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Indoor Relative Humidity vs. Outdoor Relative Humidity and Temperature

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When cold humid outdoor air is supplied and heated in a building - the relative humidity of the air is decreased.

This heating process can be visualized in a psychrometric chart or in a Mollier diagram - as below:


Example - Indoor Relative Humidity of Heated Outdoor Air

Outdoor air with dry bulb temperature 0 oC and relative humidity 90% is heated to an indoor temperature 20 oC.

The process is visualized in the Mollier diagram above with the process from A to B along the constant specific humidity - x - line. From the diagram we can estimate the indoor relative humidity to approximately 23%.

Note! - indoor supplies of vapor to the moist air - like latent heat from humans, animals or production processes - is not counted for. Latent heat supplied to the indoor air will move point B in the diagram to the right - to higher relative humidity. Sensible heat supplied to the room will move point B further up in the diagram.

Example - Indoor Relative Humidities for a given Outdoor Temperature and Relative Humidities

The table below illustrates the change in indoor relative humidity for a given outdoor temperature and relative humidities.

Indoor Relative Humidity vs. Outdoor Relative Humidity and Temperature
Relative Humidity
Relative Humidity
-7 (oC)
20 (oF)
0 21 (oC)
70 (oF)
20 3
40 6
60 8
80 11
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Related Topics

Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

Heating Systems

Design of heating systems - capacities and design of boilers, pipelines, heat exchangers, expansion systems and more.

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Health vs. Relative Humidity

The optimum relative humidity ranges for bacteria, viruses, fungi, mites, asthma.

Indoor Comfort Temperatures vs. Outdoor Temperatures

Recommended indoor comfort temperatures vs. outdoor temperatures.

Moist Air - Relative Humidity

Relative humidity in moist air is the ratio of partial vapor pressure to air pressure.

Outdoor Temperatures and Relative Humidity - US Winter and Summer Conditions

Outdoor summer and winter design temperatures and relative humidity for US states and cities.

Recommended Relative Humidity

Recommended relative humidity level.

Relative Humidity in Production and Process Environments

Recommended relative humidity in production and process environments - like libraries, breweries, storages and more.

Room Sensible Heat Factor - RSHF

Room Sensible Heat Factor - RSHF - is defined as the sensible heat load divided by the total heat load in a room

Sensible Heat Ratio - SHR

The ratio Sensible Heat (or Cooling) Load to the Total Heat (or Cooling) Load.

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