Sweating air handlers and ductwork is generally outside the conditioned space. (Conversely, sweating vents are in the conditioned space.) We can help control attic conditions by adding vents or insulating to control the temperature in the attic, but controlling moisture is our main concern.

Getting to the dew point allows the attics to sweat, which gets moisture out of the air and onto surfaces. When the air runs over the cool duct surface, the moisture condenses and becomes liquid water. (It’s just like running air over a cold evaporator coil.) Insulating the attic is usually NOT the answer because it drops the temperature even more and makes the ducts sweat even more. (Sprayfoam is a type of insulation.)

If we didn’t have to worry about costs, we could keep an air handler or ducts from sweating by applying a space heater to the surface 24/7. Radiant barriers unfortunately have a similar effect as insulation; they keep the attic cooler, but they also cause the surface temperature to drop and be more prone to condensation.

The best option to control moisture in the attic is to seal it and dehumidify it. That option will cause your power bill to rise, but it will keep the attic drier. Sealing the attic, controlling the temperature, and controlling radiant gains inside the attic will have much more of an effect on duct sweating than changing the temperature of the air inside the ducts.

However, the best options will be expensive. We shouldn’t be afraid to offer expensive options, as it isn’t our job to decide the customer’s budget for them. The most important thing we can do is explain the consequences of each option and make sure the customer makes an educated decision.

Restaurants tend to have some of the worst rusting and dripping problems around their vents because it’s very difficult to control moisture inside the conditioned space. In the summer, low setpoints, moisture loads indoors (kitchens and baths), and increased moisture loads outdoors all increase the likelihood of vents sweating during the summer.

To combat summer moisture loads, try to get the customer to reduce the outdoor moisture’s likelihood of getting into the home (by closing windows and doors), ventilating in the kitchen and bathroom, and running the HVAC system longer with a colder evaporator coil.