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Indoor Relative Humidity vs. Outdoor Relative Humidity and Temperature

Recommended indoor relative humidity vs. outdoor relative humidity and temperature.

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
Outdoor
Temperature
Outdoor
Relative Humidity
(%)
Indoor
Temperature
Indoor
Relative Humidity
(%)
-7 (oC)
20 (oF)
0 21 (oC)
70 (oF)
0
20 3
40 6
60 8
80 11

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.

Related Documents

• 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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Citation

• The Engineering ToolBox (2008). Indoor Relative Humidity vs. Outdoor Relative Humidity and Temperature. [online] Available at: https://www.engineeringtoolbox.com/temperature-relative-humidity-d_1118.html [Accessed Day Month Year].

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5.16.12

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