UPDATED: September 29, 2020
Seldom does a day go by that there isn’t a report of apartment mold somewhere in the US resulting in property loss or damage, serious health effects and legal implications. A quick Google News search for “apartment mold” returns about 75,300 results with numerous reports from across the country made within the past few weeks.
While working with hundreds of developers, property managers, mechanical engineers, and design consultants over the past few years, it has become apparent that a singular explanation as to what causes mold in apartments does not exist. There is universal agreement however, that the presence of mold results in lost revenue, remediation costs, bad publicity, health issues and potential legal consequences.
Estimates for moisture and mold-related residential failures in the United States now range in the billions of dollars annually. At the center of the debate is the issue of responsibility between landlords and tenants. By educating both landlords and tenants, we can implement moisture control solutions, maintain healthy apartments and keep mold growth outside apartments where it belongs.
Moreover, the lack of a common understanding relative to what causes mold often permeates the media, government building officials and even the court systems. This article is intended to provide a basic understanding of what causes humidity and mold growth in apartments.
Moisture Management can Help Prevent Mold
Excess humidity can cause condensation in apartments.
Condensation occurs when moisture in the air “condenses” into liquid water on a cool surface. The surface temperature at which this occurs is known as “dew point”.
In cases involving water leaks, tenants should promptly notify the landlord that water is leaking into the unit. By doing so, the tenant is acting to protect the structural integrity of the dwelling while preventing the growth of mold that will likely result from leaks that are not reported or repaired promptly. More frequently than physical leaks is the presence of excess moisture in the air (high relative humidity). A common example might be unwanted condensation on windows.
Accumulated moisture on window sills may leak into wall cavities causing hidden mold growth on wood and paper backed wallboard. The concept of condensation is important. Condensation occurs when moisture suspended in the air (water vapor) “condenses” into liquid water on a cool surface. The surface temperature at which this occurs is known as the “dew point” temperature. Dew point can be controlled by controlling BOTH room temperature and relative humidity (RH).
Four Factors must be present for Mold to Grow:
• Mold Spores
• Organic Materials (food source)
Chronic excess moisture in apartments will result in mold growth. 
Mold spores are naturally occurring and present EVERYWHERE.
Nearly every surface of an apartment and its furnishings are covered with microscopic mold spores which are unavoidably introduced through door openings, human or pet transfers and ventilation. Mold spores lie dormant until moisture activates growth. As long as moisture is available, mold will continue to grow expanding from one surface to another. The proliferation of mold causes damage to property and results in negative health consequences. Organic items (food source) include furniture, clothing and construction materials.
Moisture is the only factor that can be sufficiently controlled.
Temperature control is affected by mechanical limitations, ambient environment and tenant preference. Oversized HVAC systems are common in multifamily housing properties. It is important to note that conventional air conditioning systems operate based on sensible (temperature) input. Water vapor removal by air conditioning systems depends on how long the air conditioner runs and is secondary to the primary function which is temperature control.
A good example of this scenario would be senior housing properties. These apartments are typically smaller than the average family housing unit. The smallest conventional split air conditioner supplies 18,000 BTUs of cooling (or 1.5 tons) which may be more cooling than is required. For many senior apartments this means the tenant is forced to decide between “overcooling” and setting the thermostat to a lower temperature in order to adequately dehumidify. More often than not, the senior occupant will choose not to run the air conditioner resulting in excess moisture and in some cases mold.
Another factor to consider is high performance design. Trapped moisture has become the unintended consequence of tightly constructed, energy-efficient buildings. While tighter building envelopes drive down the sensible cooling load and save energy by reducing air conditioner run time, they also trap excess moisture inside apartments. In a tightly sealed apartment, moisture accumulates from normal occupant lifestyle activities. As stated earlier, conventional air conditioners do not operate based on water vapor content or relative humidity. The typical apartment HVAC system serves only one purpose, to raise or lower temperature. When mold shows up in apartments, it isn’t the heat, it’s the humidity.
How to Prevent Mold in Apartments
High indoor relative humidity can cause mold growth in apartments
- Do not use ventilation when outdoor dew point is above 55. 
- Make sure the clothes dryer is vented properly.
- Cook with lids to help contain steam.
- Do not dry clothing on indoor clothes lines or racks.
- Immediately clean up spills and messes.
- Reduce water vapor by using effective bathroom, kitchen, and utility room exhaust fans above common sources of moisture. Verify exhaust fans are actually moving air (hold a tissue below vent to check air flow).
- Confirm appropriate indoor relative humidity level using a reliable digital thermometer/hygrometer.
- Keep relative humidity below 60 percent at all times and use a dehumidifier when necessary. 
Why Dehumidifier Size Matters
Using a dehumidifier is the most reliable way to ensure appropriate levels of relative humidity are maintained at all times inside apartments.
Whole house dehumidifiers are appropriately sized for larger, single-family dwellings. While they will adequately remove excess moisture from apartments, common complaints include excessive noise and heat displacement as well as lack of operational control. If a tenant elects not to operate the air conditioner unit, they are unlikely to operate a dehumidifier. Whole house units are also controlled by a wall mounted dehumidistat, similar to thermostat. Additionally, whole house dehumidifiers are installed in conjunction with the exiting HVAC air handler which requires integration into the mechanical system. Installation of these systems require a licensed HVAC technician.
Portable dehumidifiers may be less costly, but present other types of challenges when attempting to control humidity inside apartments. They are inconvenient, often require manual emptying and also rely on tenant cooperation to operate. Is relying on tenants to properly maintain a portable unit worth the risk? In many situations it is impractical to expect tenants to effectively operate dehumidifiers. The complaints from tenants in regards to portable dehumidifiers range from too loud, releases hot air into the apartment, safety hazard, and even the nuisance of remembering to empty the tank. Which is counterproductive to the moisture removal process, since mold can grow almost anywhere moisture is present.
Continuous Moisture Control
Currently there is only one versatile dehumidifier specifically designed for apartments, available in the US. The award-winning, ENERGY STAR® Certified IN Wall/ ON Wall IW25-4 dehumidifier by Innovative Dehumidifier Systems. The IW25 puts apartment humidity control directly in the hands of landlords and property owners. Created to provide daily moisture control, the permanent dehumidifier system includes a tamper-proof cover that can only be removed using a specialty tool. The IW25 can be quickly installed inside an interior wall between existing studs or hung directly on wall. Once installed, it operates independently of the HVAC system to quietly and efficiently remove excess moisture from inside apartments. Designed for hands-free moisture management the dehumidification system, drains directly into current plumbing lines, or the optional internal condensate pump (purchased separately), installs into the unit to remove collected condensation.
For landlords and tenants who are searching for a cost-effective and efficient solution to prevent mold and remove humidity in apartments, the IW25 dehumidifier for homes makes a lot of sense.
 Moisture Science 101, Dan Welkin; https://www.acca.org/wp-content/uploads/2014/01/Welklin-Class-notes-Moisture-Science-101.pdf
 A Brief Guide to Mold, Moisture and Your Home, United States Environmental Protection Agency; https://www.epa.gov/mold/preventionandcontrol.html