An infiltrometer blower door test measures building tightness. Tight homes tend to have excess humidity in winter; leaky homes often have humidity problems in summer.

If there’s an indoor mold problem, all authorities agree that the mold needs to be cleaned up.  However, mold problems can be traced to moisture problems. No moisture, no mold growth.  Identifying and eliminating the underlying moisture source - or other cause of excess humidity - is just as important as mold cleanup.

What does this have to do with HVAC? A lot.

While there are many moisture sources the HVAC industry isn’t expected to diagnose or remedy - plumbing leaks, roof leaks, and exterior wall flashing problems, just to name a few - HVAC often is the culprit for mold growth. At the same time, it also can be the solution. 

As an HVACR contractor, you really can’t just turn a blind eye and say, “We’re not in the mold business.” You must know that HVAC equipment creates, transforms, transports, and/or controls moisture and humidity. Don’t try to stick your head in the sand.

An HVACR contractor must fully understand residential moisture, ventilation, infiltration, and psychrometric issues in order to properly serve its customers - and, stay out of mold trouble. All these air and humidity issues also directly affect comfort and efficiency, and are opportunities for increased sales and profits.


In cold climates, most leaky homes need a humidifier to maintain the desired minimum relative humidity of 25 percent. However, there can be too much of a good thing. Humidifiers can over-humidify a house if not properly controlled.

In cold weather, high humidity leads to condensation on cold surfaces, such as windows. In extreme cases, sweating and mold can also occur on walls in stagnant areas like closets or behind furniture. If the principal cause of the high humidity is the central humidifier, the client needs to be educated that the indoor humidity must be lowered in very cold weather. New controls are available that will automatically provide this feature.  

Know that humidifiers can cause moisture and mold problems when they develop water leaks. Inspecting for water leaks on maintenance and service calls is a good idea to keep a small problem from becoming a big one.

Know, also, that combustion gases contain a lot of water vapor. Burning 100,000 Btu of natural gas releases 1 gallon of water. If this water is released into the home, it can lead to excessive indoor humidity and mold. Therefore, inspect to ensure that vented gas appliances are actually venting.

Common causes of venting failure are chimney deterioration and back drafting caused by negative pressure in the combustion appliance zone (CAZ). Negative pressure can be caused by many things, including return duct leaks in the CAZ, large exhaust fans, supply duct leaks outside the house, powered attic ventilators, and basement zoning dampers.

Understanding how the house works as an interactive system is essential for solving many back drafting and spillage problems. Correcting them will not just get the moisture out, it will also improve IAQ by venting CO and other combustion gases.


Two-pipe, 90-plus percent furnaces and power-vented water heaters can sometimes contribute to excessive indoor humidity in the wintertime. They send less air up the chimney, thereby creating less dry air infiltration. This isn’t a reason to not sell them, as the higher humidity is usually welcome in a dry home. Just be aware that it could be a contributing factor in a home that develops excessive humidity after a change-out.  

Condensing high-efficiency furnaces produce condensate, which must be drained away. Condensate leakage can cause significant problems.

Another related moisture issue for sealed-combustion furnaces is that in very cold climates, indoor humidity can condense on the outside of the cold air intake pipe and drip onto items, typically in the basement. Either insulate the pipe or reduce the overall humidity in the house.

In the winter, a leaky house tends to be a dry house, and many are leaky. However, if a house is tight, the normal indoor-generated moisture can become trapped, leading to high indoor humidity levels.

New homebuilders have real liabilities if they build a tight house with poor mechanical ventilation. A blower door infiltration test will tell you what’s going on. All homes should have mechanical ventilation capability, and in tighter homes it should be run routinely.

An energy recovery ventilator (ERV) and/or heat recovery ventilator (HRV) is one option.  Others include quiet exhaust fans, or outside air intakes into the return, or a dehumidifier.  Inadequate bathroom ventilation is a common contributor to mold growth.


In many cases, the a/c system doesn’t do as good a job of removing moisture in the summer. Common reasons include:

• Oversized a/c equipment that short cycles, reducing moisture removal (caused by many factors).

• Higher SEER a/c equipment with lower moisture removal capability. (Some do, but some don’t.)

• Improper a/c refrigerant charge.

• High airflow (e.g., over 450 cfm per ton in a hot, humid climate), caused by mismatched equipment, defaulting to high speed for cooling, and no commissioning. This leads to inadequate dehumidification and sometimes condensate blowing off the indoor coil.

