White growth on floor joists
Three different things look the same under a torch. Efflorescence crushes to powder and dissolves in water. Mould smears. White rot leaves the wood soft when probed — and only that one is a structural problem.
You went under the house with a torch and found something white on the joists. Before you call anyone, work out which of three things it is — because one is harmless, one is a humidity problem, and one is structural.
The three-test identification
| Test | Efflorescence | Mould | White rot |
|---|---|---|---|
| Where is it? | Block, brick, concrete | Wood, cardboard, insulation | Wood only |
| Rub it | Crushes to gritty powder | Smears | Does not rub off |
| Drop of water | Dissolves | Stays, may darken | No change |
| Screwdriver probe | n/a | Wood stays firm | Wood gives or crumbles |
| Texture | Crystalline, sparkly | Fuzzy, filmy or speckled | Stringy, or wood looks bleached |
| Smell | None | Musty, earthy | Sweet, slightly fermented |
| Serious? | No, but shows water movement | Yes — humidity problem | Yes — structural |
Do the screwdriver probe on anything on wood, even if you are confident it is surface mould. It takes ten seconds and it is the only test that finds the expensive problem.
Efflorescence — the harmless one
White, crystalline, on masonry rather than wood. It is mineral salt carried by water through the block and left behind when the water evaporates.
It is not alive, not a health concern, and brushes off. But it is a record of water movement: something is pushing moisture through that block. The deposit is the receipt.
If it is on a foundation wall partway up rather than at the base, that points to hydrostatic pressure rather than splash-back, and is worth reading alongside the crack guide.
Mould — the common one
Fuzzy, filmy or speckled, on wood or on anything organic. Smears rather than crumbles. Often accompanied by the musty smell that reaches the living space.
In Tennessee crawl spaces this is overwhelmingly a humidity problem rather than a leak problem. Summer dew points above 70°F, a vented crawl space pulling that air in, condensation on surfaces cooled by the ground — and growth follows.
Remediation without moisture control fails within one season. Every time. The order is: stop water, remove growth, control humidity permanently.
White rot — the one that costs money
This is a wood-decay fungus, and it is the reason to do the probe test.
Decay fungi become active when wood moisture content exceeds roughly 20–25%. Below that they cannot proliferate; wood in normal service sits around 8–14%. A crawl space running at high relative humidity through successive Tennessee summers pushes framing into the active range and keeps it there.
What makes it dangerous is that wood loses stiffness before it loses appearance. A joist can be significantly decayed while still looking like a joist under a torch. The bounce you feel upstairs often arrives well before anything looks obviously wrong underneath.
Where to probe, in order of likelihood:
- Joist ends where they bear on the girder or sill — the wettest, least ventilated point
- The sill plate on top of the foundation wall
- Anywhere directly below a bathroom, kitchen or laundry
- Under any historic water stain
- The band joist around the perimeter
What each one costs
| Finding | What it needs | Typical cost |
|---|---|---|
| Efflorescence only | Find and fix the water path | $200 – $8,000 depending on cause |
| Surface mould, limited area | Remediation + humidity control | $1,500 – $6,000 |
| Widespread mould | Full remediation + encapsulation | $8,000 – $18,000 |
| White rot, localised | Sister or replace affected members | $1,500 – $5,000 |
| White rot, structural members | Engineer, then replacement | $5,000 – $25,000+ |
What to do this week
- Photograph it with something for scale, and note where each photo was taken
- Run the three tests — rub, water drop, screwdriver probe
- Probe the joist ends specifically, not just where the growth is visible
- Put a hygrometer down there for a week
- If the probe finds soft wood anywhere, get a structural opinion rather than a remediation quote — remediating decayed structure is treating the surface of a load-bearing problem
And if all three tests come back clean — firm wood, no smear, dissolves in water — you have efflorescence, a water path to trace, and no emergency.
Common questions
Is white mould less dangerous than black mould?
Colour is not a reliable guide to risk. Species varies within colours and colours vary within species. Any substantial growth on framing needs removing and, more importantly, needs its moisture source fixed.
What is efflorescence?
Mineral salt left behind when water moves through masonry and evaporates. It is not alive and not harmful — but it is proof water is moving through the block, which is worth knowing.
How do I test for white rot?
Push a screwdriver into the wood with moderate pressure. Sound timber resists. Decayed wood gives, crumbles or lets the tip sink in. Test several points, including the ends of joists where they sit on the girder.
Can I clean it off myself?
Surface mould on a small area, with proper protection, sometimes. But cleaning without dropping crawl space humidity below 60% means it returns within a season, and white rot cannot be cleaned at all — affected wood is removed or reinforced.
Does bleach work?
Poorly on wood. Bleach is mostly water and does not penetrate porous timber, so it removes the colour while leaving growth in the fibres — and adds moisture to a space that already has too much.