Humidity control tips

September 18, 2013

By Daryl Watters



One of the leading causes of mold in warm humid climates is humidity.
Everyone knows that leaks causes mold and that leaks must be repaired as soon as possible  to prevent mold growth. However few take elevated humidity seriously, people will live with high humidity for weeks, months, or even years and think nothing of it. Yet it is humidity that causes a thin sometimes almost invisible layer of mold to grow on clothes in our closets, on furniture, and on walls ceilings, and floors.

This surface mold is sometimes referred to as mildew, microscopic analysis will reveal the presence of mostly Aspergillus, Penicillium, and Cladosporium and others. Culturing or DNA analysis reveals much of the powdery white mold to be Penicillium chrysogenum and the black colonies to be Cladosporium. My specialty is dealing with these very specific types of mold problems. The above mold identity observations are based on my own analysis and lab analysis of hundreds of samples.

To prevent such growths from spreading throughout your entire property and potentially covering all your belongings follow these steps:

Most importantly stop humidity at it’s source. Do not keep windows and doors to the exterior open excessively on warm humid days.

Also in line with source control, ventilate bathroom air to the exterior during and for several minutes after all showers. After doing mold inspections and humidity investigations in South Florida for a decade I can safely say that outdoor air infiltration and showers are the two leading sources of humidity in homes in my area and likely yours as well.

Seal openings in AC closet walls and ceilings. You will not find this one in a book or any other source. I cannot find it anywhere but I can tell you without about that this one can and does cause humidity problems. Openings even a few inches across in an AC closet wall will allow the AC to pull humid air out of the walls. This humidity will then be free to circulate through your AC and through your property. Sure the Will remove some humidity but it can only do so much.

Set your thermostat to auto and not to on. This is another one of those tips that are not well known. Setting your thermostat to on will cause your AC to circulate humid air without drying it, this is obviously a bad thing. Setting it to auto will force the AC to dry and cool your air when it circulates it.

In commercial buildings introducing fresh outdoor air into your AC system can be beneficial, and even a building requirement, it dilutes indoor pollutants such as volatile organic compounds and body odor. However in warm humid areas outdoor air being pulled into the AC system will very often overwhelm the AC units ability to dry air and will cause condensation, humidity, and mold inside the system. Some of the buildings and homes with the foulest smelling mold problems are in such a condition because of the introduction of fresh outdoor air. This is truly a case of good intentions resulting in a very negative outcome.

Of course there are other factors to consider such as boiling foods, detached dryer vents, humidistat use, and improperly sized AC units. The above considerations however should cover most situations.  One other thing I did not mention is using a dehumidifier. These devices can be quite effective at removing humidity from your air but they do not solve the problem only mask it. It is almost always possible to cure the problem with a proper understanding of what is causing the problem.

A Accredited Mold Inspection Service, Inc. Is a mold testing and air quality consulting firm specializing in investigating mold and humidity problems in South Florida.

A Accredited Inspection Service, Inc.
6742 Forest Hill Blvd # 271, West Palm Beach, Florida 33413

The following info is from two different mold inspections conducted withen three weeks of each other.

The findings on these mold inspections and many other similar mold inspections conducted by us show how proper placement of your AC return is important in detering mold growth and mold odor in your AC ducts.

The inspector inspected the master bathroom as well as the nearby closets where past leakage occurred through a glass block window.The inspector detected two mild distinct odors in the master bathroom that appeared to be related to microbial growth.One of the odors was the result of mold growing in the duct work that services the master bedroom and master bathroom area.In this inspectors experience the specific moldy odor detected in the bathroom AC ducts is almost always created primarily by a type of mold called Cladosporium spherisperium. This is not a toxic mold and does not produce many spores but it creates musty odors that can cause health complaints. This mold like all AC mold needs two things to survive:1)   food2)   moisture.Food is from dust in your ducts so use a good filter in your AC.Moisture is from humidity drawn in through the AC return so control humidity intake.Your AC return is near the bathroom and as a result this return will allow the AC to pull much humidity in from the bathroom after showers and baths, this is called entrainment and this inspector has seen many AC mold problems resulting from of AC returns near humid bathrooms or in other areas where

excessive humidity is drawn in the AC system. Once in the cold ducts the humid air becomes even more humid this causes mold to grow in ducts.We recommend that the AC ducts be cleaned or replaced. If the humidity entrainment problem is not corrected then the mold will return so the AC return will have to be relocated to an area away from the bathroom, consult with your AC contractor about moving it to the master bedroom or master bedroom closet.In addition the bathroom would benefit from additional bathroom ceiling vents to vent humidity out of the bathroom after showers and baths.Above is a photo of your return just a few foot from your bathroom.SPECIFIC AREA FINDINGSAnother odor was detected in the bathroom, this odor was a sewer type odor and likely came from the tubs drain. Typically a P trap in your drain keeps these odors from backing out of the sewer and into our homes, but for some reason stronger than typical odors were coming up from your tubs drain or from the spa jet holes.Start by sanitizing your drain and spa jet pipes with a sanitizer but do not mix chemicals, also keep the drain stopper down when the drain is not in use, hopefully this is all that is needed, if this does not work contact a plumber.Have a plumber review and make any required repairs. This may not be a condition that every plumber is qualified to deal with as it is not the common problem of water simply evaporating from your trap, this may be something different, it may be very simple such as a dirty pipe drain. It may be complex like a missing trap.

SPECIFIC AREA FINDINGS The above mold inspection was conducted in Boca Raton Florida on 1-22-08. The same type of mold odor problem was found 3 weeks later during a mold investigation at a similar luxury home in Wellington Florida near West Palm Beach.   2-12-08 mold inspection  Musty odors were detected, musty or mold odor detection is an important part of any mold investigation because it lets us know that a little mold is present even if it cannot be visually detected. In addition, even when spore levels are low. Minor mold odors were in the following area(s) in the master bedroom when the AC unit was on. The odor detected was typical of that from AC ducts and is not like the odor produced by the many molds that grow in walls. It was detected near the ducts and only when the AC was on.  This was occurring because the AC return is near the master bathroom, this allows the AC to pull humidity into the AC after showers and baths. This humidity condenses in the AC and ducts and allows small amounts of mold to grow in the AC and ducts. This mold inspector has seen this exact same problem in similar homes, and the inspector has seen many mold problems caused under various similar circumstances when AC units pull humidity in, this is called entrainment.  To correct the condition the AC and ducts and main supply plenum for the master bedroom AC will have to be cleaned by a professional who follows NADCA standards and the AC return will have to be moved to a different location in the same zone, a location not so close to the humid bathroom.  Please be aware that this was not a serious mold problem and this does not mean that this is a moldy house, it is a nice house and did not have evidence of a serious mold probelm. This was just your typical minor musty or moldy duct odor and it may not be detected by all persons. Only sensitive persons with allergy or asthma would be affected by such a condition but based on this inspectors experience with this degree of odor some persons can be effects by such odors.

If the indoor humidity in a building is above 60%RH this may be a concern. If not corrected high humidity can contribute to mold, dust mites, and also condensation problems.

Maintain humidity between 30%RH and 60%RH.

In hot humid climates around the world the reason for humidity problems is not always, but is often related to improper AC use or improper AC design.

1) Holes in the AC closet walls, AC return walls, ceiling, or floors, are extremely common and are often ignored. AC closet holes are often made during construction or AC instillation to provide openings where AC plenums, AC refrigerant lines, condensation drain lines, and wires and wire conduits can pass through. In some climates holes may allow the AC to suck hot humid air in from the attic, crawlspace, or outdoors. When AC units pull in unwanted air we call this entrainment. Any holes in AC closet walls, ceilings, or floors, should be sealed.

2) Thermostats set to the on setting can cause a problem because the on setting causes the AC units blower fan to pull humidity in and distribute it constantly, even when the evaporator coils are not cooling and drying the air brought in by the constantly running AC fan. This is even more of a problem in commercial buildings because most commercial building AC systems are designed to intentionally pull air in from outdoors. In homes where the AC is in the garage, attic, or in an outdoor closet this is also a possible problem because the AC will pull air in through loose panels. The on setting is ok in some areas but in hot humid regions if the fan runs constantly because the thermostat is set to the on setting the amount of humidity drawn in by the AC can be more that the AC is designed to dry.

3) A buildings AC system should not provide more cooling capacity than the building was designed to have. In other words if you have to many AC units or your AC unit is to large for your home or building it can create a problem. Buildings in general should have roughly only one ton of cooling capacity provided by the AC system for each 600 square foot of conditioned area. To much cooling will cool the building and shut down the compressors long before the buildings air has had a chance to dry, thus contributing the the buildings humidity problem.

When humidity is a problem condensation often forms on the AC registers. One of the first places for mold to show up following humidity problems is in the AC ducts, and on the air handler’s blower fan, and insulation. In addition humidity related mold very often grows as small dusty white or green spots of pen asp mold only visible on black clothes and shoes in the closet.

Daryl Watters is president of A Accredited Mold Inspection Service, Inc. He provides home, mold, and indoor air quality investigations in South Florida. He is also the creator of MIR forms designed to aid inspectors in the production of computer generated indoor air quality and mold inspection reports.


Some people believe that AC ducts are destined to become moldy. While all AC ducts will likely grow some mold and will become discolored with visible mold or visible dust to some degree, it is not normal or healthy for ducts to become excessively moldy, or to produce a strong mold odor.