• Constant summertime air circulation, which causes re-evaporation of moisture from the coil and drain pan, and accelerates humid air infiltration due to duct leakage and pressure imbalances.

The answers are to size aggressively, commission properly, and discourage constant fan in the summer.

In the summer, a leaky house is often a humid one. For most of the Eastern United States, the summertime outside air contains large amounts of water vapor. The infiltration of outside air in the spring, summer, and fall can bring in significant amounts of water vapor from outside (the latent load).   For example, 100 cfm of outside air at a 70° dew point will bring over 10 gallons of water into a home over 24 hours. 

Whether the air comes in unintentionally through duct leakage or the envelope, or intentionally via mechanical ventilation, the moisture still comes in with it.


Here are some common causes of excessive air exchange that can lead to chronically high indoor humidity levels:

• Excessive summertime humid air infiltration, due to a leaky house envelope.

• Excessive summertime humid air infiltration, due to a leaky duct system.

• Negative air pressures and high infiltration rates caused, in part, by powered attic ventilators and interior door closure.

• Infiltration at the ceiling supply duct boot to sheetrock joint.

• Moisture migration from the soil and crawlspaces.

• Excessive mechanical ventilation.

If you want to control air and humidity flows, you have to understand how the whole house works as an interactive system. Duct sealing, in particular, is an essential HVAC service that improves summertime humidity control. 

Be aware that if a house has high humidity in the summertime, the answer is almost never to add ventilation. Spot ventilation for short periods of time - from steamy bathrooms, for instance - is good. Constant dilution ventilation, even with an ERV, can (and usually will) only make things worse.


Insufficient a/c maintenance is a common cause of a/c condensate drain pan overflow. Related problems are a lack of emergency drain pans, or pans that aren’t large enough or sloped properly. Condensation on cold attic condensate drain lines can lead to water dripping on the ceiling. The same can happen with refrigerant suction lines due to missing rubber insulation, which often is chewed off by rodents.

Meanwhile, condensation on ducts and air handlers in unconditioned spaces is becoming even more common. Inadequate insulation and unintentionally low airflow have always been key factors. Low airflow leads to colder air in the ducts. This is now often done intentionally for enhanced dehumidification. Unfortunately, it can lead to increased duct and air handler sweating.  

At the same time, temperatures at which the occupants keep a home can greatly affect common moisture condensation situations. The colder the house, and the colder the supply air temperature, the greater the chances are of unintentional condensation on ducts, air handlers, and registers. Many homeowners overcool their houses in an attempt to achieve comfort if their a/c system can’t remove enough humidity.

Condensation also can occur in walls and floors that aren’t nearly as cold as ducts or condensate drains. If the house is cooled to 68° and the dew point of the outdoor air is 68° or higher - which is very common in the Southeast in the summer - the entire house structure can become an evaporator coil. If the walls are covered with vinyl wallpaper, or a floor over a crawlspace has vinyl flooring, mold growth is almost inevitable.  

The answer is to get the indoor temperature up by controlling humidity. As stated before, this involves many possible solutions, including duct and house infiltration sealing, enhancing a/c dehumidification, closing damp crawlspaces, and installing supplementary dehumidification.

The drier the house, the warmer the occupants are able to keep it at, and the less sweating problems they will have - both on their skin, and on the house/HVAC components.

Remember: The HVAC system can be either a contributor or solution to a moisture problem. Understand all the issues, and you’ll keep yourself out of trouble while also increasing sales.

Sidebar: Mold Remediation

Mold remediation is not a service to offer without getting fully trained and licensed accordingly. Be aware that, in many areas, it is already a crowded field, and that it bears more resemblance to a general renovation contractor than HVAC service and replacement. It should be set up as a separate business, rather than under the same name and company.  

However, even if you don’t hold yourself out to be a mold remediation contractor, inevitably there will be times when your HVAC service and replacement personnel come in contact with, or have to remove, moldy HVAC components – equipment, ducts, coils, etc.

This is currently a gray area, with lots of potential for different interpretations by licensing authorities - and, attorneys. At a bare minimum, always ensure field personnel are issued respiratory protection. Always take precautions when removing moldy ductwork or equipment from a home. Ensure that it is done properly so mold spores are not disseminated.

If moldy components have to pass through the living space, they should be double bagged. Registers and return grilles should be sealed off before tearing out the old ducts and equipment.

Consult local state licensing authorities and your attorney about whether your routine activities fall under laws governing mold remediation.

Publication date:08/13/2007