Many techs or customers will point out microbial growth and ask “is that mold?


Rather than comment on a topic that I am not trained in (Microbiology) we can look at the root cause of this sort of thing and address the real cause, it’s a moisture problem.  

In Florida, let’s say a customer sets their inside temperature down to 66 F. And, of course, aside from the system potentially freezing when it gets down to that temperature, if you get a space too cold inside, you’re going to have a big moisture driver from the outside, and you are actually going to drop the indoor temperature below dew point in a lot of cases.

This means that somewhere there is going to be moisture. Hopefully, it’s at the vapor barrier on the outside of the house (in Florida). But if the vapor barrier is not fully intact, then it could happen on the inside walls, right behind that drywall. So this is where you will see cases when you start to get a moisture problem behind drywall. Because when you don’t have a properly sealed envelope on the outside keeping moisture from coming in, in addition to a temperature inside the space that’s lower than it ought to be, it’s also likely lower than the outdoor dew point. Theoretically, we really don’t ever want to get the inside lower than the outdoor dew point because that’s going to create condensation somewhere.

In the real world, we can’t control our customers and sometimes that’s going to happen. So what do we do about it? One thing that I’ve been talking a lot about lately is ventilating dehumidifiers. With an independent dehumidifier you can make sure your relative humidity inside the space stays 55 percent or below, while also bringing in some outside air in order to put the house under positive pressure, which helps reduce infiltration and further helps control humidity.

Obviously, ventilating dehumidifiers is a fairly extreme solution and well suited to very humid climates. If you have a leaky house, there’s a lot of things you can do to help control humidity infiltration.  Sometimes you can fix it by sealing the envelop itself, keeping the moisture out in the first place. Maybe you can convince the customer that they shouldn’t keep their temperature so low which will reduce the likelihood that surfaces hit dew-point.

In some cases, things hit dew point regardless of what customer does. An example would be a duct that’s lost its installation or the installation is compressed. We often see this with flex duct that’s running over a truss and the duct is compressed on the underside. It is common to get some moisture in those locations because the insulation is compromised, and in that area you could have a moisture problem, which often means some biological growth.


So what action do we take when the problem we are solving is a moisture problem?


The In the case of the sagging duct, we need to lift that duct up, strap it up properly so we don’t have that compression of the insulation. What do we do in the case if we have a top of an air handler where it connects to the duct is sweating because it is improperly sealed or improperly connected? We properly seal it; we properly insulate it; properly connect it so you are not going to have that moisture problem.

Is the source of the problem that the supply air is at too low a temperature? And that low temperature supplier is resulting in that vent hitting dew point? Could that be because you have a dirty air filter? Or maybe you have a bypass damper that’s open in the zoning system? Other air restrictions that are causing a really low coil temperature? Could it be because the blower settings aren’t correct?

Those are all things that can result in the temperature of the air being lower than it should be, which can result in dew point and can result in a moisture problem.

Many moisture problems occur do to moisture drivers from within the building that are not being properly addressed. This means proper design and use of kitchen ventilation and bath fans to help expel moisture from the home before it can spread to the nearby indoor environment.

We need to seek to solve problems at their source by -

  • Controlling indoor humidity to keep it at 55% of lower
  • Keeping indoor temperatures above the outdoor dew point
  •  Addressing duct insulation and connection issues
  • Maintaining a sealed building envelope to reduce infiltration
  •  Exhaust moisture from kitchens and bathrooms
  •  Look at strategies like ventilating dehumidifiers in extreme situations

For more information about DMI Companies, visit This article originally appeared in the May 2019 issue of SNIPS Magazine.