Indoor Dew Point & Surface Condensation Margin Calculator

Why compare indoor dew point with surface temperature?

Before replacing wallpaper or buying insulation to address recurring wet patches, record both the indoor air conditions and the temperature of the affected surface.
Dew point describes the temperature at which the given water vapor condition reaches saturation over liquid water.
Comparing it with a measured surface temperature helps identify conditions consistent with surface condensation.
Window glass, frame corners and walls behind furniture can have different margins even when they share the same room humidity reading.

This tool helps homeowners and families prepare a measurement record for a site assessment.
It provides a consistent calculation for comparing locations and times before discussing renovation work.
The result does not establish which repair is necessary or whether a particular product will solve the problem.

What the calculator provides

Inputs, units and supported ranges

• Air temperature: use measured room air temperature, not a thermostat setpoint.
  The supported range is 0–50°C; pair it with the RH reading from the same time.
  
• Relative humidity: enter a value above 0 and up to 100%.
  Missing input is different from zero; an extremely low reading that produces a dew point below −45°C is outside the verified formula range used here.
  
• Surface temperature: measure one wall, window or other location directly, within 0–50°C.
  Do not substitute outdoor temperature or a reading from another room.
  
• Sensor errors: each temperature error may be 0–5°C and the RH error 0–20 percentage points.
  These are product input limits, not specifications for every instrument.
  
• Notes: record the instrument model, source of its error specification, window position and heating conditions.
  Empty notes remain marked as unrecorded.
  

The Magnus formula over liquid water

The calculation uses the NPL liquid-water saturation vapor pressure approximation.
T denotes air temperature, Ts surface temperature and RH relative humidity; temperatures are in °C and vapor pressure in Pa.
ln is the natural logarithm and exp the natural exponential; intermediate values are not rounded.

Critical RH follows from the ratio of saturation vapor pressures at the surface and air temperatures.
Subtract current RH to obtain humidity headroom; a negative value means the current reading exceeds that surface threshold.
This assumes the air contacting the surface has the same vapor pressure as the sampled room air, without modeling moisture transport inside the wall or absorption by materials.

How to read the sensor range

The lowest dew point combines the lowest air temperature and RH; the highest combines their upper bounds.
The minimum margin subtracts the highest dew point from the lowest surface temperature, while the maximum uses the opposite endpoints.
These combinations assume the entered errors may vary independently and are not probability estimates or statistical confidence intervals.

An RH upper bound above 100% is limited to 100%, with an explicit notice.
If the lower bound reaches 0%, a temperature interval exceeds 0–50°C, or its dew point falls below −45°C, the sensor-range assessment is withheld.
Valid nominal results remain visible when errors are unknown, clearly distinguished from a margin supported by entered sensor bounds.

Step-by-step use

1. Select one room and surface location and measure air temperature, RH and surface temperature at the same time.
   Follow each instrument manual for stabilization and use.
   
2. Enter the readings and each applicable error specification.
   Leave unknown bounds blank rather than inventing small errors.
   
3. Read the nominal margin together with the minimum and maximum sensor results.
   Resolve missing or unsupported inputs before interpreting a withheld assessment.
   
4. Add a location, local measurement time, instrument and heating or ventilation notes, then save a snapshot.
   Enter other locations or times as separate records.
   
5. Download the TXT file and keep it with your original measurements.
   When saved records exist, only those records are exported, so add your latest changes first if you need them in the file.
   

Worked example: 20°C air and 60% RH

These fictional readings produce a liquid-water dew point of approximately 12.00°C.
The unrounded value is 11.9954538924°C, so a 12°C surface has a margin of about 0.00455°C rather than exactly zero.
The interface flags this proximity to the display rounding boundary instead of presenting the tiny positive difference as a useful safety allowance.

The assumed errors are ±0.5°C for air temperature, ±3 percentage points for RH and ±0.5°C for surface temperature.
The resulting dew-point range is 10.75–13.21°C; the critical RH range for the 12°C surface is 56.31–63.97%.
In a separate comparison with a 25°C surface, critical RH is 135.47%.
That is not a humidity recommendation: the warm surface threshold lies beyond the supported 0–100% RH range at that air temperature.

Practical comparisons and the next action

Use these records to describe conditions when seeking wallpaper, window or insulation advice, without treating the calculation as a diagnosis.
Notes about the extent of wet patches, recurrence and changes after rainfall provide additional context.
Once a site assessment supplies suitable heat-performance assumptions and quotes, a separate insulation payback calculator can examine the economics.

Measurement tips and model limits

• Water and ice have different saturation relationships.
  This tool does not calculate frost point or frost accumulation on sub-zero surfaces; a negative liquid-water dew point is not an ice assessment.
  
• Non-contact readings can depend on the surface and measurement method.
  For glass or reflective materials, follow the instrument manual and check whether the method is suitable before treating a reading as established.
  
• The sensitivity table includes only the entered sensor bounds.
  It excludes formula approximation, pressure differences, calibration issues, spatial and temporal variation and differences in air next to the surface.
  
• If wetness recurs despite a positive calculation, consider the record alongside the actual site conditions.
  The tool does not determine leaks, thermal bridges, material moisture, mold, health effects, code compliance, construction liability or renovation effectiveness.
  

Frequently asked questions