1. Humidity of indoor and outdoor air
Marianna Luoma

2. Relative humidity of air
The absolute humidity of the air is the actual amount of water vapour contained in the air
Percent of relative humidity measures how much water vapour is in the air compared to how much there would be if the air were saturated at the same temperature.
Water content and relative humidity of outdoor air is shown in the table (you will be given a copy of that table)
What is the average relative humidity of outdoor air in Jyväskylä in February? What is the average absolute humidity in February?

3. Mollier-diagram
The dry-bulb temperature, Air temperature measured with an ordinary thermometer
Relative humidity
Absolute humidity
Dew-point temperature
The wet-bulb temperature, Temperature of a wet thermometer

4. The relative humidity of indoor air in single family houses
The relative humidity of indoor air depends on the outdoor air conditions, air exchange rate and the production of humidity indoors
The water content of the indoor air is often higher than in the outdoor air
The increase in indoor air humidity was 1,8 g/m3 in single family houses in winter time (≤5 ºC) and 0,5 g/m3 during the other times of the year
The production of moisture was in general 5,9 kg/day in single family houses
As the humidity of the indoor air increases, the risk for mould problems increases, too
Reference: Puurunkoisten pientalojen kosteus- ja lämpötilaolosuhteet, ilmanvaihto ja ilmatiiviys, Tampereen teknillinen yliopisto, 2005

5. The average production of moisture kg/day (according to literature)
Human being 0,9 kg/day
Dog/cat 0,4 / 0,1 kg/day
Cooking 0,8 kg/day
Indoor plants (about 5 plants) 0,4 kg/day
Shower 0,3 kg/day
Sauna 1 kg/day
Washing and drying clothes 1 kg/day

6. Effects of indoor air humidity
Indoor air humidity has many indirect effects that are not positive
The harmful effects of some factors increase, when the relative humidity decreases.
Look at the picture. What are these factors?

7. …when the humidity decreases…
Dustiness
Airway infections, irritation of skin and mucous membranes
Static electricity chocks

8. The harmful effects of some factors increase, when the indoor air humidity increases. What are these factors?

9. … when the indoor humidity increases…
Moulds
Other microbes
Hygiene risks
Emissions from building materials
Corrosion of metals

10. The harmful effects of some factors increase, when the indoor humidity either decreases or increases. What are these factors?

11. … when the humidity either decreases or increases …
Bacteria
Virus

12. Guidelines for indoor air humidity: D2
”Buildings shall be designed and constructed in such a way that the humidity of indoor air will remain within the values specified for the intended use of the building.”
”The humidity of indoor air shall not be harmfully high on a continual basis, nor shall humidity be allowed to concentrate on structures or on their surfaces or in the ventilation system in such a way that it will cause moisture damage, growth of microbes or micro-organisms or any other health hazard.”
”In order to minimise any harmful effects that may be caused by a low relative humidity of indoor air, unnecessarily high room temperatures shall be avoided during the heating season.”

13. Finnish Indoor Air Classification 2008
The design value for indoor air relative humidity is >25 % in category S1 (the best gategory)
The indoor air humidity may occasionally drop below the design value during the winter time.
The indoor air relative humidity has to be below 60 %.
There are no values given for Gategories S2 and S3

14. Application Directive for the Decree on Housing Health. Part I
Application Directive for the Decree on Housing Health. Part I. National Supervisory Authority for Welfare and Health. 8/2016*
Indoor air humidity may change a lot depending on the outside air humidity and the activies in the residence.
Earlier, the recommended level of humidity was %, but due to the climatic reasons, even 20 % may be difficult to maintain. That does not cause a problem from the health point of view.
60 % RH may cause condensation on surfaces during the cold period of the year.
*Asumisterveysasetuksen soveltamisohje, Osa I, Ohje 8/2016, Valvira (in Finnish)

15. Continues
The indoor air humidity should not be evaluated just as relative humidity, but also as the increase of absolute humidity due to activities (breathing, showering, cooking, laundry) inside the residence.
Absolute humidity increase of over 3-4 g/m3 in indoor air increases the risk for condensation of surfaces.

16. Report CR 1752 (international)
Humidity has an influence on the thermal sensation.
However, in moderate environments the air humidity has only a modest impact on the thermal sensation.
Typically a 10 % higher relative humidity is felt as being as warm as a 0.3 ºC higher operative temperature. (page 15)
Normally few problems (for human beings) occur when the relative humidity is between 30 % and 70 %, assuming that no condensation takes place. (page 30)
Relative humidities between 30 % and 70 % in the room do not guarantee the absence of humidity problems in the building.