Reverse humidity limit
For each selected surface, the tool calculates the approximate room RH that would correspond to 80% surface RH and to saturation. This turns “is 55% RH okay?” into a surface-specific answer.
Whole-home moisture diagnostics
Whole-home humidity & condensation calculator
Compare several rooms at once instead of judging one humidity number. Calculate dew point, estimated or measured surface RH, condensation margin, the maximum room RH each surface can tolerate, and the outdoor-temperature breakpoint where that surface is modeled to enter the high-RH or condensation zone.
Runs in your browserNo account requiredPrivate measurement history
Interactive room comparison
Measure the air. Check the cold surface. Compare the whole house.
Use readings from a hygrometer. Surface temperature is optional: if you leave it blank, the tool estimates it from outdoor temperature and the selected construction preset.
Moisture attention score—Add room readings to begin
Highest dew point—
Highest surface RH—
Rooms needing attention—
Saved on this device
Measurement history
Save a snapshot to compare your rooms over time. Nothing is uploaded.
| Date | Worst room | Highest dew point | Highest surface RH | Score |
|---|
The useful part is the comparison
More than a dew-point result
A single calculator can tell you the dew point. The harder question is whether your problem comes from high indoor moisture, an unusually cold surface, or both — and which room is actually the priority.
Driver diagnosis
Each room is classified as mainly moisture-driven, cold-surface-driven, mixed, or lower concern from the values you entered. It is a prioritization aid, not a defect diagnosis.
Cold-snap breakpoint
For preset surfaces, the tool estimates the outdoor temperature at which today’s indoor conditions would push that surface to about 80% RH or to saturation. This makes a coming cold spell testable instead of generic.
History, not one screenshot
Save snapshots locally and compare the worst room, highest dew point and surface RH over time. Repeated measurements are more useful than a single morning reading.
Fast starting points
Open the lab with the problem you are measuring
Window condensation
Start with indoor temperature, RH, outside temperature and a window preset.
Basement humidity
Compare a cool basement with the warmer rooms above instead of relying on RH alone.
Bedroom overnight
Compare evening and morning snapshots to see whether dew point actually rose.
Bathroom drying
Save a post-shower snapshot, then another after ventilation to compare the drying response.
Use the tool with sensible limits
Humidity is only one part of a moisture problem
EPA guidance says indoor relative humidity should be kept below 60% and ideally around 30–50% where possible. It also emphasizes fixing the moisture source and drying wet materials quickly. WHO guidance likewise focuses on preventing persistent dampness and microbial growth rather than treating one humidity percentage as a complete diagnosis.
Common questions
Humidity and condensation questions the calculator can actually test
Why can a window condense even when room RH is below 60%?
Because condensation depends on the coldest surface, not only room RH. If the surface falls to the room dew point, saturation can occur there. Enter the window or measured surface temperature to see the margin directly.
Is dew point more useful than relative humidity?
They answer different questions. RH depends strongly on air temperature; dew point is a better indicator of the actual vapor level. For condensation, compare dew point with the coldest surface temperature.
What is the maximum safe humidity for my window?
There is no universal number for every window and weather condition. The lab calculates a surface-specific RH limit from your indoor temperature, outside temperature and selected or measured surface.
At what outdoor temperature will my window start getting risky?
For preset window and wall types, the lab estimates the outdoor-temperature breakpoint at which your current indoor temperature and RH would push the modeled interior surface to about 80% RH, and separately to saturation. Treat it as a planning estimate because real edges, frames and thermal bridges can be colder.
Why is my basement RH higher than upstairs?
Cooler air can show higher RH even with a similar vapor level. Compare dew points upstairs and downstairs; a much higher basement dew point suggests a larger moisture load, while a similar dew point points more strongly toward temperature.
Can this tell me whether I have mold?
No. It can identify conditions that deserve closer moisture investigation. It cannot inspect materials, determine duration of dampness, identify growth, or provide a health assessment.
Should I trust an estimated surface temperature?
Use it for comparisons. Real edges, frames, thermal bridges, curtains, air movement and installation details can make local surfaces colder. A careful measured surface temperature is better when the result matters.
Problem guides
Dew-point guide →Read less, measure more
Window condensation guide
What glazing estimates can and cannot tell you, and where the coldest point may be.
Basement humidity guide
Separate cool-air RH from a genuinely higher moisture load.
Bedroom humidity guide
Use repeated morning/evening readings instead of one overnight percentage.
Bathroom humidity guide
Focus on the drying response after a shower rather than only the peak.