IndoorComfortKit

Is the air outside drier?

Compare temperature and humidity on both sides of the window. A higher humidity percentage outside can still mean drier air.

Room tools
Indoor and outdoor readings

Your readings

Use temperature and relative humidity from each location, taken at roughly the same time.

Indoor air

Away from vents, direct sun and wet surfaces.

°C

0 to 45 °C

%

1 to 100% RH

Outdoor air

Use a shaded, sheltered reading near your home.

°C

-20 to 50 °C

%

1 to 100% RH

No location access or automatic weather lookup. Inputs stay in memory during room-tool and guide navigation; refreshing clears them.

Which air is drier?

Enter both temperature and humidity readings, then compare their calculated dew points.

This compares moisture only. Before opening windows, consider outdoor air quality, rain, security and heating or cooling needs. It does not predict ventilation time, final room humidity or condensation on a surface.

View calculation checks (new tab) Fixed inputs check software output, not sensor accuracy or equipment performance. Your current inputs stay in this tab.

Compare up to four locations

Rank rooms and outdoor air by calculated dew point, with absolute humidity beside each reading. A warmer room can have lower RH and still contain more water vapor.

Readings taken around the same time

Location comparison method

Each location uses the existing Magnus dew point calculation. Absolute humidity = 216.7 × Pv / (T + 273.15) g/m³, with Pv in hPa, T in °C, and Pv = RH / 100 × 6.112 × exp(17.62T / (243.12 + T)). These ideal-gas and liquid-water approximations cover the tool's −40 to 50 °C input range, not frost point. Compare samples at similar pressure. No new readings or timestamps are inferred.

Why compare dew points?

Relative humidity changes with temperature. Two percentages alone cannot show which air has more moisture. At similar air pressure, a lower dew point indicates drier air. That is the comparison this tool makes; it does not measure airflow or the room itself.

A higher percentage can still mean drier air

In this calculated example, indoor air at 21 °C and 55% RH has a dew point of 11.6 °C. Outdoor air at 10 °C and 80% RH has a dew point of 6.7 °C. The outdoor percentage is higher, but its dew point is 4.9 °C lower. These are illustrative inputs, not a weather report or a home observation.

Will opening windows lower indoor humidity?

It may help when the outdoor dew point is lower. In cold weather, weigh that possible moisture benefit against heat loss and the other window-opening checks beside the result. A higher outdoor dew point suggests that opening windows could add moisture, even if the outside temperature feels pleasant.

If the rounded dew points match, the tool shows no clear moisture advantage. Check the meter positions and repeat the readings. After a shower or cooking, record the activity and a later indoor reading separately; this comparison cannot identify the moisture source or tell you how many minutes to ventilate.

For a basement, compare the result with the room's repeated readings and moisture clues. Drier outdoor air does not resolve seepage or tell you whether a cold wall will stay dry.

Start with comparable readings

Keep the indoor meter away from vents and direct sun. For outdoors, use equipment rated for outdoor conditions and allow it to settle in a shaded, sheltered position. A weather station far from your home, or a forecast for a different time, may not describe the air at your window. Do not expose an indoor-only meter to rain.

Small differences can be smaller than the sensors’ measurement errors. Repeat the readings before acting on a close result. The tool does not assume a sensor accuracy or attach a confidence interval to an unknown device.

Calculation method and limits

Each side uses the existing IndoorComfortKit Magnus approximation:

γ = ln(RH / 100) + 17.62 × T / (243.12 + T)
Dew point = 243.12 × γ / (17.62 − γ)

T is temperature in Celsius. Each dew point is rounded to 0.1 °C before comparison. Equal rounded values are described as matching at this precision, not as proof that the two air samples are identical. Fahrenheit values and differences are converted from the Celsius results.

Supported inputs are 0–45 °C indoors, −20–50 °C outdoors, and 1–100% RH. These are this tool’s input limits. RH is taken relative to liquid water, including below freezing; the calculation does not estimate frost point. It assumes comparable atmospheric pressure and does not model compressed air or high-altitude pressure differences.

For the outdoor sample at indoor temperature, RH₂ = RH₁ × exp(17.62 × T₁ / (243.12 + T₁) − 17.62 × T₂ / (243.12 + T₂)). This assumes unchanged pressure and water-vapor content up to saturation. It is not a prediction of mixed room air. If the calculated ratio reaches 100%, condensation can change the moisture content, so the tool reports saturation instead of an RH above 100%.

Room moisture sources, airflow, surface temperatures and weather changes affect what happens after ventilation. A drier outdoor reading does not guarantee a dry room, a comfortable temperature or safe conditions. At or below 30% indoor RH, the result reminds you that further drying may not be useful; this is a site-defined context prompt, not a ventilation rule.

Sources and scope

IndoorComfortKit source registry and existing methods

Written and maintained by IndoorComfortKit Last reviewed: