Relative humidity

August 2021

1. Description

Relative humidity (%) expresses the degree of saturation of the air from the ratio between the amount of water vapor contained in the air (absolute humidity) and the maximum amount of water vapor that the air would be able to contain at the same temperature (saturation humidity). Its value, therefore, depends on the indoor air temperature and is a clear indicator of the amount of water vapor present in the indoor air.
Absolute humidity (g/m³) is the amount of water vapor contained in a given volume of air.

2. Recommended levels

The ideal relative humidity for a normal indoor temperature in a living space is between 45 and 50% humidity, with a range of recommended values between 40 and 60%.

3. My inBiot Ranks

‍4. Unit of measure

The relative humidity of indoor air is expressed as the ratio (%) between the amount of water the air actually contains at a given temperature and the amount it could contain if it were saturated at the same temperature.

5. Sources

The most common sources of high relative humidity in an indoor space are due to excess water vapour contained in the indoor air, which can be caused by:

  • Condensation: this is one of the main causes of the appearance of humidity in an interior space. This is generated when the interior air, charged with water vapor due to the normal use of a building (breathing, evaporation of water in kitchens or bathrooms...), comes into contact with the cold surfaces of exterior enclosures.
  • Capillarity: capillary dampness occurs due to the natural process of capillary rise of water contained in the subsoil through the building materials in contact with the ground (foundations, screeds, baseboards), without an adequate horizontal waterproof barrier to prevent this capillary rise.
  • Construction pathologies due to incorrect execution of construction details and/or waterproofing, which lead to water seepage inside the building or to the wetting of the construction materials. This means, on the one hand, the loss of their basic physical properties, as well as a higher risk of mold proliferation due to a consequent high relative humidity rate.
  • External weather conditions with high ambient humidity.
  • Building use: Human occupation is a source of water vapour emission. A person at rest emits approximately 60 grams of water vapour per hour, depending on indoor conditions of humidity and temperature, as well as activity.

6. Benefits of optimal levels  

During the process of breathing in and out, with a physiologically adequate humidity of 45-50%, the exhaled air is able to absorb more water vapor than with a higher relative humidity. And this fact is beneficial because during exhalation we expel toxic substances resulting from our own metabolic activity.
The relative humidity should be maintained between 40 and 60%. Occasional and slightly lower values should not be cause for alarm. The relative humidity depends on the ambient temperature, so a slight drop in this parameter will increase the RH.

7. Risks of inadequate levels

The humidity contained in the air greatly modifies its properties and greatly influences the comfort of people. From a physiological point of view, it is more difficult to breathe in a room with high humidity. The proportion of pathogenic germs in the air also increases and there is a greater likelihood of mould growth, with a higher risk of developing fungal diseases and allergic reactions due to contact with spores and moulds. Humid air causes a feeling of fatigue and generates unpleasant odours. In addition, there is an increased deterioration of building materials, higher energy consumption and an increased risk of mould growth.

On the other hand, very low humidity levels contribute to irritation and dryness of the respiratory and ocular mucous membranes - dry throat, colds, dry eyes, skin complaints. In addition, there is a greater proliferation of dust and micro-organisms in suspension and electrostatic charge on the surface of walls and furniture.

8. Recommendations for improvement

The basic recommendations to ameliorate the effects of excessively high or low relative humidity are mainly based on the following recommendations:

  • Avoid any situation with RH >60%. This involves monitoring the main sources of moisture generation inside the building, possible thermal bridges, surface and interstitial condensation, possible leaks or possible construction pathologies that cause this high relative humidity.
  • Use finishing materials and equipment that do not generate or accumulate dust.
  • Use filtering and vacuuming systems with high efficiency HEPA type filters and properly clean any accumulation of dust in radiators to avoid airborne dust.
  • Encourage the use of materials with hygroscopic properties that allow the damping of humidity fluctuations due to normal use of the building (kitchens, bathrooms, bedrooms).
  • Optimize the ventilation system to ensure the necessary spot ventilation for peak indoor humidity production and continuous ventilation for optimal indoor air quality.
  • For continuously low relative humidity, the use of houseplants is recommended.
  • The use of humidifiers or dehumidifiers should be monitored because of the high risk of promoting the growth of germs. If the use of humidifiers or dehumidifiers is essential, it is recommended that their operation be programmed on the basis of actual measured values.

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