Ventilation and moisture
Indoor vs outdoor humidity calculator
Enter temperature and humidity for both places to see which air is drier.
Would outside air bring in less moisture?
Example result
Outdoor air contains less moisture
At 50°F and 90% RH, outdoor air contains less water vapor per mass of dry air than indoor air at 68°F and 60% RH. Replacing indoor air with this drier air can help remove moisture.
Indoor dew point: 53.6°F. Outdoor dew point: 47.2°F.
This compares moisture, not drying time or ventilation needs. Weather, outdoor air quality and heating or cooling costs still matter.
Formula, input ranges and limitations
The bars compare water vapor per kilogram of dry air at 101.325 kPa, approximately sea-level atmospheric pressure. Indoor temperature covers 32–104°F (0–40°C); outdoor temperature covers −4–104°F (−20–40°C). RH covers 5–100%, except combinations that put liquid-water dew point below −49°F (−45°C), outside the approximation’s range.
RH inputs use saturation over liquid water, including below freezing. The output is liquid-water dew point, not frost point; a weather source using RH relative to ice is not interchangeable. The calculation uses NPL’s liquid-water Magnus relationship and the ideal-mixture humidity-ratio equation. Differences within 0.2 g/kg are labeled similar: this is a display convention, not sensor accuracy.
The tool does not estimate airflow, eventual indoor humidity, heat loss or condensation on a particular surface.
Which humidity number should you compare?
Compare dew points or moisture per kilogram of dry air, rather than RH percentages at different temperatures. Cold air at 90% RH can contain less moisture than warmer air at 60% RH.
Try the cold-day example: inside air at 68°F and 60% RH contains about 8.7 g of vapor per kilogram of dry air. Outside air at 50°F and 90% RH contains about 6.8 g/kg. Exchanging those samples offers a way to remove indoor moisture, despite the higher outdoor percentage.
What this says about opening windows
Drier outdoor air can reduce moisture as it replaces indoor air. Wetter outdoor air adds a moisture load. That comparison does not decide ventilation needs, air quality, rain exposure, security or the heating and cooling cost.
The result cannot supply an airing time: wind, the opening, mixing between rooms and ongoing moisture sources are not measured. Read when opening windows reduces humidity for contrasting examples, or use the dew-point calculator to investigate a cold surface.
Method: NPL vapor-pressure relationships and ASHRAE’s ideal-mixture humidity ratio. The EPA moisture-control guide explains the indoor/outdoor dew-point comparison.