Mold spores from AC ducts result in health problems and even mold odors in the absence of spores have been shown to cause health problems. Keeping ducts free of excessive mold is important.

The information in this blog is intended to provide readers with information on:

1) Some reasons why AC ducts become moldy.

2) The importance of proper sizing of AC units.

3) In addition it draws attention to a very common indoor air quality problem called entrainment. Entrainment is when AC units pull contaminated air from inside attics or walls or other undesirable areas.

The following information is taken from a Broward County mold inspection we conducted on 11-17-07 it shows an actual example of how ducts became moldy and gives recommendations at the end.

The property inspected is a one story single family Broward county house built in 1974.

Client reported mold growing on AC vent registers. The mold started a few weeks ago. The client reported that a new larger AC unit was installed about 3 months prior to the mold starting. This inspection is for mold conditions at the AC registers only.

mold humidity and AC instillation

Location: Garage:

In the above referenced area the inspector observed a new air conditioner. The serial number on the unit indicates that it is a 4 ton unit, thus is it rated to cool a 2400 sf house. This AC unit is just a little large for this house.

The inspector also observed a new condenser / compressor unit. The serial number indicates that it is a 3.5 ton unit thus designed to cool a 2100 sf house. This is also slightly large for the house.

You have a newer larger AC unit.

The size of this newer AC unit is a major reason for why you have humidity related cladosporium on your AC registers that were previously not moldy.

The previous AC unit in use about 3 months ago was smaller than this AC system thus would have been more efficient at drying the houses air before cooling the houses air and shutting down. If an AC is to large it shuts off after the air is cooled but before the air is properly dried / dehumidified. Air that is not dehumidified and has a humidity above 60%RH to 65%RH can and often does cause humidity problems in Florida homes.

The above photo is of cladosporium mold growing in your registers.

Another reason why you have mold problems now is because of the two holes in your AC return. These holes in the wall studs allow the AC to pull humidity into the AC each time the AC turns on, this phenomenon is called entrainment and is a common cause of mold and humidity problems.

Yes all metal studs have these holes, but it is not typical for these open holes in the studs to be exposed and in such close proximity to an AC return.

Where does the humidity in your walls come from? it comes room the attic, the attic in turn gets the humidity from outside via the soffit vents like the one in the above photo.

Why did this problem not occur till recently? the older smaller AC was more efficient at drying air as stated above and thus was able to handle the humidity load created by these openings.

AC duct mold air conditioner mold

More photos of mold on and around your AC registers. Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold. This mold problem is caused by elevated humidity in your ducts.

This is not likely to cause a toxic reaction, but this mold may result in allergic reactions or act as an asthma trigger in sensitive persons. The real concern is if it is growing excessively deep in your ducts then you will have much more mold than you can see on the registers.

Mold growth in the ducts can and often causes health complaints.

Excessive mold was not observed in your AC unit but you are recommended to have your ducts cleaned in accordance with NADCA standards or replaced.

More photos of mold on and around your AC registers. Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold.

Based on this inspectors observations and conclusions your inspector recommends using good quality foil mastic tape or other methods to seal two openings in your AC return wall studs.

Have a AC company install a humidistat and set it to around 50% to 60%RH always maintain indoor humidity between 30% and 60%RH.

Most people live with a little mold in the ducts and have no problems. Mold spore levels in your air was very low. No mold odors were detected. Much of the mold appeared to be primarily in the ends of the ducts just behind the registers.

Have an AC company clean the mold from the registers and from the ends of the ducts just behind the registers.

Duct removal or good professional cleaning in accordance with NADCA standards is recommended if additional hidden mold is discovered inside the ducts during cleaning.

Daryl Watters is president of A Accredited Mold Inspection Service, Inc. He provides home, mold, and indoor air quality investigations in South Florida. He is also the creator of MIR forms designed to aid inspectors in the production of computer generated indoor air quality and mold inspection reports.

In coastal Florida if your doors and windows are weathered and leak excessive amounts of humid air, a heavenly ocean view can allow for conditions that result in a hellish AC mold problem.

The information in this blog give a text book example of how warm tropical ocean breezes infiltrating into an AC system can result in humidity, condensation, and mold in and around your AC ducts and registers.

This information is from a Broward county Ft Lauderdale mold inspection that took place on 11-8-07.

Client moved in as a renter 7 days prior to this inspection. Client reported that his fiancee moved into the property with him and enjoyed the property, but because of severe allergic reactions that occurred while in the property she moved out after just one day.

Client requested this investigation to help determine if mold or other similar bio-allergen may be at the property that may account for her complaints.

The inspector observed that mold was a problem in the AC unit and in the AC ducts at the property, this mold requires food and moisture to live.

