intelligent FRESH AIR systems

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Frequently Asked Questions

5.  Why are Relative Humidity (RH) levels important in your home?

It is accepted that the indoor relative humidity often exceeds outdoor relative humidity. This is mainly due to activities within the home ( showers, cooking, washing, dryer, etc)

Optimum Relative Humidity Chart
(Click on Chart)  

The optimum RH zone for the wellbeing of the building as well as the health of its occupants is between 30 and 55 % relative humidity.

Outside of this range the growth of bacteria, fungi, viruses as well as increased chemical offgassing and ozone production is promoted.

Allowing outside air to enter the house through fixed vents (uncontrolled ventilation) or the use of windows and doors will only improve humidity levels in the home occasionally. However, the relative humidity level of the outside air is often too high or too low to create a healthy indoor environment. (see RH Levels Chart below for Sydney and Melbourne.)

To add to these problems, on still days insufficient airflow occurs and so pollution levels can increase in the home. Windy days could provide sufficient air changes to remove indoor pollutants, however comfort and energy costs can also be compromised by outdoor drafts reducing the effectiveness of heating or cooling.

A phenomenon known as 'Short-circuiting' can also reduce the effectiveness of 'natural' ventilation through fixed vents. This occurs when fresh air flowing in through fixed vents is not able to flow throughout the entire home. Long hallways, closed doors and other barriers limit the flow and mixing of air. Also much of the fresh air entering the home can often leave through another vent before thoroughly mixing with the indoor air. The result is 'dead spots' where air is stagnant or exchanged less often.

Another disadvantage of the fixed vents system is the uncontrolled entering of pollen, and traffic pollution into the indoor environment together with the fresh air.

The above problems can only be solved by creating a well sealed building envelope and introducing a balanced mechanical ventilation system. This will give the occupant full control over indoor air quality, relative humidity and comfort levels in the home.

Outdoor Relative Humidity (RH) levels - Sydney & Melbourne - 1996

Sydney   Melbourne
Month Max Min Max Min
January 69 61 60 44
February 73 64 64 46
March 72 62 67 47
April 71 57 72 51
May 72 55 78 58
June 74 57 80 63
July 69 50 79 61
August 66 50 74 57
September 62 51 68 52
October 61 56 63 50
November 62 57 62 48
December 65 59 60 45

These levels are monthly averages. Readings are taken at 9 am and 3 pm. Statistics on February 3, 1997 For long term data for any location in Australia see the Bureau of Meteorology

Note - most months register in both cities at a level that is above the optimum (healthy) zone of 30% to 55% RH.

If you suspect unhealthy conditions in your home due to excessive humidity, then there is a tool known as a Thermo-Hygrometer to find out what is going on. This handy and healthy tool registers (digitally) indoor and outdoor temperature and indoor relative humidity levels. It memorises the minimum and maximum temperature and relative humidity readings as well as providing a read out of present conditions. Using a Thermo-Hygrometer is the best way to learn about and understand your indoor environment.

Sources:
* Canadian Home Builders Association, Building Manual,    section 2: Aspects of building Science.
* Residential Indoor Air Quality & Energy Efficiency. Peter du Pont & John Morrill,
   American Council for Energy Efficient Economy
* Australian Bureau of Meteorology

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