Water activity, and why it decides what grows
Water activity (aw) is the share of water in a material available to an organism, on a 0 to 1 scale. Mold growth depends on it rather than room humidity: most indoor species need about 0.80 aw.
What the number means
Water activity is defined as the vapor pressure of water in a material divided by the vapor pressure of pure water at the same temperature. Pure water is 1.0. A bone-dry material approaches 0. Everything a building is made of sits somewhere between, and moves up and down as conditions change.
The practical translation is this: aw is the water a microorganism can actually get at. Wood can hold a substantial quantity of water that is chemically bound and unavailable, which is why moisture content — the weight of water as a fraction of dry material — is a different measurement that does not map cleanly onto growth. Two materials at the same moisture content can have quite different water activities, and it is the water activity that the mold responds to.
At equilibrium, the water activity of a surface equals the relative humidity of the thin layer of air immediately against it, expressed as a fraction. That is the bridge between the two ideas, and it is also the trap.
Why room humidity misleads
A hygrometer in the middle of a room reports the relative humidity of the air it sits in. Relative humidity is temperature-dependent: the same absolute quantity of water vapour reads as a higher relative humidity in colder air, because cold air holds less.
So consider a wall with a thermal bridge — a stud, a lintel, an uninsulated corner — in a room at 21°C and 55% relative humidity. The room is comfortable and the reading is unremarkable. If the surface of that cold spot sits at 13°C, the air against it is at roughly 91% relative humidity, which is a surface water activity around 0.91. That is enough for Chaetomium globosum. The room reading never showed it.
The same reasoning applies to the back of a cold exterior wall, to the underside of a floor over a vented crawl space, and to any cavity where warm moist air meets a cold plane. This is what a dew point margin is measuring, and it is why moisture risk modelling looks at building geometry and climate together rather than at a single humidity figure.
The thresholds that matter
No single number separates growth from no growth, because the threshold is species-specific and shifts with temperature and substrate. But the bands are stable enough to reason with, and they are the reason a species list reads as a moisture history.
The published minima below are the lowest water activities at which growth or germination has been demonstrated under laboratory conditions. Real buildings are colder, dirtier and more variable than a laboratory, and growth at a minimum is typically extremely slow — a colony that takes months rather than days. Treat these as the floor of what is possible, not as what is likely.
What this changes about assessment
If growth tracks available water at a surface, then the useful questions about a property are about water: where it comes from, where it collects, how fast the assembly dries, and whether anything has kept a surface above its threshold for long enough.
That is a question about climate load, terrain and drainage, construction era and materials, and water event history — all of which exist as records for an address. It is why a risk estimate is possible without a sample, and equally why a risk estimate is not a finding of mold. It describes the conditions, which is exactly what the water activity concept says is decisive.
Approximate minimum water activity for growth, by species
| Species | Minimum aw reported | Typical indoor niche | What it indicates |
|---|---|---|---|
| Aspergillus penicillioides | 0.585–0.632 (germination and cell division) | Settled dust, bedding, mattress dust | Dust accumulation and chronic mild damp |
| Wallemia sebi | 0.69–0.75 | House dust, dry and salted foods | Dust reservoir; rarely active wet growth |
| Aspergillus versicolor | 0.78–0.80 | Damp gypsum board, carpet and upholstery dust | Sustained mild dampness or condensation |
| Chaetomium globosum | Above ~0.90 | Drywall paper, wet framing, cellulose | Wetting that lasted rather than dried |
| Stachybotrys chartarum | ~0.89–0.94 minimum, optimum above 0.98 | Chronically saturated cellulose | Prolonged free water at that location |
Reading a surface: room conditions and the surface they produce
| Room air | Surface temperature | Approximate surface aw | What can grow there |
|---|---|---|---|
| 21°C, 50% RH | 21°C (same as room) | 0.50 | Nothing on this list |
| 21°C, 50% RH | 16°C (cool exterior wall) | ~0.68 | Extreme xerophiles only |
| 21°C, 55% RH | 13°C (thermal bridge) | ~0.91 | Most of this list, including Chaetomium |
| 21°C, 60% RH | 12°C (cold corner) | Condensing | Everything, given time |
Sources
- Stevenson et al., Aspergillus penicillioides differentiation and cell division at 0.585 water activity (Environmental Microbiology, 2017)
- Stevenson et al., Water-, pH- and temperature relations of germination for extreme xerophiles (Microbial Biotechnology, 2017)
- Jurjević et al. / INSPQ, Stachybotrys chartarum species fact sheet
- Zalar et al., The genus Wallemia — from contamination of food to health threat
- WHO Guidelines for Indoor Air Quality: Dampness and Mould (2009)
Questions people ask
- What water activity does mold need to grow?
- Most indoor molds need about 0.80 aw or above. A few xerophilic species grow well below that — Wallemia sebi from roughly 0.69, and Aspergillus penicillioides has been shown dividing at 0.585. The wet-building species need 0.90 and above.
- Is water activity the same as moisture content?
- No. Moisture content is how much water a material holds by weight; water activity is how much of that water is available. Materials with identical moisture content can differ in water activity, and it is water activity that determines growth.
- What relative humidity causes mold?
- There is no single figure, because what matters is the humidity at the surface rather than in the room. Keeping room relative humidity below about 60% is sound general guidance, but a cold surface in a 55% room can still sit at 0.90 water activity.
- Can mold grow without a leak?
- Yes. Condensation on cold surfaces, high indoor moisture production with poor ventilation, and ground moisture rising through a slab all raise surface water activity without any leak. This is why the xerophilic species are so common in ordinary houses.