Indoor Humidity Risk Calculator

Inputs

Use measured indoor RH, or estimate it from moisture sources and ventilation.

Tip

Select the coldest surface you are concerned about.

Indoor temperature (°C) *

Typical: 18–28°C.

Outdoor temperature (°C) *

Used to estimate cold-surface temperature.

Humidity input mode *

Estimation needs outdoor RH and ACH.

Indoor relative humidity (%) *

Measured with a hygrometer.

Outdoor relative humidity (%)

Needed only when estimating indoor RH.

Room volume (m³) *

Length × width × height.

Ventilation rate (ACH) *

Typical homes: 0.2–1.0 ACH.

Surface of concern *

Choose the coldest likely surface.

Envelope input method *

U-value relates to heat loss and cold surfaces.

Typical U-value preset

Preset drives the surface temperature estimate.

Custom U-value (W/m²·K)

Lower U-value usually reduces condensation risk.

Occupants

People add moisture via breathing and activities.

Moisture level

Used only when estimating indoor RH.

Extra moisture (g/h)

Include cooking, plants, wet materials, etc.

Active moisture hours/day

Hours/day with higher moisture generation.

Dehumidifier removal (L/day)

Set to 0 if not used.

Dehumidifier hours/day

Used only with dehumidifier removal.

Reset

Example data table

These examples show how colder outdoor temperatures and weaker envelopes raise risk.

Indoor T (°C)	Indoor RH (%)	Outdoor T (°C)	U-value	ACH	Surface	Risk category	Condensation likely
24	50	8	1.2	0.6	Wall	Moderate	No
22	60	0	2.2	0.3	Window	High	Yes

Formula used

Saturation vapor pressure: e_s(T) = 6.112 × exp(17.62T / (243.12 + T)) (hPa).
Indoor vapor pressure: e = RH × e_s(T_i).
Dew point: computed by inverting the Magnus relation using ln(e/6.112).
Estimated interior surface temperature: T_s = T_i − U(T_i − T_o)R_si.
Surface relative humidity: RH_s = 100 × e / e_s(T_s).
Optional RH estimate: steady-state moisture balance with ACH, room volume, moisture generation, and dehumidifier removal.

How to use this calculator

Enter indoor and outdoor temperature values for the current conditions.
Choose measured mode if you know indoor relative humidity.
Choose estimate mode to predict RH from moisture and ventilation.
Select the coldest surface you are evaluating, such as windows.
Pick a typical envelope preset, or enter a known U-value.
Press Calculate Risk and review the risk summary.
Download the CSV or PDF report if you need documentation.

Moisture sources during construction

Indoor humidity often spikes while buildings are being finished. Fresh concrete, plaster, paint, and wet trades release water for days. Occupants and workers add moisture through breathing, cooking, and showers. Even a small room can climb above 55% RH when ventilation is limited. Track extra moisture and active hours to represent daily bursts instead of a single moment reading.

Dew point for quick decisions

Dew point turns temperature and RH into one control value. At 24°C and 50% RH, the dew point is about 13°C. If a surface drops below that temperature, water can form. The calculator displays dew point and the estimated surface temperature so you can see the condensation margin immediately and adjust targets before damage begins.

Cold surfaces and U-value

Risk rises at windows, corners, slab edges, and thermal bridges. The tool estimates interior surface temperature from indoor and outdoor temperatures, a U-value, and a standard internal surface resistance. A lower U-value generally keeps surfaces warmer. For example, moving from a weak envelope (around 2.2 W/m²·K) to an insulated assembly (around 0.35 W/m²·K) can raise surface temperature enough to restore a positive margin under the same indoor humidity.

Ventilation and moisture balance

In estimate mode, indoor humidity is predicted from a steady-state moisture balance. Moisture generation from occupants and activities is divided by ventilation flow, with optional dehumidifier removal subtracted. Increasing air changes per hour from 0.3 to 0.8 improves dilution and reduces predicted RH. Use realistic runtimes for fans and dehumidifiers so the daily average reflects actual operation.

Using risk outputs in reports

The score combines surface RH, condensation margin, ventilation, and moisture intensity into a 0–100 indicator. Moderate risk suggests monitoring and minor adjustments, while high or severe risk indicates likely condensation or persistent surface humidity that can support mold growth, especially during cold nights and early mornings. Download the CSV or PDF to document assumptions, compare design options, and communicate actions such as source control, insulation continuity, and controlled ventilation.

FAQs

1) What indoor RH range helps reduce damage risk?

Many teams aim for about 30–55% RH in occupied spaces. In cooler weather, lower targets can reduce window sweating and damp corners.

2) Why is outdoor temperature included in the inputs?

Outdoor temperature influences how cold interior surfaces become. Colder outdoor air can reduce surface temperature and shrink the margin to the dew point.

3) When should I use estimated humidity mode?

Use it for scenario testing or when you lack a reliable indoor RH reading. It uses occupants, moisture sources, ventilation, and dehumidifier removal to predict indoor RH.

4) What does a negative condensation margin indicate?

A negative Ts−Td means the estimated surface temperature is below the dew point. Condensation is likely on that surface if conditions persist.

5) How should I interpret the surface RH value?

Surface RH estimates moisture conditions right at the cold surface. Values near or above 80% can support mold growth over time, depending on duration and material sensitivity.

6) What are fast ways to reduce the risk score?

Limit moisture sources, run exhaust during wet tasks, increase effective air exchange, and use dehumidification with proper drainage. Improve cold spots with insulation continuity and air-sealing.

Built for planning and screening. Verify with field measurements for critical decisions.