Grey mould in a crawl space
Usually a maturing colony or a mixed community rather than a distinct species.
1 of the 3 likely answers here is not mould at all.
The species matters less than you think
- Public health guidance is to remove all indoor mould regardless of type. The CDC and EPA do not recommend routine species testing for ordinary households, because the response does not change: find the water, stop it, remove the growth.
- “Toxic black mould” is a marketing phrase, not a category. Stachybotrys is real and does need soaked material to grow, so finding it tells you something useful about your building. But the health effects that are well established — allergy, asthma aggravation, irritation — are not specific to it.
- Mould without water does not persist. Cleaning growth while leaving the moisture source is why it comes back. The leak, the condensation or the ventilation is the actual problem.
Call a professional for areas larger than about 10 square feet, for anything in HVAC ductwork, after sewage or flood water, or if anyone in the household is immunocompromised or has a lung condition.
The 3 likely answers
1.Efflorescence
Not mouldA white or off-white crystalline crust on brick, concrete or masonry, sometimes fluffy and needle-like. It dissolves in water and often reappears elsewhere as the wall dries.
How to know it is this one
Dissolves in water. Mould does not — this is the definitive test.
What it means: Water is migrating through the masonry. The deposit is harmless but the moisture behind it will eventually support real mould on adjacent materials.
More on efflorescence →2.Dry Rot
InvestigateWhite or grey cotton-wool sheets over timber, with grey-white strands that can run across masonry, and sometimes a flat rusty-orange fruiting body with a wrinkled surface. Affected wood darkens, shrinks and cracks into cubes.
How to know it is this one
Wood cracks into cube-shaped blocks across and along the grain.
What it means: Structural timber decay. This is a building surveyor and a builder, not a cleaning product. It needs the moisture source fixed and affected timber cut out well beyond the visible damage.
More on dry rot →3.White Mould
RoutineWhite fuzzy or cottony growth, sometimes with visible thread-like strands, on damp wood, soil, stored items and food. Many colonies darken as they mature, so white often means young rather than a distinct species.
How to know it is this one
Fuzzy or cottony with visible three-dimensional growth, not a flat crust.
What it means: Sustained damp. On timber in a crawl space, check whether the wood is soft — if it is, you may be looking at decay rather than surface growth.
More on white mould →Still not sure?
The key adds texture and surface, which separate these far better than colour does.
Identify it step by step