Enter readings
Example data table
| RH (%) | Moisture (%) | Air (°C) | Wall (°C) | Mold | Leaks | Vent | Drain | Dehum | Index | Band |
|---|---|---|---|---|---|---|---|---|---|---|
| 48 | 11 | 21 | 20 | 0 | 0 | 4 | 4 | 3 | 7 | Dry |
| 58 | 15 | 22 | 18 | 1 | 1 | 3 | 3 | 2 | 25 | Slightly Damp |
| 65 | 19 | 23 | 17 | 2 | 2 | 2 | 2 | 1 | 44.6 | Moderately Damp |
| 74 | 24 | 24 | 16 | 3 | 3 | 1 | 1 | 1 | 74.3 | Very Damp |
| 82 | 30 | 24 | 14 | 4 | 4 | 1 | 0 | 0 | 93.6 | Severe |
Formula used
The index is a weighted score from 0 to 100. Each factor is normalized to 0–1, then multiplied by its weight, and scaled to 100.
- Humidity: 45% → 0, 80% → 1 (clamped).
- Moisture: 10% → 0, 30% → 1 (clamped).
- Condensation: dew point − wall temperature; −2°C → 0, +4°C → 1.
- Issue ratings: mold and intrusion use rating/5.
- Control ratings: ventilation, drainage, dehumidifier use (5−rating)/5.
| Factor | Weight |
|---|---|
| Relative humidity | 0.18 |
| Material moisture | 0.22 |
| Condensation risk | 0.18 |
| Visible mold | 0.12 |
| Water intrusion | 0.12 |
| Ventilation (lack) | 0.08 |
| Drainage (lack) | 0.06 |
| Dehumidifier (lack) | 0.04 |
| Total | 1.00 |
How to use this calculator
- Measure humidity and air temperature after 30 minutes of stable conditions.
- Measure the coldest wall temperature, especially near corners or rim joists.
- Take material moisture readings on suspect areas and record the average.
- Rate mold, leaks, ventilation, drainage, and dehumidifier use from 0 to 5.
- Click Calculate Index, then export PDF/CSV for records.
- 0–20: Generally dry. Maintain controls.
- 21–40: Minor dampness. Watch seasonal swings.
- 41–60: Moderate risk. Plan targeted fixes.
- 61–80: High risk. Address sources and controls.
- 81–100: Severe. Inspect structure and remediate promptly.
Why dampness indexing improves basement decisions
Moisture complaints often come from multiple causes, not one defect. The index combines humidity, material moisture, and condensation potential into a single score that is easier to compare across seasons and rooms. Contractors can document conditions before repairs and verify outcomes after sealing, drainage work, or ventilation upgrades.
Key readings that drive real-world performance
Relative humidity above 60% raises corrosion risk, odors, and mold probability, while low wall temperatures increase condensation on cold surfaces. Material moisture readings help distinguish airborne humidity from water migration through concrete, joints, or framing.
Condensation margin and insulation strategy
The calculator estimates dew point and compares it with the coldest wall temperature. When dew point is higher than the wall, condensation becomes likely. Improving rim-joist insulation, air sealing, and thermal breaks can raise surface temperature and reduce the margin quickly.
Control levers: drainage, air exchange, and drying
Drainage and grading reduce bulk water load and damp foundation walls. Ventilation improves air mixing and removes humid air, but it must be controlled in humid weather. A properly drained dehumidifier stabilizes humidity during rainy periods, curing cycles, and winter stack-effect events.
Example data for reporting and benchmarking
Use the sample rows below to understand how the score responds to changing site conditions. Higher humidity, higher material moisture, and a positive dew-point margin generally push the index upward.
| RH | Moisture | Air | Wall | Mold | Leaks | Typical Index |
|---|---|---|---|---|---|---|
| 55% | 14% | 22°C | 18°C | 1 | 1 | ≈ 33 |
| 65% | 19% | 23°C | 17°C | 2 | 2 | ≈ 52 |
| 82% | 30% | 24°C | 14°C | 4 | 4 | ≈ 86 |
FAQs
1) What does the Dampness Index represent?
It is a 0–100 screening score that blends humidity, material moisture, condensation risk, and site/control ratings. Use it to compare conditions over time and prioritize corrective actions.
2) What humidity should I aim for in a basement?
Most basements perform best around 45–55% relative humidity. Sustained readings above 60% increase the likelihood of odors, corrosion, and mold, especially when surfaces are cool.
3) Why does wall temperature matter?
A cold wall can drop below the air’s dew point, creating condensation even when leaks are absent. Raising surface temperature with insulation and air sealing often reduces dampness quickly.
4) How do I rate ventilation, drainage, and dehumidifier use?
Use 0 for poor control and 5 for strong control. For example, 5 can mean steady air exchange, downspouts extended away, and a dehumidifier on continuous drain with regular run-time.
5) My moisture meter shows high values on concrete. Is that normal?
Concrete can read higher than wood because it stores moisture. Look for trends, compare multiple locations, and focus on areas near joints and cracks. Rising readings after rain suggest water migration.
6) Can the calculator size a dehumidifier for me?
If you enter area, it provides a rough liters-per-day suggestion based on the index. Treat it as a starting point; real capacity depends on infiltration, temperature, and moisture sources.
7) When should I call a professional?
Seek expert help for persistent standing water, structural cracking, extensive mold growth, or electrical/HVAC concerns. A qualified professional can test for hidden moisture pathways and specify durable repairs.