Mold loves humidity

Mold Loves Humidity

Mold Loves Humidity

In general, mold loves humidity! Humidity levels in your home should be between 40 to 60%.  More specifically, between 20 and 40% during the winter months and less than 60% during the rest of the year. This percentage reflects the amount of moisture in the air. However, checking your humidistat level is not enough in maintaining a healthy indoor environment.

Humidifiers contain water and as such, are a potential place for microbial growth.   Many homes have a humidifier attached to their furnace which releases humidity during the heating season.  There is a waterline that feeds the humidifier when the water level drops.  When the pan is full the floater shuts the water off.  Basically works like the water tank of a toilet.  A drum wrapped in pads absorbs the water from the pan as it rotates and forced air from the furnace fan travels through the wet pads sending humidity into the heating system to be delivered throughout the house.  Next to the humidifier is a humidity thermostat that controls the humidifier.

If not maintained properly, the potential for mold growth increases.  People who are sensitive to air quality issues will suffer as a consequence.  

In addition, if your ductwork is dirty, it becomes a suitable place for mold growth.  But I digress, the humidifier which is connected to your furnace must be maintained regularly.  That is, the pads should be changed monthly, the water reservoir drained and cleaned of lime and other residue deposits.   Whether there is a high or low level of humidity, mold cannot grow in air. Rather, it is the level of continued humidity found on surfaces that determines the potential for mold growth.  The relative humidity in your home may be 60% or less, but that does not eliminate the possibility of mold growth.  There could be areas of the house where humidity levels on substrates are above 70%.  Inspectors are equipped to check for areas of high humidity and potential locations for mold and or mold growth.

One of the most important components of a forced air furnace is the air filter.  A clogged filter will cause the furnace to run harder and longer.  If using disposable filters, change monthly.  If the system uses an electronic filter, have it maintained by a qualified technician as there are deadly high voltage conditions present.  Electronic filters are usually maintained seasonally.  Always turn off the electrical supply before servicing.

In a previous article I mentioned how a house works as a system.  The same applies here.  Let’s say that the furnace humidifier is properly maintained and every precaution followed.  If your ductwork is dirty, humidity gets trapped in the dust buildup and over time develops into the growth of mold.  Have the ductwork of your house checked to see if it requires cleaning.  There are companies that specialize in ductwork cleaning.  

I personally am not a fan of furnace installed humidifiers.  Most of these units that I come across are lacking in one thing or the other. Whether it be poor maintenance or poor functionality.  For those that are in good working order, are usually lacking proper regular maintenance.  That is, these are maintained seasonally by the technician when he comes to check on the furnace, and assume he checks the humidifier, and not regularly/monthly maintenance by the home owner.  It’s one of those things that are out of sight out of mind to the home owner.

There are other sources of humidity released into the house during the winter months such as from taking showers, cooking and house plants.  This activity in my view is more than enough to supply the house with ample humidity.  Furthermore, not using the furnace humidifier reduces the potential of unhealthy air.  

Remember, even if the relative humidity is within the acceptable parameters, keep in mind surfaces that are constantly damp to the touch.  Also, dampness may be found behind a wall or a ceiling or under or behind objects.  Inspectors have tools to detect potential issues that the naked eye cannot.  Another sign of high levels of humidity and thus potential mold growth, is a musty like smell in the air.  Not all molds are dangerous.  However,  if mold is discovered, it should be tested by a specialist to determine the type and level of toxicity.

Geoffrey Darwent CPI

Condominium Management & Residential Home Inspector, CPI

Gestion de condominium & Inspecteur des immeubles résidentiels, IPC

Assuré / Insured

Member InterNACHI

Membre InterNACHI-Québec                                                                                                                                   



Feature photo by Pietro de Grandi


Geoffrey Darwent
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