction.
Why it matters more now
Older Australian houses were leaky and poorly insulated. That was uncomfortable and expensive, and it also meant continuous accidental ventilation and warm-ish internal surfaces that rarely dropped below dew point.
Better insulation and tighter construction removed both of those accidental protections. Insulation makes internal surfaces colder on the outside of the insulation layer, and reduced leakage removes the accidental ventilation that was carrying moisture away.
Neither is a reason to build worse. They are reasons to manage moisture deliberately rather than relying on the building leaking.
The three things that prevent it
Air seal the envelope properly. This stops the largest moisture transport mechanism. Continuous air barrier, sealed penetrations, sealed window perimeters.
Ventilate for the moisture load. Extract at source in wet areas, discharging outdoors, not into a roof space. Make-up air provided. NCC 2022 requires most of this.
Let assemblies dry. Correct membrane permeance for the climate zone, drained and ventilated cavities, no vapour barriers on both sides, and drying time before closing up.
Thermal bridges and visible mould
The mould an occupant reports is usually surface mould at a thermal bridge: a corner, a window reveal, behind a wardrobe on an external wall.
Those locations have colder internal surfaces than the wall around them. If the surface drops below the dew point of the room air, moisture condenses there.
The occupant sees a mould patch in an apparently random spot. The cause is a cold surface, and the fix is thermal, not chemical. Cleaning it treats the symptom.
For builders specifically
The commercial case is straightforward. A mould callback five years after handover is expensive to investigate, expensive to remediate, and difficult to attribute cleanly.
The measures that prevent it, correct membrane class, continuous air sealing, ventilated cavities and compliant exhaust, cost very little at construction stage relative to the remediation cost.
Documenting the envelope with photographs before closing up is also worth the twenty minutes. If a moisture claim arrives years later, a photographic record of a properly detailed assembly is the difference between a conversation and a dispute.