Dew Point & Surface Risk
See whether a cold surface is likely to collect moisture under current room conditions.
For glass, pipes, concrete, or metal, take the coldest repeatable surface reading. Reflective surfaces can mislead infrared thermometers.
The surface is above the calculated dew point. More margin means less condensation risk if conditions remain stable.
The 3°F and 8°F display bands are Moisture Atlas reminders to recheck measurements, not code limits or a mold-safety assessment.
A separate hygrometer and a suitable surface thermometer can make repeat checks more useful.
As an Amazon Associate I earn from qualifying purchases.Two ways to create margin
Condensation forms when a surface reaches the air’s dew point. Lowering relative humidity lowers the dew point; air sealing, insulation, and room-air movement can raise surface temperature.
Measure room air
Keep the hygrometer away from vents, direct sun, the dehumidifier outlet, and the wet surface.
Find the cold spot
Check window edges, metal frames, pipes, slab edges, corners, and areas behind furniture.
Change the mechanism
Reduce vapor sources and improve cold surfaces instead of repeatedly wiping the symptom.
Learn more: Dew Point and Home Condensation, Explained →
The formula and its limits
This tool uses a Magnus approximation for ordinary indoor temperatures, not a building assembly or mold-growth simulation. With air temperature T in °C and RH in percent, it calculates γ = ln(RH/100) + 17.625T/(243.04 + T), then dew point = 243.04γ/(17.625 − γ). Fahrenheit inputs are converted before the calculation and the result is converted back.
Surface margin means measured surface temperature minus calculated dew point. A zero or negative margin indicates that condensation is physically possible under those inputs. The tool’s 3°F and 8°F caution bands are editorial prompts for rechecking a close reading; they are not code requirements, measurement tolerances, or proof that mold cannot grow.
Worked example: change vapor, not just the display
At 70°F and 50% RH, the calculated dew point is about 50.5°F. A measured 48°F window has a margin of −2.5°F. At the same air and surface temperature but 40% RH, dew point falls to about 44.6°F and the margin rises to +3.4°F. These are calculated examples, not measurements made in a tested home.
Take readings together
Record air temperature and RH near the affected area at the same time as surface temperature, away from a heater or dehumidifier outlet. Let the humidity instrument settle according to its instructions. Infrared readings on reflective glass or metal can be misleading; follow the instrument’s surface and emissivity guidance. If readings are close to the boundary, repeat them rather than treating a tenth of a degree as certainty.
A positive margin does not rule out rain entry, plumbing leaks, earlier condensation, hidden cold spots, or damp materials. Stop monitoring and address active leaks, sewage, electrical water hazards, or visible contamination promptly. The worksheet below is for recording safe observations, not delaying repairs.
Definition of dew point and the difference from RH: National Weather Service ↗. Method checked October 10, 2026.
Your measurement record
Record the same location before and after one change. For water collection, include the unit and collection interval (for example, pints over 6 hours). Do not compare different intervals as if they were daily capacity. This blank worksheet contains your observations, not test results from Moisture Atlas.
On a small screen, scroll the table sideways to reach every field.
| Time / location | Air °F | RH % | Surface °F / water collected | Weather, equipment and wet spots |
|---|---|---|---|---|
Entries stay in this page and are not sent to our server. Download before leaving or reloading: this worksheet does not save your entries.