Measure the room, not the window
Take temperature and RH from the occupied part of the room, away from a radiator, direct sun, shower steam or an open window. Give the device time to settle.
Humidity, dew point & condensation
Check indoor humidity and condensation risk room by room
Enter a room temperature, humidity reading and the surface you are worried about. The lab turns those numbers into a plain-English condensation check, a room-RH target and a cold-weather breakpoint — then lets you compare more rooms if you want the whole-home picture.
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.
Start with one room
You only need three things
Use the numbers from a simple hygrometer. You do not need a surface thermometer — that field is optional.
- Room temperatureRead the thermometer where you spend time.
- Relative humidityUse the RH % from the same device and same moment.
- What gets cold?Choose your window or exterior wall. We estimate the surface if you leave its temperature blank.
Room attention index—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.
Dew-point change—Latest vs first saved reading
Surface-RH change—Percentage points
Attention-score change—Latest vs first
Flagged snapshots—Surface RH ≥80% or score ≥35
Outdoor-temp change—Weather context when available
Trend review compares snapshots with the same room name. It shows movement in your saved readings, not proof that a repair or ventilation change caused it. Try to measure at similar times and conditions.
| Date | Worst room | Highest dew point | Highest surface RH | Score |
|---|
Make the numbers useful
Full measuring guide →What to measure — and how to read the result
You do not need building-science equipment. A basic hygrometer plus one consistent routine is enough to make the comparison meaningful.
Choose the coldest likely surface
For condensation, think glass edge, frame, reveal, exterior-wall corner or a wall behind furniture. If you can measure that surface, enter it; otherwise use the preset as a planning estimate.
Repeat at the useful moment
Bedroom: compare evening with morning. Bathroom: measure after a shower and again while it dries. Windows: repeat during colder weather. Patterns are more useful than one isolated reading.
Read these first
Surface RH tells you how moisture-loaded the selected cold surface is. Suggested room-RH ceiling turns that into a practical humidity target for this modeled surface.
Then watch the weather breakpoint
Cold-snap breakpoint estimates the outdoor temperature where today’s indoor conditions would push the preset surface to about 80% RH. It is a planning threshold, not a weather forecast.
Only then open technical details
Dew-point margin, estimated surface temperature and saturation thresholds are there when you want to inspect the physics — not as the first thing you must interpret.
Turn the result into a next step
What should I do with this result?
Do not chase one “perfect” humidity percentage. First decide whether the stronger signal is room moisture, a cold surface, or both.
Room RH and surface RH both look reasonable
Save the snapshot and repeat it when the weather gets colder or the room is used differently. A trend is more useful than forcing humidity lower for no reason.
Room humidity is the stronger signal
Look first at moisture sources and air exchange: occupants overnight, showers, cooking, drying laundry, humidifiers or a room that stays closed. Compare dew point with another room.
The room is not especially humid, but the surface is cold
Measure the actual coldest spot if you can. Window edges, frames, reveals, exterior corners and walls behind furniture can be colder than the preset estimate.
High room moisture and a cold surface occur together
Work on both sides of the equation: reduce excess indoor moisture and improve the temperature/airflow around the cold surface. Persistent wetting or damage needs source investigation.
These are measurement priorities, not a building diagnosis. If you have active leaks, wet materials, visible growth or recurring damage, investigate the moisture source rather than relying on the calculator alone.
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.
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.
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.
Room trend review
Save snapshots locally, choose the same room across visits and compare dew point, surface RH, attention score and outdoor-temperature context instead of relying on one screenshot.
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.
Where should I place a hygrometer?
Use an open position in the occupied part of the room, away from a radiator, open window, direct sun, humidifier, shower steam or cooking plume. Keep temperature and RH from the same device and same moment. See the measuring guide for a repeatable workflow.
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.
How to measure indoor humidity
Where to put a hygrometer, when to take readings and how to avoid misleading measurements.