Calculator Inputs
Saved Runs
| Time | Project | Location | Air | RH% | Surface | Dew Pt | Margin | Surface RH | Risk | Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| No saved runs yet. Calculate with “Save to Log” enabled. | ||||||||||
Example Data Table
| Scenario | Air | RH% | Surface | Dew Pt | Surface RH | Note |
|---|---|---|---|---|---|---|
| Winter bedroom, closed door | 20.0 °C | 55 | 14.0 °C | 10.7 °C | 73% | Usually safe; monitor corners behind furniture. |
| Bathroom after shower | 24.0 °C | 75 | 18.0 °C | 19.3 °C | 100% | Condensation likely; exhaust and warm surfaces. |
| New plaster drying, limited ventilation | 22.0 °C | 65 | 16.0 °C | 15.1 °C | 86% | High risk if prolonged; manage moisture load. |
Formula Used
Td = (b·γ)/(a−γ)
a = 17.27, b = 237.7, T and Td in °C
e(T) = 6.112 × exp((17.67·T)/(T+243.5))
How to Use This Calculator
- Measure indoor air temperature and relative humidity.
- Measure the coldest surface temperature you can access.
- Click Calculate to see dew point and margin.
- Use Surface RH and Risk to prioritize fixes.
- Enable Save to Log to keep a running record.
- Download CSV or PDF for site notes and reporting.
Why Dew Point Matters on Site
Dew point converts temperature and relative humidity into a single moisture threshold. When a surface drops below this threshold, condensation can form if the room feels comfortable. In construction, this affects glazing edges, slab perimeters, steel members, and insulated cavities. Tracking dew point helps schedule drying, choose vapor control layers, and verify that temporary heating is not hiding moisture problems.
Surface Temperature and Thermal Bridges
Mold risk is driven by the coldest surface, not the thermostat setting. Thermal bridges at lintels, column lines, fasteners, and balcony connections create localized cold spots. Use an infrared thermometer to scan corners and junctions, then enter the lowest reading. Improving insulation continuity, air sealing, and air movement raises surface temperature and increases the dew point margin.
Using Surface RH to Predict Mold
Surface relative humidity estimates how close a surface is to saturation. Values above 80% sustained for long periods are treated as mold‑prone, especially on porous finishes like gypsum and timber. This calculator derives surface RH from the vapor pressure implied by the dew point and compares it with saturation at the surface temperature. The result is a practical screening metric for detailing and commissioning checks.
Drying Strategy: Ventilation vs Dehumidification
Ventilation works best when outdoor absolute humidity is lower than indoor conditions. If outdoor air carries more moisture, ventilation can worsen drying and elevate dew point. Dehumidification reduces moisture regardless of outdoor weather, but requires controlled air paths and sufficient runtime. Compare indoor and outdoor readings in the optional fields, then choose the approach that lowers absolute humidity while keeping surfaces warm.
Documentation and Quality Control
Log each run with project and location labels to build a moisture profile across rooms and elevations. Use the CSV for trend reviews, and the PDF for site reports, handover packs, and subcontractor coordination. When risk is high, document mitigation steps and recheck after changes to heating, ventilation, or insulation. Consistent records support defect prevention and responsibility during closeout.
FAQs
What is the safest dew‑point margin for interior finishes?
A larger margin is safer. Many teams aim for several degrees of separation between the coldest surface and the dew point, then verify performance during the coldest expected conditions and moisture‑producing activities.
Why does mold risk increase even without visible condensation?
Surfaces can stay damp at high surface humidity without liquid water droplets. Dust and porous materials absorb moisture, and microbes can grow when the surface remains above roughly 80% relative humidity for long periods.
How do I measure surface temperature accurately?
Use an infrared thermometer, measure multiple spots, and avoid reflective metal surfaces or sunlight. Confirm the coldest junctions: corners, window frames, slab edges, and behind furniture. Enter the lowest stable reading.
Can outdoor air dry a building in humid climates?
Sometimes, but only when outdoor absolute humidity is lower than indoors. A cool, humid day can contain more moisture than warm indoor air. Compare g/m³ values; if outdoors is higher, prioritize dehumidification and controlled ventilation.
Does insulation alone eliminate mold risk?
Insulation helps by warming surfaces, but air leaks can still deliver moist air to cold layers. Combine insulation with air sealing, vapor control appropriate to climate, and reliable ventilation to keep both moisture and temperature in balance.
How often should I recheck conditions during construction?
Recheck after major changes: HVAC startup, temporary heat, wet trades, weather shifts, or enclosure completion. For sensitive finishes, daily checks during drying phases are common, then weekly verification until stable readings are achieved.