As is almost always the case in moldy AC units the mold in the AC and ducts received it’s moisture from elevated humidity and from condensation in the AC and ducts. It recieves it’s food from dust that bypasses the filter.

Humidity entered the AC and ducts by first entering from outside.

The view outside the unit is excellent, it is this high ocean front vantage point combined with leaky sliding glass doors that provided the moist breezes that allowed this mold problem to occur.

In addition the master bathroom is not vented to the exterior, whenever someone takes a shower that shower steam can enter the AC unit and form condensation as well. Either the non vented bathroom, or the leaky sliding glass doors overlooking the ocean alone would have been enough to cause a problem.

The sliding glass doors did not fit air tight, in fact the inspector could feel a strong ocean breeze blowing through the sliding glass doors from 2 foot away even though the doors were closed as tight as possible. Humidity blowing indoors from outdoors often results in mold and humidity problems, this is sometimes called the paston effect.

Gaps in these old doors allow much moist ocean air in.

Mold requires not just moisture but also food. The mold in the AC system and ducts at this property received it’s food from dust as does most AC mold.

In the above photo you can see a hole where dusty air can bypass the AC filters and enter the AC unit.

Dust can also by pass the filters because they are very low quality and do not fit properly, dust goes around the filters.

AC mold and condinsation problems

During the mold inspection the inspector observed that condensation sometimes forms on the AC registers.

The inspector observed the water stains left behind by condensation dripping off the AC registers. Note that condensation forming on AC registers is not normal, this condition indicates that so much humidity was present in the property that it condensed out on cold metal & glass registers that typically reach around 60F or so when in operation. The natural progression of this condition is for mold and condensation and water damage to form on the drywall around the register if the humidity problem is not corrected.

Elevated humidity and condensation inside ducts and AC units causes mold to grow in ducts and AC units just as it developed on the visible sections of the outside of registers. It is common for homes with mold on registers to have much more mold inside the ducts and AC unit because the interior of the ducts and AC is much colder thus much wetter than the surfaces of the registers, also the interior is duster thus has more food for mold growth.

Because mold in the AC unit has better growth conditions than mold on the registers it exhibits dimorphism. In other words these mold look different in different environments. Most of the mold in the AC unit is the exact same type that formed on the registers but being in a more healthy environment with more food and water inside the AC unit it grows more thick and lush and sometimes takes on a fluffy grey velvety appearance. In each case the mold is cladosporium sphearisperium.

Mold inside AC units does not produce many spores but instead grows and reproduce more vegetatively. This is significant because mold in AC units do not produce many spores but will grow profusely feeding off condensation and dust. AC mold by products are not lots of spores but are Mvoc’s, these Mvoc’s are foul smelling moldy odors that are released by the molds and this is why the clients fiancee got sick.

It is a well accepted fact that Mvoc’s or mold odors produced by AC type molds and other molds can and very often do result in strong allergy like reactions and irritations.

In this inspectors experience AC units that grow mold in the amounts seen at this property very often produce health complaints in building occupants.

In conclusion the AC unit’s ducts, registers, blower fan, evaporator coil, condensation pan, wires, and most of all interior fiberglass insulation are infested with more than normal amounts of mold growth, and the grow is extremely consistent with conditions that this inspector has observed to result in both odors and health complaints.

Because the AC unit is extremely contaminated with mold and produces strong foul odors it is recommended that the unit be professionally cleaned in accordance with NADCA standards, or the unit should simply be replaced because cleaning the unit will likely result in damage to the very rusty pan and coils.

Because of the amount of contamination in the ducts it is recommended that the ducts be replaced. Cleaning fiberglass ducts is not recommended they are supposed to be replaced if contaminated. Note that the interior of all sections of the ducts will not be visibly moldy but expect mold contamination to have spread to many different duct sections.

The following photos are of mold on AC registers, in the AC ducts, and in the AC unit at this property. Note that the ducts should all be bright yellow and resemble a tennis ball in color and texture.

AC duct mold

Typical cladosporium mold colonies on an AC register. Nearly all registers at the property were visibly moldy.

black air conditioner mold spots

Close up of cladosporium mold colonies on the AC register.

HVAC mold

Close up of mold in the duct, this duct used to be yellow.

Mold in the AC unit. The white mold is likely pen asp and the grey mold is cladosporium sphearisperium, it is the same type of mold observed as black spots on the register, but it looks different inside the AC because of different environmental conditions inside the AC.

Mold on the AC unit panel insulation.

Fluffy grey cladosporium sphearisperium mold on the blower fan inside the AC, this is very common.

About the Author:
Daryl Watters is a certified indoor environmentalist CIE # 1952 who provides mold testing and IAQ consultation services in South East Florida.